3 <div style="border: 2px solid red; padding:5px;">
4 This guide is a work in progress. Until it is ready, we highly recommend that
5 you read the <a href="tutorial.html">Tutorial</a> instead. This work-in-progress Guide is being
6 displayed here in line with Rust's open development policy. Please open any
7 issues you find as usual.
12 Hey there! Welcome to the Rust guide. This is the place to be if you'd like to
13 learn how to program in Rust. Rust is a systems programming language with a
14 focus on "high-level, bare-metal programming": the lowest level control a
15 programming language can give you, but with zero-cost, higher level
16 abstractions, because people aren't computers. We really think Rust is
17 something special, and we hope you do too.
19 To show you how to get going with Rust, we're going to write the traditional
20 "Hello, World!" program. Next, we'll introduce you to a tool that's useful for
21 writing real-world Rust programs and libraries: "Cargo." After that, we'll talk
22 about the basics of Rust, write a little program to try them out, and then learn
29 The first step to using Rust is to install it! There are a number of ways to
30 install Rust, but the easiest is to use the `rustup` script. If you're on
31 Linux or a Mac, all you need to do is this (note that you don't need to type
32 in the `$`s, they just indicate the start of each command):
35 $ curl -s https://static.rust-lang.org/rustup.sh | sudo sh
38 (If you're concerned about `curl | sudo sh`, please keep reading. Disclaimer
41 If you're on Windows, please [download this .exe and run
42 it](https://static.rust-lang.org/dist/rust-nightly-install.exe).
44 If you decide you don't want Rust anymore, we'll be a bit sad, but that's okay.
45 Not every programming language is great for everyone. Just pass an argument to
49 $ curl -s https://static.rust-lang.org/rustup.sh | sudo sh -s -- --uninstall
52 If you used the Windows installer, just re-run the `.exe` and it will give you
55 You can re-run this script any time you want to update Rust. Which, at this
56 point, is often. Rust is still pre-1.0, and so people assume that you're using
59 This brings me to one other point: some people, and somewhat rightfully so, get
60 very upset when we tell you to `curl | sudo sh`. And they should be! Basically,
61 when you do this, you are trusting that the good people who maintain Rust
62 aren't going to hack your computer and do bad things. That's a good instinct!
63 If you're one of those people, please check out the documentation on [building
64 Rust from Source](https://github.com/rust-lang/rust#building-from-source), or
65 [the official binary downloads](http://www.rust-lang.org/install.html). And we
66 promise that this method will not be the way to install Rust forever: it's just
67 the easiest way to keep people updated while Rust is in its alpha state.
69 Oh, we should also mention the officially supported platforms:
71 * Windows (7, 8, Server 2008 R2), x86 only
72 * Linux (2.6.18 or later, various distributions), x86 and x86-64
73 * OSX 10.7 (Lion) or greater, x86 and x86-64
75 We extensively test Rust on these platforms, and a few others, too, like
76 Android. But these are the ones most likely to work, as they have the most
79 Finally, a comment about Windows. Rust considers Windows to be a first-class
80 platform upon release, but if we're honest, the Windows experience isn't as
81 integrated as the Linux/OS X experience is. We're working on it! If anything
82 does not work, it is a bug. Please let us know if that happens. Each and every
83 commit is tested against Windows just like any other platform.
85 If you've got Rust installed, you can open up a shell, and type this:
91 You should see some output that looks something like this:
94 rustc 0.12.0-pre (443a1cd 2014-06-08 14:56:52 -0700)
97 If you did, Rust has been installed successfully! Congrats!
99 If not, there are a number of places where you can get help. The easiest is
100 [the #rust IRC channel on irc.mozilla.org](irc://irc.mozilla.org/#rust), which
101 you can access through
102 [Mibbit](http://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust). Click
103 that link, and you'll be chatting with other Rustaceans (a silly nickname we
104 call ourselves), and we can help you out. Other great resources include [our
105 mailing list](https://mail.mozilla.org/listinfo/rust-dev), [the /r/rust
106 subreddit](http://www.reddit.com/r/rust), and [Stack
107 Overflow](http://stackoverflow.com/questions/tagged/rust).
111 Now that you have Rust installed, let's write your first Rust program. It's
112 traditional to make your first program in any new language one that prints the
113 text "Hello, world!" to the screen. The nice thing about starting with such a
114 simple program is that you can verify that your compiler isn't just installed,
115 but also working properly. And printing information to the screen is a pretty
118 The first thing that we need to do is make a file to put our code in. I like
119 to make a projects directory in my home directory, and keep all my projects
120 there. Rust does not care where your code lives.
122 This actually leads to one other concern we should address: this tutorial will
123 assume that you have basic familiarity with the command line. Rust does not
124 require that you know a whole ton about the command line, but until the
125 language is in a more finished state, IDE support is spotty. Rust makes no
126 specific demands on your editing tooling, or where your code lives.
128 With that said, let's make a directory in our projects directory.
137 If you're on Windows and not using PowerShell, the `~` may not work. Consult
138 the documentation for your shell for more details.
140 Let's make a new source file next. I'm going to use the syntax `editor
141 filename` to represent editing a file in these examples, but you should use
142 whatever method you want. We'll call our file `hello_world.rs`:
145 $ editor hello_world.rs
148 Rust files always end in a `.rs` extension. If you're using more than one word
149 in your file name, use an underscore. `hello_world.rs` versus `goodbye.rs`.
151 Now that you've got your file open, type this in:
155 println!("Hello, world!");
159 Save the file, and then type this into your terminal window:
162 $ rustc hello_world.rs
163 $ ./hello_world # or hello_world.exe on Windows
167 Success! Let's go over what just happened in detail.
175 These two lines define a **function** in Rust. The `main` function is special:
176 it's the beginning of every Rust program. The first line says "I'm declaring a
177 function named `main`, which takes no arguments and returns nothing." If there
178 were arguments, they would go inside the parentheses (`(` and `)`), and because
179 we aren't returning anything from this function, we've dropped that notation
180 entirely. We'll get to it later.
182 You'll also note that the function is wrapped in curly braces (`{` and `}`).
183 Rust requires these around all function bodies. It is also considered good
184 style to put the opening curly brace on the same line as the function
185 declaration, with one space in between.
187 Next up is this line:
190 println!("Hello, world!");
193 This line does all of the work in our little program. There are a number of
194 details that are important here. The first is that it's indented with four
195 spaces, not tabs. Please configure your editor of choice to insert four spaces
196 with the tab key. We provide some sample configurations for various editors
197 [here](https://github.com/rust-lang/rust/tree/master/src/etc).
199 The second point is the `println!()` part. This is calling a Rust **macro**,
200 which is how metaprogramming is done in Rust. If it were a function instead, it
201 would look like this: `println()`. For our purposes, we don't need to worry
202 about this difference. Just know that sometimes, you'll see a `!`, and that
203 means that you're calling a macro instead of a normal function. One last thing
204 to mention: Rust's macros are significantly different than C macros, if you've
205 used those. Don't be scared of using macros. We'll get to the details
206 eventually, you'll just have to trust us for now.
208 Next, `"Hello, world!"` is a **string**. Strings are a surprisingly complicated
209 topic in a systems programming language, and this is a **statically allocated**
210 string. We will talk more about different kinds of allocation later. We pass
211 this string as an argument to `println!`, which prints the string to the
214 Finally, the line ends with a semicolon (`;`). Rust is an **expression
215 oriented** language, which means that most things are expressions. The `;` is
216 used to indicate that this expression is over, and the next one is ready to
217 begin. Most lines of Rust code end with a `;`. We will cover this in-depth
218 later in the tutorial.
220 Finally, actually **compiling** and **running** our program. We can compile
221 with our compiler, `rustc`, by passing it the name of our source file:
224 $ rustc hello_world.rs
227 This is similar to `gcc` or `clang`, if you come from a C or C++ background. Rust
228 will output a binary executable. You can see it with `ls`:
232 hello_world hello_world.rs
239 hello_world.exe hello_world.rs
242 There are now two files: our source code, with the `.rs` extension, and the
243 executable (`hello_world.exe` on Windows, `hello_world` everywhere else)
246 $ ./hello_world # or hello_world.exe on Windows
249 This prints out our `Hello, world!` text to our terminal.
251 If you come from a dynamically typed language like Ruby, Python, or JavaScript,
252 you may not be used to these two steps being separate. Rust is an
253 **ahead-of-time compiled language**, which means that you can compile a
254 program, give it to someone else, and they don't need to have Rust installed.
255 If you give someone a `.rb` or `.py` or `.js` file, they need to have
256 Ruby/Python/JavaScript installed, but you just need one command to both compile
257 and run your program. Everything is a tradeoff in language design, and Rust has
260 Congratulations! You have officially written a Rust program. That makes you a
261 Rust programmer! Welcome.
263 Next, I'd like to introduce you to another tool, Cargo, which is used to write
264 real-world Rust programs. Just using `rustc` is nice for simple things, but as
265 your project grows, you'll want something to help you manage all of the options
266 that it has, and to make it easy to share your code with other people and
271 [Cargo](http://crates.io) is a tool that Rustaceans use to help manage their
272 Rust projects. Cargo is currently in an alpha state, just like Rust, and so it
273 is still a work in progress. However, it is already good enough to use for many
274 Rust projects, and so it is assumed that Rust projects will use Cargo from the
277 Cargo manages three things: building your code, downloading the dependencies
278 your code needs, and building the dependencies your code needs. At first, your
279 program doesn't have any dependencies, so we'll only be using the first part of
280 its functionality. Eventually, we'll add more. Since we started off by using
281 Cargo, it'll be easy to add later.
283 Let's convert Hello World to Cargo. The first thing we need to do to begin
284 using Cargo is to install Cargo. Luckily for us, the script we ran to install
285 Rust includes Cargo by default. If you installed Rust some other way, you may
286 want to [check the Cargo
287 README](https://github.com/rust-lang/cargo#installing-cargo-from-nightlies)
288 for specific instructions about installing it.
290 To Cargo-ify our project, we need to do two things: Make a `Cargo.toml`
291 configuration file, and put our source file in the right place. Let's
296 $ mv hello_world.rs src/hello_world.rs
299 Cargo expects your source files to live inside a `src` directory. That leaves
300 the top level for other things, like READMEs, licence information, and anything
301 not related to your code. Cargo helps us keep our projects nice and tidy. A
302 place for everything, and everything in its place.
304 Next, our configuration file:
310 Make sure to get this name right: you need the capital `C`!
319 authors = [ "Your name <you@example.com>" ]
326 This file is in the [TOML](https://github.com/toml-lang/toml) format. Let's let
327 it explain itself to you:
329 > TOML aims to be a minimal configuration file format that's easy to read due
330 > to obvious semantics. TOML is designed to map unambiguously to a hash table.
331 > TOML should be easy to parse into data structures in a wide variety of
334 TOML is very similar to INI, but with some extra goodies.
336 Anyway, there are two **table**s in this file: `package` and `bin`. The first
337 tells Cargo metadata about your package. The second tells Cargo that we're
338 interested in building a binary, not a library (though we could do both!), as
339 well as what it is named.
341 Once you have this file in place, we should be ready to build! Try this:
345 Compiling hello_world v0.0.1 (file:///home/yourname/projects/hello_world)
346 $ ./target/hello_world
350 Bam! We build our project with `cargo build`, and run it with
351 `./target/hello_world`. This hasn't bought us a whole lot over our simple use
352 of `rustc`, but think about the future: when our project has more than one
353 file, we would need to call `rustc` twice, and pass it a bunch of options to
354 tell it to build everything together. With Cargo, as our project grows, we can
355 just `cargo build` and it'll work the right way.
357 You'll also notice that Cargo has created a new file: `Cargo.lock`.
365 This file is used by Cargo to keep track of dependencies in your application.
366 Right now, we don't have any, so it's a bit sparse. You won't ever need
367 to touch this file yourself, just let Cargo handle it.
369 That's it! We've successfully built `hello_world` with Cargo. Even though our
370 program is simple, it's using much of the real tooling that you'll use for the
371 rest of your Rust career.
373 Now that you've got the tools down, let's actually learn more about the Rust
374 language itself. These are the basics that will serve you well through the rest
375 of your time with Rust.
379 The first thing we'll learn about are 'variable bindings.' They look like this:
385 In many languages, this is called a 'variable.' But Rust's variable bindings
386 have a few tricks up their sleeves. Rust has a very powerful feature called
387 'pattern matching' that we'll get into detail with later, but the left
388 hand side of a `let` expression is a full pattern, not just a variable name.
389 This means we can do things like:
392 let (x, y) = (1i, 2i);
395 After this expression is evaluated, `x` will be one, and `y` will be two.
396 Patterns are really powerful, but this is about all we can do with them so far.
397 So let's just keep this in the back of our minds as we go forward.
399 By the way, in these examples, `i` indicates that the number is an integer.
401 Rust is a statically typed language, which means that we specify our types up
402 front. So why does our first example compile? Well, Rust has this thing called
403 "[Hindley-Milner type
404 inference](http://en.wikipedia.org/wiki/Hindley%E2%80%93Milner_type_system)",
405 named after some really smart type theorists. If you clicked that link, don't
406 be scared: what this means for you is that Rust will attempt to infer the types
407 in your program, and it's pretty good at it. If it can infer the type, Rust
408 doesn't require you to actually type it out.
410 We can add the type if we want to. Types come after a colon (`:`):
416 If I asked you to read this out loud to the rest of the class, you'd say "`x`
417 is a binding with the type `int` and the value `five`."
419 By default, bindings are **immutable**. This code will not compile:
426 It will give you this error:
429 error: re-assignment of immutable variable `x`
434 If you want a binding to be mutable, you can use `mut`:
441 There is no single reason that bindings are immutable by default, but we can
442 think about it through one of Rust's primary focuses: safety. If you forget to
443 say `mut`, the compiler will catch it, and let you know that you have mutated
444 something you may not have cared to mutate. If bindings were mutable by
445 default, the compiler would not be able to tell you this. If you _did_ intend
446 mutation, then the solution is quite easy: add `mut`.
448 There are other good reasons to avoid mutable state when possible, but they're
449 out of the scope of this guide. In general, you can often avoid explicit
450 mutation, and so it is preferable in Rust. That said, sometimes, mutation is
451 what you need, so it's not verboten.
453 Let's get back to bindings. Rust variable bindings have one more aspect that
454 differs from other languages: bindings are required to be initialized with a
455 value before you're allowed to use them. If we try...
461 ...we'll get an error:
464 src/hello_world.rs:2:9: 2:10 error: cannot determine a type for this local variable: unconstrained type
465 src/hello_world.rs:2 let x;
469 Giving it a type will compile, though:
475 Let's try it out. Change your `src/hello_world.rs` file to look like this:
481 println!("Hello world!");
485 You can use `cargo build` on the command line to build it. You'll get a warning,
486 but it will still print "Hello, world!":
489 Compiling hello_world v0.0.1 (file:///home/you/projects/hello_world)
490 src/hello_world.rs:2:9: 2:10 warning: unused variable: `x`, #[warn(unused_variable)] on by default
491 src/hello_world.rs:2 let x: int;
495 Rust warns us that we never use the variable binding, but since we never use it,
496 no harm, no foul. Things change if we try to actually use this `x`, however. Let's
497 do that. Change your program to look like this:
503 println!("The value of x is: {}", x);
507 And try to build it. You'll get an error:
511 Compiling hello_world v0.0.1 (file:///home/you/projects/hello_world)
512 src/hello_world.rs:4:39: 4:40 error: use of possibly uninitialized variable: `x`
513 src/hello_world.rs:4 println!("The value of x is: {}", x);
515 note: in expansion of format_args!
516 <std macros>:2:23: 2:77 note: expansion site
517 <std macros>:1:1: 3:2 note: in expansion of println!
518 src/hello_world.rs:4:5: 4:42 note: expansion site
519 error: aborting due to previous error
520 Could not compile `hello_world`.
523 Rust will not let us use a value that has not been initialized. So why let us
524 declare a binding without initializing it? You'd think our first example would
525 have errored. Well, Rust is smarter than that. Before we get to that, let's talk
526 about this stuff we've added to `println!`.
528 If you include two curly braces (`{}`, some call them moustaches...) in your
529 string to print, Rust will interpret this as a request to interpolate some sort
530 of value. **String interpolation** is a computer science term that means "stick
531 in the middle of a string." We add a comma, and then `x`, to indicate that we
532 want `x` to be the value we're interpolating. The comma is used to separate
533 arguments we pass to functions and macros, if you're passing more than one.
535 When you just use the curly braces, Rust will attempt to display the
536 value in a meaningful way by checking out its type. If you want to specify the
537 format in a more detailed manner, there are a [wide number of options
538 available](std/fmt/index.html). For now, we'll just stick to the default:
539 integers aren't very complicated to print.
541 So, we've cleared up all of the confusion around bindings, with one exception:
542 why does Rust let us declare a variable binding without an initial value if we
543 must initialize the binding before we use it? And how does it know that we have
544 or have not initialized the binding? For that, we need to learn our next
549 Rust's take on `if` is not particularly complex, but it's much more like the
550 `if` you'll find in a dynamically typed language than in a more traditional
551 systems language. So let's talk about it, to make sure you grasp the nuances.
553 `if` is a specific form of a more general concept, the 'branch.' The name comes
554 from a branch in a tree: a decision point, where depending on a choice,
555 multiple paths can be taken.
557 In the case of `if`, there is one choice that leads down two paths:
563 println!("x is five!");
567 If we changed the value of `x` to something else, this line would not print.
568 More specifically, if the expression after the `if` evaluates to `true`, then
569 the block is executed. If it's `false`, then it is not.
571 If you want something to happen in the `false` case, use an `else`:
577 println!("x is five!");
579 println!("x is not five :(");
583 This is all pretty standard. However, you can also do this:
596 Which we can (and probably should) write like this:
601 let y = if x == 5i { 10i } else { 15i };
604 This reveals two interesting things about Rust: it is an expression-based
605 language, and semicolons are different than in other 'curly brace and
606 semicolon'-based languages. These two things are related.
608 ## Expressions vs. Statements
610 Rust is primarily an expression based language. There are only two kinds of
611 statements, and everything else is an expression.
613 So what's the difference? Expressions return a value, and statements do not.
614 In many languages, `if` is a statement, and therefore, `let x = if ...` would
615 make no sense. But in Rust, `if` is an expression, which means that it returns
616 a value. We can then use this value to initialize the binding.
618 Speaking of which, bindings are a kind of the first of Rust's two statements.
619 The proper name is a **declaration statement**. So far, `let` is the only kind
620 of declaration statement we've seen. Let's talk about that some more.
622 In some languages, variable bindings can be written as expressions, not just
623 statements. Like Ruby:
629 In Rust, however, using `let` to introduce a binding is _not_ an expression. The
630 following will produce a compile-time error:
633 let x = (let y = 5i); // found `let` in ident position
636 The compiler is telling us here that it was expecting to see the beginning of
637 an expression, and a `let` can only begin a statement, not an expression.
639 Note that assigning to an already-bound variable (e.g. `y = 5i`) is still an
640 expression, although its value is not particularly useful. Unlike C, where an
641 assignment evaluates to the assigned value (e.g. `5i` in the previous example),
642 in Rust the value of an assignment is the unit type `()` (which we'll cover later).
644 The second kind of statement in Rust is the **expression statement**. Its
645 purpose is to turn any expression into a statement. In practical terms, Rust's
646 grammar expects statements to follow other statements. This means that you use
647 semicolons to separate expressions from each other. This means that Rust
648 looks a lot like most other languages that require you to use semicolons
649 at the end of every line, and you will see semicolons at the end of almost
650 every line of Rust code you see.
652 What is this exception that makes us say 'almost?' You saw it already, in this
658 let y: int = if x == 5i { 10i } else { 15i };
661 Note that I've added the type annotation to `y`, to specify explicitly that I
662 want `y` to be an integer.
664 This is not the same as this, which won't compile:
669 let y: int = if x == 5i { 10i; } else { 15i; };
672 Note the semicolons after the 10 and 15. Rust will give us the following error:
675 error: mismatched types: expected `int` but found `()` (expected int but found ())
678 We expected an integer, but we got `()`. `()` is pronounced 'unit', and is a
679 special type in Rust's type system. `()` is different than `null` in other
680 languages, because `()` is distinct from other types. For example, in C, `null`
681 is a valid value for a variable of type `int`. In Rust, `()` is _not_ a valid
682 value for a variable of type `int`. It's only a valid value for variables of
683 the type `()`, which aren't very useful. Remember how we said statements don't
684 return a value? Well, that's the purpose of unit in this case. The semicolon
685 turns any expression into a statement by throwing away its value and returning
688 There's one more time in which you won't see a semicolon at the end of a line
689 of Rust code. For that, we'll need our next concept: functions.
693 You've already seen one function so far, the `main` function:
700 This is the simplest possible function declaration. As we mentioned before,
701 `fn` says 'this is a function,' followed by the name, some parenthesis because
702 this function takes no arguments, and then some curly braces to indicate the
703 body. Here's a function named `foo`:
710 So, what about taking arguments? Here's a function that prints a number:
713 fn print_number(x: int) {
714 println!("x is: {}", x);
718 Here's a complete program that uses `print_number`:
725 fn print_number(x: int) {
726 println!("x is: {}", x);
730 As you can see, function arguments work very similar to `let` declarations:
731 you add a type to the argument name, after a colon.
733 Here's a complete program that adds two numbers together and prints them:
740 fn print_sum(x: int, y: int) {
741 println!("sum is: {}", x + y);
745 You separate arguments with a comma, both when you call the function, as well
746 as when you declare it.
748 Unlike `let`, you _must_ declare the types of function arguments. This does
752 fn print_number(x, y) {
753 println!("x is: {}", x + y);
760 hello.rs:5:18: 5:19 error: expected `:` but found `,`
761 hello.rs:5 fn print_number(x, y) {
764 This is a deliberate design decision. While full-program inference is possible,
765 languages which have it, like Haskell, often suggest that documenting your
766 types explicitly is a best-practice. We agree that forcing functions to declare
767 types while allowing for inference inside of function bodies is a wonderful
768 compromise between full inference and no inference.
770 What about returning a value? Here's a function that adds one to an integer:
773 fn add_one(x: int) -> int {
778 Rust functions return exactly one value, and you declare the type after an
779 'arrow', which is a dash (`-`) followed by a greater-than sign (`>`).
781 You'll note the lack of a semicolon here. If we added it in:
784 fn add_one(x: int) -> int {
789 We would get an error:
792 error: not all control paths return a value
793 fn add_one(x: int) -> int {
797 note: consider removing this semicolon:
802 Remember our earlier discussions about semicolons and `()`? Our function claims
803 to return an `int`, but with a semicolon, it would return `()` instead. Rust
804 realizes this probably isn't what we want, and suggests removing the semicolon.
806 This is very much like our `if` statement before: the result of the block
807 (`{}`) is the value of the expression. Other expression-oriented languages,
808 such as Ruby, work like this, but it's a bit unusual in the systems programming
809 world. When people first learn about this, they usually assume that it
810 introduces bugs. But because Rust's type system is so strong, and because unit
811 is its own unique type, we have never seen an issue where adding or removing a
812 semicolon in a return position would cause a bug.
814 But what about early returns? Rust does have a keyword for that, `return`:
817 fn foo(x: int) -> int {
818 if x < 5 { return x; }
824 Using a `return` as the last line of a function works, but is considered poor
828 fn foo(x: int) -> int {
829 if x < 5 { return x; }
835 There are some additional ways to define functions, but they involve features
836 that we haven't learned about yet, so let's just leave it at that for now.
841 Now that we have some functions, it's a good idea to learn about comments.
842 Comments are notes that you leave to other programmers to help explain things
843 about your code. The compiler mostly ignores them.
845 Rust has two kinds of comments that you should care about: **line comment**s
846 and **doc comment**s.
849 // Line comments are anything after '//' and extend to the end of the line.
851 let x = 5i; // this is also a line comment.
853 // If you have a long explanation for something, you can put line comments next
854 // to each other. Put a space between the // and your comment so that it's
858 The other kind of comment is a doc comment. Doc comments use `///` instead of
859 `//`, and support Markdown notation inside:
862 /// `hello` is a function that prints a greeting that is personalized based on
867 /// * `name` - The name of the person you'd like to greet.
872 /// let name = "Steve";
873 /// hello(name); // prints "Hello, Steve!"
875 fn hello(name: &str) {
876 println!("Hello, {}!", name);
880 When writing doc comments, adding sections for any arguments, return values,
881 and providing some examples of usage is very, very helpful.
883 You can use the `rustdoc` tool to generate HTML documentation from these doc
884 comments. We will talk more about `rustdoc` when we get to modules, as
885 generally, you want to export documentation for a full module.
887 # Compound Data Types
889 Rust, like many programming languages, has a number of different data types
890 that are built-in. You've already done some simple work with integers and
891 strings, but next, let's talk about some more complicated ways of storing data.
895 The first compound data type we're going to talk about are called **tuple**s.
896 Tuples are an ordered list of a fixed size. Like this:
899 let x = (1i, "hello");
902 The parenthesis and commas form this two-length tuple. Here's the same code, but
903 with the type annotated:
906 let x: (int, &str) = (1, "hello");
909 As you can see, the type of a tuple looks just like the tuple, but with each
910 position having a type name rather than the value. Careful readers will also
911 note that tuples are heterogeneous: we have an `int` and a `&str` in this tuple.
912 You haven't seen `&str` as a type before, and we'll discuss the details of
913 strings later. In systems programming languages, strings are a bit more complex
914 than in other languages. For now, just read `&str` as "a string slice," and
915 we'll learn more soon.
917 You can access the fields in a tuple through a **destructuring let**. Here's
921 let (x, y, z) = (1i, 2i, 3i);
923 println!("x is {}", x);
926 Remember before when I said the left hand side of a `let` statement was more
927 powerful than just assigning a binding? Here we are. We can put a pattern on
928 the left hand side of the `let`, and if it matches up to the right hand side,
929 we can assign multiple bindings at once. In this case, `let` 'destructures,'
930 or 'breaks up,' the tuple, and assigns the bits to three bindings.
932 This pattern is very powerful, and we'll see it repeated more later.
934 The last thing to say about tuples is that they are only equivalent if
935 the arity, types, and values are all identical.
938 let x = (1i, 2i, 3i);
939 let y = (2i, 3i, 4i);
948 This will print `no`, as the values aren't equal.
950 One other use of tuples is to return multiple values from a function:
953 fn next_two(x: int) -> (int, int) { (x + 1i, x + 2i) }
956 let (x, y) = next_two(5i);
957 println!("x, y = {}, {}", x, y);
961 Even though Rust functions can only return one value, a tuple _is_ one value,
962 that happens to be made up of two. You can also see in this example how you
963 can destructure a pattern returned by a function, as well.
965 Tuples are a very simple data structure, and so are not often what you want.
966 Let's move on to their bigger sibling, structs.
970 A struct is another form of a 'record type,' just like a tuple. There's a
971 difference: structs give each element that they contain a name, called a
972 'field' or a 'member.' Check it out:
981 let origin = Point { x: 0i, y: 0i };
983 println!("The origin is at ({}, {})", origin.x, origin.y);
987 There's a lot going on here, so let's break it down. We declare a struct with
988 the `struct` keyword, and then with a name. By convention, structs begin with a
989 capital letter and are also camel cased: `PointInSpace`, not `Point_In_Space`.
991 We can create an instance of our struct via `let`, as usual, but we use a `key:
992 value` style syntax to set each field. The order doesn't need to be the same as
993 in the original declaration.
995 Finally, because fields have names, we can access the field through dot
996 notation: `origin.x`.
998 The values in structs are immutable, like other bindings in Rust. However, you
999 can use `mut` to make them mutable:
1008 let mut point = Point { x: 0i, y: 0i };
1012 println!("The point is at ({}, {})", point.x, point.y);
1016 This will print `The point is at (5, 0)`.
1018 ## Tuple Structs and Newtypes
1020 Rust has another data type that's like a hybrid between a tuple and a struct,
1021 called a **tuple struct**. Tuple structs do have a name, but their fields
1026 struct Color(int, int, int);
1027 struct Point(int, int, int);
1030 These two will not be equal, even if they have the same values:
1033 let black = Color(0, 0, 0);
1034 let origin = Point(0, 0, 0);
1037 It is almost always better to use a struct than a tuple struct. We would write
1038 `Color` and `Point` like this instead:
1054 Now, we have actual names, rather than positions. Good names are important,
1055 and with a struct, we have actual names.
1057 There _is_ one case when a tuple struct is very useful, though, and that's a
1058 tuple struct with only one element. We call this a 'newtype,' because it lets
1059 you create a new type that's a synonym for another one:
1064 let length = Inches(10);
1066 let Inches(integer_length) = length;
1067 println!("length is {} inches", integer_length);
1070 As you can see here, you can extract the inner integer type through a
1071 destructuring `let`.
1075 Finally, Rust has a "sum type", an **enum**. Enums are an incredibly useful
1076 feature of Rust, and are used throughout the standard library. This is an enum
1077 that is provided by the Rust standard library:
1087 An `Ordering` can only be _one_ of `Less`, `Equal`, or `Greater` at any given
1088 time. Here's an example:
1091 fn cmp(a: int, b: int) -> Ordering {
1093 else if a > b { Greater }
1101 let ordering = cmp(x, y);
1103 if ordering == Less {
1105 } else if ordering == Greater {
1106 println!("greater");
1107 } else if ordering == Equal {
1113 `cmp` is a function that compares two things, and returns an `Ordering`. We
1114 return either `Less`, `Greater`, or `Equal`, depending on if the two values
1115 are greater, less, or equal.
1117 The `ordering` variable has the type `Ordering`, and so contains one of the
1118 three values. We can then do a bunch of `if`/`else` comparisons to check
1121 However, repeated `if`/`else` comparisons get quite tedious. Rust has a feature
1122 that not only makes them nicer to read, but also makes sure that you never
1123 miss a case. Before we get to that, though, let's talk about another kind of
1124 enum: one with values.
1126 This enum has two variants, one of which has a value:
1139 Value(n) => println!("x is {:d}", n),
1140 Missing => println!("x is missing!"),
1144 Value(n) => println!("y is {:d}", n),
1145 Missing => println!("y is missing!"),
1150 This enum represents an `int` that we may or may not have. In the `Missing`
1151 case, we have no value, but in the `Value` case, we do. This enum is specific
1152 to `int`s, though. We can make it usable by any type, but we haven't quite
1155 You can have any number of values in an enum:
1158 enum OptionalColor {
1159 Color(int, int, int),
1164 Enums with values are quite useful, but as I mentioned, they're even more
1165 useful when they're generic across types. But before we get to generics, let's
1166 talk about how to fix this big `if`/`else` statements we've been writing. We'll
1167 do that with `match`.
1171 Often, a simple `if`/`else` isn't enough, because you have more than two
1172 possible options. And `else` conditions can get incredibly complicated. So
1173 what's the solution?
1175 Rust has a keyword, `match`, that allows you to replace complicated `if`/`else`
1176 groupings with something more powerful. Check it out:
1182 1 => println!("one"),
1183 2 => println!("two"),
1184 3 => println!("three"),
1185 4 => println!("four"),
1186 5 => println!("five"),
1187 _ => println!("something else"),
1191 `match` takes an expression, and then branches based on its value. Each 'arm' of
1192 the branch is of the form `val => expression`. When the value matches, that arm's
1193 expression will be evaluated. It's called `match` because of the term 'pattern
1194 matching,' which `match` is an implementation of.
1196 So what's the big advantage here? Well, there are a few. First of all, `match`
1197 does 'exhaustiveness checking.' Do you see that last arm, the one with the
1198 underscore (`_`)? If we remove that arm, Rust will give us an error:
1201 error: non-exhaustive patterns: `_` not covered
1204 In other words, Rust is trying to tell us we forgot a value. Because `x` is an
1205 integer, Rust knows that it can have a number of different values. For example,
1206 `6i`. But without the `_`, there is no arm that could match, and so Rust refuses
1207 to compile. `_` is sort of like a catch-all arm. If none of the other arms match,
1208 the arm with `_` will. And since we have this catch-all arm, we now have an arm
1209 for every possible value of `x`, and so our program will now compile.
1211 `match` statements also destructure enums, as well. Remember this code from the
1215 fn cmp(a: int, b: int) -> Ordering {
1217 else if a > b { Greater }
1225 let ordering = cmp(x, y);
1227 if ordering == Less {
1229 } else if ordering == Greater {
1230 println!("greater");
1231 } else if ordering == Equal {
1237 We can re-write this as a `match`:
1240 fn cmp(a: int, b: int) -> Ordering {
1242 else if a > b { Greater }
1251 Less => println!("less"),
1252 Greater => println!("greater"),
1253 Equal => println!("equal"),
1258 This version has way less noise, and it also checks exhaustively to make sure
1259 that we have covered all possible variants of `Ordering`. With our `if`/`else`
1260 version, if we had forgotten the `Greater` case, for example, our program would
1261 have happily compiled. If we forget in the `match`, it will not. Rust helps us
1262 make sure to cover all of our bases.
1264 `match` is also an expression, which means we can use it on the right hand side
1265 of a `let` binding. We could also implement the previous line like this:
1268 fn cmp(a: int, b: int) -> Ordering {
1270 else if a > b { Greater }
1278 let result = match cmp(x, y) {
1280 Greater => "greater",
1284 println!("{}", result);
1288 In this case, it doesn't make a lot of sense, as we are just making a temporary
1289 string where we don't need to, but sometimes, it's a nice pattern.
1293 Looping is the last basic construct that we haven't learned yet in Rust. Rust has
1294 two main looping constructs: `for` and `while`.
1298 The `for` loop is used to loop a particular number of times. Rust's `for` loops
1299 work a bit differently than in other systems languages, however. Rust's `for`
1300 loop doesn't look like this C `for` loop:
1303 for (x = 0; x < 10; x++) {
1304 printf( "%d\n", x );
1311 for x in range(0i, 10i) {
1312 println!("{:d}", x);
1316 In slightly more abstract terms,
1319 for var in expression {
1324 The expression is an iterator, which we will discuss in more depth later in the
1325 guide. The iterator gives back a series of elements. Each element is one
1326 iteration of the loop. That value is then bound to the name `var`, which is
1327 valid for the loop body. Once the body is over, the next value is fetched from
1328 the iterator, and we loop another time. When there are no more values, the
1331 In our example, the `range` function is a function, provided by Rust, that
1332 takes a start and an end position, and gives an iterator over those values. The
1333 upper bound is exclusive, though, so our loop will print `0` through `9`, not
1336 Rust does not have the "C style" `for` loop on purpose. Manually controlling
1337 each element of the loop is complicated and error prone, even for experienced C
1338 developers. There's an old joke that goes, "There are two hard problems in
1339 computer science: naming things, cache invalidation, and off-by-one errors."
1340 The joke, of course, being that the setup says "two hard problems" but then
1341 lists three things. This happens quite a bit with "C style" `for` loops.
1343 We'll talk more about `for` when we cover **iterator**s, later in the Guide.
1347 The other kind of looping construct in Rust is the `while` loop. It looks like
1352 let mut done = false;
1357 if x % 5 == 0 { done = true; }
1361 `while` loops are the correct choice when you're not sure how many times
1364 If you need an infinite loop, you may be tempted to write this:
1370 Rust has a dedicated keyword, `loop`, to handle this case:
1376 Rust's control-flow analysis treats this construct differently than a
1377 `while true`, since we know that it will always loop. The details of what
1378 that _means_ aren't super important to understand at this stage, but in
1379 general, the more information we can give to the compiler, the better it
1380 can do with safety and code generation. So you should always prefer
1381 `loop` when you plan to loop infinitely.
1383 ## Ending iteration early
1385 Let's take a look at that `while` loop we had earlier:
1389 let mut done = false;
1394 if x % 5 == 0 { done = true; }
1398 We had to keep a dedicated `mut` boolean variable binding, `done`, to know
1399 when we should skip out of the loop. Rust has two keywords to help us with
1400 modifying iteration: `break` and `continue`.
1402 In this case, we can write the loop in a better way with `break`:
1410 if x % 5 == 0 { break; }
1414 We now loop forever with `loop`, and use `break` to break out early.
1416 `continue` is similar, but instead of ending the loop, goes to the next
1417 iteration: This will only print the odd numbers:
1420 for x in range(0i, 10i) {
1421 if x % 2 == 0 { continue; }
1423 println!("{:d}", x);
1427 Both `continue` and `break` are valid in both kinds of loops.
1431 Strings are an important concept for any programmer to master. Rust's string
1432 handling system is a bit different than in other languages, due to its systems
1433 focus. Any time you have a data structure of variable size, things can get
1434 tricky, and strings are a re-sizable data structure. That said, Rust's strings
1435 also work differently than in some other systems languages, such as C.
1437 Let's dig into the details. A **string** is a sequence of unicode scalar values
1438 encoded as a stream of UTF-8 bytes. All strings are guaranteed to be
1439 validly-encoded UTF-8 sequences. Additionally, strings are not null-terminated
1440 and can contain null bytes.
1442 Rust has two main types of strings: `&str` and `String`.
1444 The first kind is a `&str`. This is pronounced a 'string slice.' String literals
1445 are of the type `&str`:
1448 let string = "Hello there.";
1451 This string is statically allocated, meaning that it's saved inside our
1452 compiled program, and exists for the entire duration it runs. The `string`
1453 binding is a reference to this statically allocated string. String slices
1454 have a fixed size, and cannot be mutated.
1456 A `String`, on the other hand, is an in-memory string. This string is
1457 growable, and is also guaranteed to be UTF-8.
1460 let mut s = "Hello".to_string();
1463 s.push_str(", world.");
1467 You can coerce a `String` into a `&str` with the `as_slice()` method:
1470 fn takes_slice(slice: &str) {
1471 println!("Got: {}", slice);
1475 let s = "Hello".to_string();
1476 takes_slice(s.as_slice());
1480 To compare a String to a constant string, prefer `as_slice()`...
1483 fn compare(string: String) {
1484 if string.as_slice() == "Hello" {
1490 ... over `to_string()`:
1493 fn compare(string: String) {
1494 if string == "Hello".to_string() {
1500 Converting a `String` to a `&str` is cheap, but converting the `&str` to a
1501 `String` involves allocating memory. No reason to do that unless you have to!
1503 That's the basics of strings in Rust! They're probably a bit more complicated
1504 than you are used to, if you come from a scripting language, but when the
1505 low-level details matter, they really matter. Just remember that `String`s
1506 allocate memory and control their data, while `&str`s are a reference to
1507 another string, and you'll be all set.
1511 Like many programming languages, Rust has a list type for when you want a list
1512 of things. But similar to strings, Rust has different types to represent this
1513 idea: `Vec<T>` (a 'vector'), `[T, .. N]` (an 'array'), and `&[T]` (a 'slice').
1516 Vectors are similar to `String`s: they have a dynamic length, and they
1517 allocate enough memory to fit. You can create a vector with the `vec!` macro:
1520 let nums = vec![1i, 2i, 3i];
1523 Notice that unlike the `println!` macro we've used in the past, we use square
1524 brackets (`[]`) with `vec!`. Rust allows you to use either in either situation,
1525 this is just convention.
1527 You can create an array with just square brackets:
1530 let nums = [1i, 2i, 3i];
1533 So what's the difference? An array has a fixed size, so you can't add or
1537 let mut nums = vec![1i, 2i, 3i];
1538 nums.push(4i); // works
1540 let mut nums = [1i, 2i, 3i];
1541 nums.push(4i); // error: type `[int, .. 3]` does not implement any method
1542 // in scope named `push`
1545 The `push()` method lets you append a value to the end of the vector. But
1546 since arrays have fixed sizes, adding an element doesn't make any sense.
1547 You can see how it has the exact type in the error message: `[int, .. 3]`.
1548 An array of `int`s, with length 3.
1550 Similar to `&str`, a slice is a reference to another array. We can get a
1551 slice from a vector by using the `as_slice()` method:
1554 let vec = vec![1i, 2i, 3i];
1555 let slice = vec.as_slice();
1558 All three types implement an `iter()` method, which returns an iterator. We'll
1559 talk more about the details of iterators later, but for now, the `iter()` method
1560 allows you to write a `for` loop that prints out the contents of a vector, array,
1564 let vec = vec![1i, 2i, 3i];
1566 for i in vec.iter() {
1571 This code will print each number in order, on its own line.
1573 You can access a particular element of a vector, array, or slice by using
1574 **subscript notation**:
1577 let names = ["Graydon", "Brian", "Niko"];
1579 println!("The second name is: {}", names[1]);
1582 These subscripts start at zero, like in most programming languages, so the
1583 first name is `names[0]` and the second name is `names[1]`. The above example
1584 prints `The second name is Brian`.
1586 There's a whole lot more to vectors, but that's enough to get started. We have
1587 now learned all of the most basic Rust concepts. We're ready to start building
1588 our guessing game, but we need to know how to do one last thing first: get
1589 input from the keyboard. You can't have a guessing game without the ability to
1594 Getting input from the keyboard is pretty easy, but uses some things
1595 we haven't seen before. Here's a simple program that reads some input,
1596 and then prints it back out:
1602 println!("Type something!");
1604 let input = std::io::stdin().read_line().ok().expect("Failed to read line");
1606 println!("{}", input);
1610 Let's go over these chunks, one by one:
1616 This calls a function, `stdin()`, that lives inside the `std::io` module. As
1617 you can imagine, everything in `std` is provided by Rust, the 'standard
1618 library.' We'll talk more about the module system later.
1620 Since writing the fully qualified name all the time is annoying, we can use
1621 the `use` statement to import it in:
1629 However, it's considered better practice to not import individual functions, but
1630 to import the module, and only use one level of qualification:
1638 Let's update our example to use this style:
1644 println!("Type something!");
1646 let input = io::stdin().read_line().ok().expect("Failed to read line");
1648 println!("{}", input);
1658 The `read_line()` method can be called on the result of `stdin()` to return
1659 a full line of input. Nice and easy.
1662 .ok().expect("Failed to read line");
1665 Do you remember this code?
1678 Value(n) => println!("x is {:d}", n),
1679 Missing => println!("x is missing!"),
1683 Value(n) => println!("y is {:d}", n),
1684 Missing => println!("y is missing!"),
1689 We had to match each time, to see if we had a value or not. In this case,
1690 though, we _know_ that `x` has a `Value`. But `match` forces us to handle
1691 the `missing` case. This is what we want 99% of the time, but sometimes, we
1692 know better than the compiler.
1694 Likewise, `read_line()` does not return a line of input. It _might_ return a
1695 line of input. It might also fail to do so. This could happen if our program
1696 isn't running in a terminal, but as part of a cron job, or some other context
1697 where there's no standard input. Because of this, `read_line` returns a type
1698 very similar to our `OptionalInt`: an `IoResult<T>`. We haven't talked about
1699 `IoResult<T>` yet because it is the **generic** form of our `OptionalInt`.
1700 Until then, you can think of it as being the same thing, just for any type, not
1703 Rust provides a method on these `IoResult<T>`s called `ok()`, which does the
1704 same thing as our `match` statement, but assuming that we have a valid value.
1705 If we don't, it will terminate our program. In this case, if we can't get
1706 input, our program doesn't work, so we're okay with that. In most cases, we
1707 would want to handle the error case explicitly. The result of `ok()` has a
1708 method, `expect()`, which allows us to give an error message if this crash
1711 We will cover the exact details of how all of this works later in the Guide.
1712 For now, this gives you enough of a basic understanding to work with.
1714 Back to the code we were working on! Here's a refresher:
1720 println!("Type something!");
1722 let input = io::stdin().read_line().ok().expect("Failed to read line");
1724 println!("{}", input);
1728 With long lines like this, Rust gives you some flexibility with the whitespace.
1729 We _could_ write the example like this:
1735 println!("Type something!");
1737 let input = io::stdin()
1740 .expect("Failed to read line");
1742 println!("{}", input);
1746 Sometimes, this makes things more readable. Sometimes, less. Use your judgement
1749 That's all you need to get basic input from the standard input! It's not too
1750 complicated, but there are a number of small parts.
1754 Okay! We've got the basics of Rust down. Let's write a bigger program.
1756 For our first project, we'll implement a classic beginner programming problem:
1757 the guessing game. Here's how it works: Our program will generate a random
1758 integer between one and a hundred. It will then prompt us to enter a guess.
1759 Upon entering our guess, it will tell us if we're too low or too high. Once we
1760 guess correctly, it will congratulate us, and print the number of guesses we've
1761 taken to the screen. Sound good?
1765 Let's set up a new project. Go to your projects directory. Remember how we
1766 had to create our directory structure and a `Cargo.toml` for `hello_world`? Cargo
1767 has a command that does that for us. Let's give it a shot:
1771 $ cargo new guessing_game --bin
1775 We pass the name of our project to `cargo new`, and then the `--bin` flag,
1776 since we're making a binary, rather than a library.
1778 Check out the generated `Cargo.toml`:
1783 name = "guessing_game"
1785 authors = ["Your Name <you@example.com>"]
1788 Cargo gets this information from your environment. If it's not correct, go ahead
1791 Finally, Cargo generated a hello, world for us. Check out `src/main.rs`:
1795 println!("Hello, world!");
1799 Let's try compiling what Cargo gave us:
1803 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1806 Excellent! Open up your `src/main.rs` again. We'll be writing all of
1807 our code in this file. We'll talk about multiple-file projects later on in the
1810 Before we move on, let me show you one more Cargo command: `run`. `cargo run`
1811 is kind of like `cargo build`, but it also then runs the produced exectuable.
1816 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
1817 Running `target/guessing_game`
1821 Great! The `run` command comes in handy when you need to rapidly iterate on a project.
1822 Our game is just such a project, we need to quickly test each iteration before moving on to the next one.
1824 ## Processing a Guess
1826 Let's get to it! The first thing we need to do for our guessing game is
1827 allow our player to input a guess. Put this in your `src/main.rs`:
1833 println!("Guess the number!");
1835 println!("Please input your guess.");
1837 let input = io::stdin().read_line()
1839 .expect("Failed to read line");
1841 println!("You guessed: {}", input);
1845 You've seen this code before, when we talked about standard input. We
1846 import the `std::io` module with `use`, and then our `main` function contains
1847 our program's logic. We print a little message announcing the game, ask the
1848 user to input a guess, get their input, and then print it out.
1850 Because we talked about this in the section on standard I/O, I won't go into
1851 more details here. If you need a refresher, go re-read that section.
1853 ## Generating a secret number
1855 Next, we need to generate a secret number. To do that, we need to use Rust's
1856 random number generation, which we haven't talked about yet. Rust includes a
1857 bunch of interesting functions in its standard library. If you need a bit of
1858 code, it's possible that it's already been written for you! In this case,
1859 we do know that Rust has random number generation, but we don't know how to
1862 Enter the docs. Rust has a page specifically to document the standard library.
1863 You can find that page [here](std/index.html). There's a lot of information on
1864 that page, but the best part is the search bar. Right up at the top, there's
1865 a box that you can enter in a search term. The search is pretty primitive
1866 right now, but is getting better all the time. If you type 'random' in that
1867 box, the page will update to [this
1868 one](http://doc.rust-lang.org/std/index.html?search=random). The very first
1870 [std::rand::random](http://doc.rust-lang.org/std/rand/fn.random.html). If we
1871 click on that result, we'll be taken to its documentation page.
1873 This page shows us a few things: the type signature of the function, some
1874 explanatory text, and then an example. Let's modify our code to add in the
1882 println!("Guess the number!");
1884 let secret_number = (rand::random() % 100i) + 1i;
1886 println!("The secret number is: {}", secret_number);
1888 println!("Please input your guess.");
1890 let input = io::stdin().read_line()
1892 .expect("Failed to read line");
1895 println!("You guessed: {}", input);
1899 The first thing we changed was to `use std::rand`, as the docs
1900 explained. We then added in a `let` expression to create a variable binding
1901 named `secret_number`, and we printed out its result.
1903 Also, you may wonder why we are using `%` on the result of `rand::random()`.
1904 This operator is called 'modulo', and it returns the remainder of a division.
1905 By taking the modulo of the result of `rand::random()`, we're limiting the
1906 values to be between 0 and 99. Then, we add one to the result, making it from 1
1907 to 100. Using modulo can give you a very, very small bias in the result, but
1908 for this example, it is not important.
1910 Let's try to compile this using `cargo build`:
1914 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1915 src/main.rs:7:26: 7:34 error: the type of this value must be known in this context
1916 src/main.rs:7 let secret_number = (rand::random() % 100i) + 1i;
1918 error: aborting due to previous error
1921 It didn't work! Rust says "the type of this value must be known in this
1922 context." What's up with that? Well, as it turns out, `rand::random()` can
1923 generate many kinds of random values, not just integers. And in this case, Rust
1924 isn't sure what kind of value `random()` should generate. So we have to help
1925 it. With number literals, we just add an `i` onto the end to tell Rust they're
1926 integers, but that does not work with functions. There's a different syntax,
1927 and it looks like this:
1930 rand::random::<int>();
1933 This says "please give me a random `int` value." We can change our code to use
1941 println!("Guess the number!");
1943 let secret_number = (rand::random::<int>() % 100i) + 1i;
1945 println!("The secret number is: {}", secret_number);
1947 println!("Please input your guess.");
1949 let input = io::stdin().read_line()
1951 .expect("Failed to read line");
1954 println!("You guessed: {}", input);
1958 Try running our new program a few times:
1962 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
1963 Running `target/guessing_game`
1965 The secret number is: 7
1966 Please input your guess.
1969 $ ./target/guessing_game
1971 The secret number is: 83
1972 Please input your guess.
1975 $ ./target/guessing_game
1977 The secret number is: -29
1978 Please input your guess.
1983 Wait. Negative 29? We wanted a number between one and a hundred! We have two
1984 options here: we can either ask `random()` to generate an unsigned integer, which
1985 can only be positive, or we can use the `abs()` function. Let's go with the
1986 unsigned integer approach. If we want a random positive number, we should ask for
1987 a random positive number. Our code looks like this now:
1994 println!("Guess the number!");
1996 let secret_number = (rand::random::<uint>() % 100u) + 1u;
1998 println!("The secret number is: {}", secret_number);
2000 println!("Please input your guess.");
2002 let input = io::stdin().read_line()
2004 .expect("Failed to read line");
2007 println!("You guessed: {}", input);
2015 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
2016 Running `target/guessing_game`
2018 The secret number is: 57
2019 Please input your guess.
2024 Great! Next up: let's compare our guess to the secret guess.
2026 ## Comparing guesses
2028 If you remember, earlier in the tutorial, we made a `cmp` function that compared
2029 two numbers. Let's add that in, along with a `match` statement to compare the
2030 guess to the secret guess:
2037 println!("Guess the number!");
2039 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2041 println!("The secret number is: {}", secret_number);
2043 println!("Please input your guess.");
2045 let input = io::stdin().read_line()
2047 .expect("Failed to read line");
2050 println!("You guessed: {}", input);
2052 match cmp(input, secret_number) {
2053 Less => println!("Too small!"),
2054 Greater => println!("Too big!"),
2055 Equal => { println!("You win!"); },
2059 fn cmp(a: int, b: int) -> Ordering {
2061 else if a > b { Greater }
2066 If we try to compile, we'll get some errors:
2070 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2071 src/main.rs:20:15: 20:20 error: mismatched types: expected `int` but found `collections::string::String` (expected int but found struct collections::string::String)
2072 src/main.rs:20 match cmp(input, secret_number) {
2074 src/main.rs:20:22: 20:35 error: mismatched types: expected `int` but found `uint` (expected int but found uint)
2075 src/main.rs:20 match cmp(input, secret_number) {
2077 error: aborting due to 2 previous errors
2080 This often happens when writing Rust programs, and is one of Rust's greatest
2081 strengths. You try out some code, see if it compiles, and Rust tells you that
2082 you've done something wrong. In this case, our `cmp` function works on integers,
2083 but we've given it unsigned integers. In this case, the fix is easy, because
2084 we wrote the `cmp` function! Let's change it to take `uint`s:
2091 println!("Guess the number!");
2093 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2095 println!("The secret number is: {}", secret_number);
2097 println!("Please input your guess.");
2099 let input = io::stdin().read_line()
2101 .expect("Failed to read line");
2104 println!("You guessed: {}", input);
2106 match cmp(input, secret_number) {
2107 Less => println!("Too small!"),
2108 Greater => println!("Too big!"),
2109 Equal => { println!("You win!"); },
2113 fn cmp(a: uint, b: uint) -> Ordering {
2115 else if a > b { Greater }
2120 And try compiling again:
2124 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2125 src/main.rs:20:15: 20:20 error: mismatched types: expected `uint` but found `collections::string::String` (expected uint but found struct collections::string::String)
2126 src/main.rs:20 match cmp(input, secret_number) {
2128 error: aborting due to previous error
2131 This error is similar to the last one: we expected to get a `uint`, but we got
2132 a `String` instead! That's because our `input` variable is coming from the
2133 standard input, and you can guess anything. Try it:
2136 $ ./target/guessing_game
2138 The secret number is: 73
2139 Please input your guess.
2144 Oops! Also, you'll note that we just ran our program even though it didn't compile.
2145 This works because the older version we did successfully compile was still lying
2146 around. Gotta be careful!
2148 Anyway, we have a `String`, but we need a `uint`. What to do? Well, there's
2149 a function for that:
2152 let input = io::stdin().read_line()
2154 .expect("Failed to read line");
2155 let input_num: Option<uint> = from_str(input.as_slice());
2158 The `from_str` function takes in a `&str` value and converts it into something.
2159 We tell it what kind of something with a type hint. Remember our type hint with
2160 `random()`? It looked like this:
2163 rand::random::<uint>();
2166 There's an alternate way of providing a hint too, and that's declaring the type
2170 let x: uint = rand::random();
2173 In this case, we say `x` is a `uint` explicitly, so Rust is able to properly
2174 tell `random()` what to generate. In a similar fashion, both of these work:
2177 let input_num = from_str::<Option<uint>>("5");
2178 let input_num: Option<uint> = from_str("5");
2181 In this case, I happen to prefer the latter, and in the `random()` case, I prefer
2182 the former. I think the nested `<>`s make the first option especially ugly and
2183 a bit harder to read.
2185 Anyway, with us now converting our input to a number, our code looks like this:
2192 println!("Guess the number!");
2194 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2196 println!("The secret number is: {}", secret_number);
2198 println!("Please input your guess.");
2200 let input = io::stdin().read_line()
2202 .expect("Failed to read line");
2203 let input_num: Option<uint> = from_str(input.as_slice());
2207 println!("You guessed: {}", input_num);
2209 match cmp(input_num, secret_number) {
2210 Less => println!("Too small!"),
2211 Greater => println!("Too big!"),
2212 Equal => { println!("You win!"); },
2216 fn cmp(a: uint, b: uint) -> Ordering {
2218 else if a > b { Greater }
2227 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2228 src/main.rs:22:15: 22:24 error: mismatched types: expected `uint` but found `core::option::Option<uint>` (expected uint but found enum core::option::Option)
2229 src/main.rs:22 match cmp(input_num, secret_number) {
2231 error: aborting due to previous error
2234 Oh yeah! Our `input_num` has the type `Option<uint>`, rather than `uint`. We
2235 need to unwrap the Option. If you remember from before, `match` is a great way
2236 to do that. Try this code:
2243 println!("Guess the number!");
2245 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2247 println!("The secret number is: {}", secret_number);
2249 println!("Please input your guess.");
2251 let input = io::stdin().read_line()
2253 .expect("Failed to read line");
2254 let input_num: Option<uint> = from_str(input.as_slice());
2256 let num = match input_num {
2259 println!("Please input a number!");
2265 println!("You guessed: {}", num);
2267 match cmp(num, secret_number) {
2268 Less => println!("Too small!"),
2269 Greater => println!("Too big!"),
2270 Equal => { println!("You win!"); },
2274 fn cmp(a: uint, b: uint) -> Ordering {
2276 else if a > b { Greater }
2281 We use a `match` to either give us the `uint` inside of the `Option`, or we
2282 print an error message and return. Let's give this a shot:
2286 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
2287 Running `target/guessing_game`
2289 The secret number is: 17
2290 Please input your guess.
2292 Please input a number!
2295 Uh, what? But we did!
2297 ... actually, we didn't. See, when you get a line of input from `stdin()`,
2298 you get all the input. Including the `\n` character from you pressing Enter.
2299 So, `from_str()` sees the string `"5\n"` and says "nope, that's not a number,
2300 there's non-number stuff in there!" Luckily for us, `&str`s have an easy
2301 method we can use defined on them: `trim()`. One small modification, and our
2302 code looks like this:
2309 println!("Guess the number!");
2311 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2313 println!("The secret number is: {}", secret_number);
2315 println!("Please input your guess.");
2317 let input = io::stdin().read_line()
2319 .expect("Failed to read line");
2320 let input_num: Option<uint> = from_str(input.as_slice().trim());
2322 let num = match input_num {
2325 println!("Please input a number!");
2331 println!("You guessed: {}", num);
2333 match cmp(num, secret_number) {
2334 Less => println!("Too small!"),
2335 Greater => println!("Too big!"),
2336 Equal => { println!("You win!"); },
2340 fn cmp(a: uint, b: uint) -> Ordering {
2342 else if a > b { Greater }
2351 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
2352 Running `target/guessing_game`
2354 The secret number is: 58
2355 Please input your guess.
2361 Nice! You can see I even added spaces before my guess, and it still figured
2362 out that I guessed 76. Run the program a few times, and verify that guessing
2363 the number works, as well as guessing a number too small.
2365 The Rust compiler helped us out quite a bit there! This technique is called
2366 "lean on the compiler," and it's often useful when working on some code. Let
2367 the error messages help guide you towards the correct types.
2369 Now we've got most of the game working, but we can only make one guess. Let's
2370 change that by adding loops!
2374 As we already discussed, the `loop` keyword gives us an infinite loop. So
2382 println!("Guess the number!");
2384 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2386 println!("The secret number is: {}", secret_number);
2390 println!("Please input your guess.");
2392 let input = io::stdin().read_line()
2394 .expect("Failed to read line");
2395 let input_num: Option<uint> = from_str(input.as_slice().trim());
2397 let num = match input_num {
2400 println!("Please input a number!");
2406 println!("You guessed: {}", num);
2408 match cmp(num, secret_number) {
2409 Less => println!("Too small!"),
2410 Greater => println!("Too big!"),
2411 Equal => { println!("You win!"); },
2416 fn cmp(a: uint, b: uint) -> Ordering {
2418 else if a > b { Greater }
2423 And try it out. But wait, didn't we just add an infinite loop? Yup. Remember
2424 that `return`? If we give a non-number answer, we'll `return` and quit. Observe:
2428 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
2429 Running `target/guessing_game`
2431 The secret number is: 59
2432 Please input your guess.
2436 Please input your guess.
2440 Please input your guess.
2444 Please input your guess.
2446 Please input a number!
2449 Ha! `quit` actually quits. As does any other non-number input. Well, this is
2450 suboptimal to say the least. First, let's actually quit when you win the game:
2457 println!("Guess the number!");
2459 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2461 println!("The secret number is: {}", secret_number);
2465 println!("Please input your guess.");
2467 let input = io::stdin().read_line()
2469 .expect("Failed to read line");
2470 let input_num: Option<uint> = from_str(input.as_slice().trim());
2472 let num = match input_num {
2475 println!("Please input a number!");
2481 println!("You guessed: {}", num);
2483 match cmp(num, secret_number) {
2484 Less => println!("Too small!"),
2485 Greater => println!("Too big!"),
2487 println!("You win!");
2494 fn cmp(a: uint, b: uint) -> Ordering {
2496 else if a > b { Greater }
2501 By adding the `return` line after the `You win!`, we'll exit the program when
2502 we win. We have just one more tweak to make: when someone inputs a non-number,
2503 we don't want to quit, we just want to ignore it. Change that `return` to
2512 println!("Guess the number!");
2514 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2516 println!("The secret number is: {}", secret_number);
2520 println!("Please input your guess.");
2522 let input = io::stdin().read_line()
2524 .expect("Failed to read line");
2525 let input_num: Option<uint> = from_str(input.as_slice().trim());
2527 let num = match input_num {
2530 println!("Please input a number!");
2536 println!("You guessed: {}", num);
2538 match cmp(num, secret_number) {
2539 Less => println!("Too small!"),
2540 Greater => println!("Too big!"),
2542 println!("You win!");
2549 fn cmp(a: uint, b: uint) -> Ordering {
2551 else if a > b { Greater }
2556 Now we should be good! Let's try:
2560 Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
2561 Running `target/guessing_game`
2563 The secret number is: 61
2564 Please input your guess.
2568 Please input your guess.
2572 Please input your guess.
2574 Please input a number!
2575 Please input your guess.
2581 Awesome! With one tiny last tweak, we have finished the guessing game. Can you
2582 think of what it is? That's right, we don't want to print out the secret number.
2583 It was good for testing, but it kind of ruins the game. Here's our final source:
2590 println!("Guess the number!");
2592 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2596 println!("Please input your guess.");
2598 let input = io::stdin().read_line()
2600 .expect("Failed to read line");
2601 let input_num: Option<uint> = from_str(input.as_slice().trim());
2603 let num = match input_num {
2606 println!("Please input a number!");
2612 println!("You guessed: {}", num);
2614 match cmp(num, secret_number) {
2615 Less => println!("Too small!"),
2616 Greater => println!("Too big!"),
2618 println!("You win!");
2625 fn cmp(a: uint, b: uint) -> Ordering {
2627 else if a > b { Greater }
2634 At this point, you have successfully built the Guessing Game! Congratulations!
2636 You've now learned the basic syntax of Rust. All of this is relatively close to
2637 various other programming languages you have used in the past. These
2638 fundamental syntactical and semantic elements will form the foundation for the
2639 rest of your Rust education.
2641 Now that you're an expert at the basics, it's time to learn about some of
2642 Rust's more unique features.
2644 # Crates and Modules
2646 Rust features a strong module system, but it works a bit differently than in
2647 other programming languages. Rust's module system has two main components:
2648 **crate**s, and **module**s.
2650 A crate is Rust's unit of independent compilation. Rust always compiles one
2651 crate at a time, producing either a library or an executable. However, executables
2652 usually depend on libraries, and many libraries depend on other libraries as well.
2653 To support this, crates can depend on other crates.
2655 Each crate contains a hierarchy of modules. This tree starts off with a single
2656 module, called the **crate root**. Within the crate root, we can declare other
2657 modules, which can contain other modules, as deeply as you'd like.
2659 Note that we haven't mentioned anything about files yet. Rust does not impose a
2660 particular relationship between your filesystem structure and your module
2661 structure. That said, there is a conventional approach to how Rust looks for
2662 modules on the file system, but it's also overrideable.
2664 Enough talk, let's build something! Let's make a new project called `modules`.
2668 $ cargo new modules --bin
2671 Let's double check our work by compiling:
2675 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2676 Running `target/modules`
2680 Excellent! So, we already have a single crate here: our `src/main.rs` is a crate.
2681 Everything in that file is in the crate root. A crate that generates an executable
2682 defines a `main` function inside its root, as we've done here.
2684 Let's define a new module inside our crate. Edit `src/main.rs` to look
2689 println!("Hello, world!");
2694 println!("Hello, world!");
2699 We now have a module named `hello` inside of our crate root. Modules use
2700 `snake_case` naming, like functions and variable bindings.
2702 Inside the `hello` module, we've defined a `print_hello` function. This will
2703 also print out our hello world message. Modules allow you to split up your
2704 program into nice neat boxes of functionality, grouping common things together,
2705 and keeping different things apart. It's kinda like having a set of shelves:
2706 a place for everything and everything in its place.
2708 To call our `print_hello` function, we use the double colon (`::`):
2711 hello::print_hello();
2714 You've seen this before, with `io::stdin()` and `rand::random()`. Now you know
2715 how to make your own. However, crates and modules have rules about
2716 **visibility**, which controls who exactly may use the functions defined in a
2717 given module. By default, everything in a module is private, which means that
2718 it can only be used by other functions in the same module. This will not
2723 hello::print_hello();
2728 println!("Hello, world!");
2736 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2737 src/main.rs:2:5: 2:23 error: function `print_hello` is private
2738 src/main.rs:2 hello::print_hello();
2742 To make it public, we use the `pub` keyword:
2746 hello::print_hello();
2750 pub fn print_hello() {
2751 println!("Hello, world!");
2760 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2761 Running `target/modules`
2767 There's a common pattern when you're building an executable: you build both an
2768 executable and a library, and put most of your logic in the library. That way,
2769 other programs can use that library to build their own functionality.
2771 Let's do that with our project. If you remember, libraries and executables
2772 are both crates, so while our project has one crate now, let's make a second:
2773 one for the library, and one for the executable.
2775 To make the second crate, open up `src/lib.rs` and put this code in it:
2779 pub fn print_hello() {
2780 println!("Hello, world!");
2785 And change your `src/main.rs` to look like this:
2788 extern crate modules;
2791 modules::hello::print_hello();
2795 There's been a few changes. First, we moved our `hello` module into its own
2796 file, `src/lib.rs`. This is the file that Cargo expects a library crate to
2797 be named, by convention.
2799 Next, we added an `extern crate modules` to the top of our `src/main.rs`. This,
2800 as you can guess, lets Rust know that our crate relies on another, external
2801 crate. We also had to modify our call to `print_hello`: now that it's in
2802 another crate, we need to specify that crate first.
2804 This doesn't _quite_ work yet. Try it:
2808 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2809 /home/you/projects/modules/src/lib.rs:2:5: 4:6 warning: code is never used: `print_hello`, #[warn(dead_code)] on by default
2810 /home/you/projects/modules/src/lib.rs:2 pub fn print_hello() {
2811 /home/you/projects/modules/src/lib.rs:3 println!("Hello, world!");
2812 /home/you/projects/modules/src/lib.rs:4 }
2813 /home/you/projects/modules/src/main.rs:4:5: 4:32 error: function `print_hello` is private
2814 /home/you/projects/modules/src/main.rs:4 modules::hello::print_hello();
2815 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
2816 error: aborting due to previous error
2817 Could not compile `modules`.
2820 First, we get a warning that some code is never used. Odd. Next, we get an error:
2821 `print_hello` is private, so we can't call it. Notice that the first error came
2822 from `src/lib.rs`, and the second came from `src/main.rs`: cargo is smart enough
2823 to build it all with one command. Also, after seeing the second error, the warning
2824 makes sense: we never actually call `hello_world`, because we're not allowed to!
2826 Just like modules, crates also have private visibility by default. Any modules
2827 inside of a crate can only be used by other modules in the crate, unless they
2828 use `pub`. In `src/lib.rs`, change this line:
2840 And everything should work:
2844 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2845 Running `target/modules`
2849 Let's do one more thing: add a `goodbye` module as well. Imagine a `src/lib.rs`
2850 that looks like this:
2854 pub fn print_hello() {
2855 println!("Hello, world!");
2860 pub fn print_goodbye() {
2861 println!("Goodbye for now!");
2866 Now, these two modules are pretty small, but imagine we've written a real, large
2867 program: they could both be huge. So maybe we want to move them into their own
2868 files. We can do that pretty easily, and there are two different conventions
2869 for doing it. Let's give each a try. First, make `src/lib.rs` look like this:
2876 This tells Rust that this crate has two public modules: `hello` and `goodbye`.
2878 Next, make a `src/hello.rs` that contains this:
2881 pub fn print_hello() {
2882 println!("Hello, world!");
2886 When we include a module like this, we don't need to make the `mod` declaration
2887 in `hello.rs`, because it's already been declared in `lib.rs`. `hello.rs` just
2888 contains the body of the module which is defined (by the `pub mod hello`) in
2889 `lib.rs`. This helps prevent 'rightward drift': when you end up indenting so
2890 many times that your code is hard to read.
2892 Finally, make a new directory, `src/goodbye`, and make a new file in it,
2893 `src/goodbye/mod.rs`:
2896 pub fn print_goodbye() {
2897 println!("Bye for now!");
2901 Same deal, but we can make a folder with a `mod.rs` instead of `mod_name.rs` in
2902 the same directory. If you have a lot of modules, nested folders can make
2903 sense. For example, if the `goodbye` module had its _own_ modules inside of
2904 it, putting all of that in a folder helps keep our directory structure tidy.
2905 And in fact, if you place the modules in separate files, they're required to be
2906 in separate folders.
2908 This should all compile as usual:
2912 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2915 We've seen how the `::` operator can be used to call into modules, but when
2916 we have deep nesting like `modules::hello::say_hello`, it can get tedious.
2917 That's why we have the `use` keyword.
2919 `use` allows us to bring certain names into another scope. For example, here's
2923 extern crate modules;
2926 modules::hello::print_hello();
2930 We could instead write this:
2933 extern crate modules;
2935 use modules::hello::print_hello;
2942 By bringing `print_hello` into scope, we don't need to qualify it anymore. However,
2943 it's considered proper style to do write this code like like this:
2946 extern crate modules;
2951 hello::print_hello();
2955 By just bringing the module into scope, we can keep one level of namespacing.
2959 Traditionally, testing has not been a strong suit of most systems programming
2960 languages. Rust, however, has very basic testing built into the language
2961 itself. While automated testing cannot prove that your code is bug-free, it is
2962 useful for verifying that certain behaviors work as intended.
2964 Here's a very basic test:
2968 fn is_one_equal_to_one() {
2973 You may notice something new: that `#[test]`. Before we get into the mechanics
2974 of testing, let's talk about attributes.
2978 Rust's testing system uses **attribute**s to mark which functions are tests.
2979 Attributes can be placed on any Rust **item**. Remember how most things in
2980 Rust are an expression, but `let` is not? Item declarations are also not
2981 expressions. Here's a list of things that qualify as an item:
2992 You haven't learned about all of these things yet, but that's the list. As
2993 you can see, functions are at the top of it.
2995 Attributes can appear in three ways:
2997 1. A single identifier, the attribute name. `#[test]` is an example of this.
2998 2. An identifier followed by an equals sign (`=`) and a literal. `#[cfg=test]`
2999 is an example of this.
3000 3. An identifier followed by a parenthesized list of sub-attribute arguments.
3001 `#[cfg(unix, target_word_size = "32")]` is an example of this, where one of
3002 the sub-arguments is of the second kind.
3004 There are a number of different kinds of attributes, enough that we won't go
3005 over them all here. Before we talk about the testing-specific attributes, I
3006 want to call out one of the most important kinds of attributes: stability
3009 ## Stability attributes
3011 Rust provides six attributes to indicate the stability level of various
3012 parts of your library. The six levels are:
3014 * deprecated: this item should no longer be used. No guarantee of backwards
3016 * experimental: This item was only recently introduced or is otherwise in a
3017 state of flux. It may change significantly, or even be removed. No guarantee
3018 of backwards-compatibility.
3019 * unstable: This item is still under development, but requires more testing to
3020 be considered stable. No guarantee of backwards-compatibility.
3021 * stable: This item is considered stable, and will not change significantly.
3022 Guarantee of backwards-compatibility.
3023 * frozen: This item is very stable, and is unlikely to change. Guarantee of
3024 backwards-compatibility.
3025 * locked: This item will never change unless a serious bug is found. Guarantee
3026 of backwards-compatibility.
3028 All of Rust's standard library uses these attribute markers to communicate
3029 their relative stability, and you should use them in your code, as well.
3030 There's an associated attribute, `warn`, that allows you to warn when you
3031 import an item marked with certain levels: deprecated, experimental and
3032 unstable. For now, only deprecated warns by default, but this will change once
3033 the standard library has been stabilized.
3035 You can use the `warn` attribute like this:
3041 And later, when you import a crate:
3044 extern crate some_crate;
3047 You'll get a warning if you use something marked unstable.
3049 You may have noticed an exclamation point in the `warn` attribute declaration.
3050 The `!` in this attribute means that this attribute applies to the enclosing
3051 item, rather than to the item that follows the attribute. So this `warn`
3052 attribute declaration applies to the enclosing crate itself, rather than
3053 to whatever item statement follows it:
3056 // applies to the crate we're in
3059 extern crate some_crate;
3061 // applies to the following `fn`.
3070 Let's write a very simple crate in a test-driven manner. You know the drill by
3071 now: make a new project:
3075 $ cargo new testing --bin
3083 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3084 Running `target/testing`
3088 Great. Rust's infrastructure supports tests in two sorts of places, and they're
3089 for two kinds of tests: you include **unit test**s inside of the crate itself,
3090 and you place **integration test**s inside a `tests` directory. "Unit tests"
3091 are small tests that test one focused unit, "integration tests" tests multiple
3092 units in integration. That said, this is a social convention, they're no different
3093 in syntax. Let's make a `tests` directory:
3099 Next, let's create an integration test in `tests/lib.rs`:
3108 It doesn't matter what you name your test functions, though it's nice if
3109 you give them descriptive names. You'll see why in a moment. We then use a
3110 macro, `assert!`, to assert that something is true. In this case, we're giving
3111 it `false`, so this test should fail. Let's try it!
3115 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3116 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3117 /home/you/projects/testing/src/main.rs:1 fn main() {
3118 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3119 /home/you/projects/testing/src/main.rs:3 }
3123 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3131 ---- foo stdout ----
3132 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
3139 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
3141 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
3144 Lots of output! Let's break this down:
3148 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3151 You can run all of your tests with `cargo test`. This runs both your tests in
3152 `tests`, as well as the tests you put inside of your crate.
3155 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3156 /home/you/projects/testing/src/main.rs:1 fn main() {
3157 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3158 /home/you/projects/testing/src/main.rs:3 }
3161 Rust has a **lint** called 'warn on dead code' used by default. A lint is a
3162 bit of code that checks your code, and can tell you things about it. In this
3163 case, Rust is warning us that we've written some code that's never used: our
3164 `main` function. Of course, since we're running tests, we don't use `main`.
3165 We'll turn this lint off for just this function soon. For now, just ignore this
3171 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3174 Wait a minute, zero tests? Didn't we define one? Yup. This output is from
3175 attempting to run the tests in our crate, of which we don't have any.
3176 You'll note that Rust reports on several kinds of tests: passed, failed,
3177 ignored, and measured. The 'measured' tests refer to benchmark tests, which
3178 we'll cover soon enough!
3185 Now we're getting somewhere. Remember when we talked about naming our tests
3186 with good names? This is why. Here, it says 'test foo' because we called our
3187 test 'foo.' If we had given it a good name, it'd be more clear which test
3188 failed, especially as we accumulate more tests.
3193 ---- foo stdout ----
3194 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
3201 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
3203 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
3206 After all the tests run, Rust will show us any output from our failed tests.
3207 In this instance, Rust tells us that our assertion failed, with false. This was
3210 Whew! Let's fix our test:
3219 And then try to run our tests again:
3223 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3224 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3225 /home/you/projects/testing/src/main.rs:1 fn main() {
3226 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3227 /home/you/projects/testing/src/main.rs:3 }
3231 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3237 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3240 Nice! Our test passes, as we expected. Let's get rid of that warning for our `main`
3241 function. Change your `src/main.rs` to look like this:
3246 println!("Hello, world");
3250 This attribute combines two things: `cfg` and `not`. The `cfg` attribute allows
3251 you to conditionally compile code based on something. The following item will
3252 only be compiled if the configuration says it's true. And when Cargo compiles
3253 our tests, it sets things up so that `cfg(test)` is true. But we want to only
3254 include `main` when it's _not_ true. So we use `not` to negate things:
3255 `cfg(not(test))` will only compile our code when the `cfg(test)` is false.
3257 With this attribute, we won't get the warning:
3261 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3265 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3271 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3274 Nice. Okay, let's write a real test now. Change your `tests/lib.rs`
3279 fn math_checks_out() {
3280 let result = add_three_times_four(5i);
3282 assert_eq!(32i, result);
3286 And try to run the test:
3290 Compiling testing v0.0.1 (file:///home/youg/projects/testing)
3291 /home/youg/projects/testing/tests/lib.rs:3:18: 3:38 error: unresolved name `add_three_times_four`.
3292 /home/youg/projects/testing/tests/lib.rs:3 let result = add_three_times_four(5i);
3293 ^~~~~~~~~~~~~~~~~~~~
3294 error: aborting due to previous error
3295 Build failed, waiting for other jobs to finish...
3296 Could not compile `testing`.
3298 To learn more, run the command again with --verbose.
3301 Rust can't find this function. That makes sense, as we didn't write it yet!
3303 In order to share this codes with our tests, we'll need to make a library crate.
3304 This is also just good software design: as we mentioned before, it's a good idea
3305 to put most of your functionality into a library crate, and have your executable
3306 crate use that library. This allows for code re-use.
3308 To do that, we'll need to make a new module. Make a new file, `src/lib.rs`,
3313 pub fn add_three_times_four(x: int) -> int {
3318 We're calling this file `lib.rs` because it has the same name as our project,
3319 and so it's named this, by convention.
3321 We'll then need to use this crate in our `src/main.rs`:
3324 extern crate testing;
3328 println!("Hello, world");
3332 Finally, let's import this function in our `tests/lib.rs`:
3335 extern crate testing;
3336 use testing::add_three_times_four;
3339 fn math_checks_out() {
3340 let result = add_three_times_four(5i);
3342 assert_eq!(32i, result);
3346 Let's give it a run:
3350 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3354 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3359 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3363 test math_checks_out ... ok
3365 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3368 Great! One test passed. We've got an integration test showing that our public
3369 method works, but maybe we want to test some of the internal logic as well.
3370 While this function is simple, if it were more complicated, you can imagine
3371 we'd need more tests. So let's break it up into two helper functions, and
3372 write some unit tests to test those.
3374 Change your `src/lib.rs` to look like this:
3377 pub fn add_three_times_four(x: int) -> int {
3378 times_four(add_three(x))
3381 fn add_three(x: int) -> int { x + 3 }
3383 fn times_four(x: int) -> int { x * 4 }
3386 If you run `cargo test`, you should get the same output:
3390 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3394 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3399 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3403 test math_checks_out ... ok
3405 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3408 If we tried to write a test for these two new functions, it wouldn't
3412 extern crate testing;
3413 use testing::add_three_times_four;
3414 use testing::add_three;
3417 fn math_checks_out() {
3418 let result = add_three_times_four(5i);
3420 assert_eq!(32i, result);
3424 fn test_add_three() {
3425 let result = add_three(5i);
3427 assert_eq!(8i, result);
3431 We'd get this error:
3434 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3435 /home/you/projects/testing/tests/lib.rs:3:5: 3:24 error: function `add_three` is private
3436 /home/you/projects/testing/tests/lib.rs:3 use testing::add_three;
3440 Right. It's private. So external, integration tests won't work. We need a
3441 unit test. Open up your `src/lib.rs` and add this:
3444 pub fn add_three_times_four(x: int) -> int {
3445 times_four(add_three(x))
3448 fn add_three(x: int) -> int { x + 3 }
3450 fn times_four(x: int) -> int { x * 4 }
3454 use super::add_three;
3455 use super::times_four;
3458 fn test_add_three() {
3459 let result = add_three(5i);
3461 assert_eq!(8i, result);
3465 fn test_times_four() {
3466 let result = times_four(5i);
3468 assert_eq!(20i, result);
3473 Let's give it a shot:
3477 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3480 test test::test_times_four ... ok
3481 test test::test_add_three ... ok
3483 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3488 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3492 test math_checks_out ... ok
3494 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3497 Cool! We now have two tests of our internal functions. You'll note that there
3498 are three sets of output now: one for `src/main.rs`, one for `src/lib.rs`, and
3499 one for `tests/lib.rs`. There's one interesting thing that we haven't talked
3500 about yet, and that's these lines:
3503 use super::add_three;
3504 use super::times_four;
3507 Because we've made a nested module, we can import functions from the parent
3508 module by using `super`. Sub-modules are allowed to 'see' private functions in
3509 the parent. We sometimes call this usage of `use` a 're-export,' because we're
3510 exporting the name again, somewhere else.
3512 We've now covered the basics of testing. Rust's tools are primitive, but they
3513 work well in the simple cases. There are some Rustaceans working on building
3514 more complicated frameworks on top of all of this, but thery're just starting
3519 In systems programming, pointers are an incredibly important topic. Rust has a
3520 very rich set of pointers, and they operate differently than in many other
3521 languages. They are important enough that we have a specific [Pointer
3522 Guide](guide-pointers.html) that goes into pointers in much detail. In fact,
3523 while you're currently reading this guide, which covers the language in broad
3524 overview, there are a number of other guides that put a specific topic under a
3525 microscope. You can find the list of guides on the [documentation index
3526 page](index.html#guides).
3528 In this section, we'll assume that you're familiar with pointers as a general
3529 concept. If you aren't, please read the [introduction to
3530 pointers](guide-pointers.html#an-introduction) section of the Pointer Guide,
3531 and then come back here. We'll wait.
3533 Got the gist? Great. Let's talk about pointers in Rust.
3537 The most primitive form of pointer in Rust is called a **reference**.
3538 References are created using the ampersand (`&`). Here's a simple
3546 `y` is a reference to `x`. To dereference (get the value being referred to
3547 rather than the reference itself) `y`, we use the asterisk (`*`):
3556 Like any `let` binding, references are immutable by default.
3558 You can declare that functions take a reference:
3561 fn add_one(x: &int) -> int { *x + 1 }
3564 assert_eq!(6, add_one(&5));
3568 As you can see, we can make a reference from a literal by applying `&` as well.
3569 Of course, in this simple function, there's not a lot of reason to take `x` by
3570 reference. It's just an example of the syntax.
3572 Because references are immutable, you can have multiple references that
3573 **alias** (point to the same place):
3581 We can make a mutable reference by using `&mut` instead of `&`:
3588 Note that `x` must also be mutable. If it isn't, like this:
3598 6:19 error: cannot borrow immutable local variable `x` as mutable
3603 We don't want a mutable reference to immutable data! This error message uses a
3604 term we haven't talked about yet, 'borrow.' We'll get to that in just a moment.
3606 This simple example actually illustrates a lot of Rust's power: Rust has
3607 prevented us, at compile time, from breaking our own rules. Because Rust's
3608 references check these kinds of rules entirely at compile time, there's no
3609 runtime overhead for this safety. At runtime, these are the same as a raw
3610 machine pointer, like in C or C++. We've just double-checked ahead of time
3611 that we haven't done anything dangerous.
3613 Rust will also prevent us from creating two mutable references that alias.
3622 It gives us this error:
3625 error: cannot borrow `x` as mutable more than once at a time
3628 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3631 note: previous borrow ends here
3640 This is a big error message. Let's dig into it for a moment. There are three
3641 parts: the error and two notes. The error says what we expected, we cannot have
3642 two pointers that point to the same memory.
3644 The two notes give some extra context. Rust's error messages often contain this
3645 kind of extra information when the error is complex. Rust is telling us two
3646 things: first, that the reason we cannot **borrow** `x` as `z` is that we
3647 previously borrowed `x` as `y`. The second note shows where `y`'s borrowing
3652 In order to truly understand this error, we have to learn a few new concepts:
3653 **ownership**, **borrowing**, and **lifetimes**.
3655 ## Ownership, borrowing, and lifetimes
3657 Whenever a resource of some kind is created, something must be responsible
3658 for destroying that resource as well. Given that we're discussing pointers
3659 right now, let's discuss this in the context of memory allocation, though
3660 it applies to other resources as well.
3662 When you allocate heap memory, you need a mechanism to free that memory. Many
3663 languages let the programmer control the allocation, and then use a garbage
3664 collector to handle the deallocation. This is a valid, time-tested strategy,
3665 but it's not without its drawbacks. Because the programmer does not have to
3666 think as much about deallocation, allocation becomes something commonplace,
3667 because it's easy. And if you need precise control over when something is
3668 deallocated, leaving it up to your runtime can make this difficult.
3670 Rust chooses a different path, and that path is called **ownership**. Any
3671 binding that creates a resource is the **owner** of that resource. Being an
3672 owner gives you three privileges, with two restrictions:
3674 1. You control when that resource is deallocated.
3675 2. You may lend that resource, immutably, to as many borrowers as you'd like.
3676 3. You may lend that resource, mutably, to a single borrower. **BUT**
3677 4. Once you've done so, you may not also lend it out otherwise, mutably or
3679 5. You may not lend it out mutably if you're currently lending it to someone.
3681 What's up with all this 'lending' and 'borrowing'? When you allocate memory,
3682 you get a pointer to that memory. This pointer allows you to manipulate said
3683 memory. If you are the owner of a pointer, then you may allow another
3684 binding to temporarily borrow that pointer, and then they can manipulate the
3685 memory. The length of time that the borrower is borrowing the pointer
3686 from you is called a **lifetime**.
3688 If two distinct bindings share a pointer, and the memory that pointer points to
3689 is immutable, then there are no problems. But if it's mutable, both pointers
3690 can attempt to write to the memory at the same time, causing a **race
3691 condition**. Therefore, if someone wants to mutate something that they've
3692 borrowed from you, you must not have lent out that pointer to anyone else.
3694 Rust has a sophisticated system called the **borrow checker** to make sure that
3695 everyone plays by these rules. At compile time, it verifies that none of these
3696 rules are broken. If there's no problem, our program compiles successfully, and
3697 there is no runtime overhead for any of this. The borrow checker works only at
3698 compile time. If the borrow checker did find a problem, it will report a
3699 **lifetime error**, and your program will refuse to compile.
3701 That's a lot to take in. It's also one of the _most_ important concepts in
3702 all of Rust. Let's see this syntax in action:
3706 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3708 // other code here...
3710 } // privilege 1: when x goes out of scope, this memory is deallocated
3712 /// this function borrows an integer. It's given back automatically when the
3713 /// function returns.
3714 fn foo(x: &int) -> &int { x }
3717 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3719 // privilege 2: you may lend that resource, to as many borrowers as you'd like
3723 foo(&x); // functions can borrow too!
3725 let a = &x; // we can do this alllllll day!
3729 let mut x = 5i; // x is the owner of this integer, which is memory on the stack.
3731 let y = &mut x; // privilege 3: you may lend that resource to a single borrower,
3736 If you are a borrower, you get a few privileges as well, but must also obey a
3739 1. If the borrow is immutable, you may read the data the pointer points to.
3740 2. If the borrow is mutable, you may read and write the data the pointer points to.
3741 3. You may lend the pointer to someone else in an immutable fashion, **BUT**
3742 4. When you do so, they must return it to you before you must give your own
3745 This last requirement can seem odd, but it also makes sense. If you have to
3746 return something, and you've lent it to someone, they need to give it back to
3747 you for you to give it back! If we didn't, then the owner could deallocate
3748 the memory, and the person we've loaned it out to would have a pointer to
3749 invalid memory. This is called a 'dangling pointer.'
3751 Let's re-examine the error that led us to talk about all of this, which was a
3752 violation of the restrictions placed on owners who lend something out mutably.
3764 error: cannot borrow `x` as mutable more than once at a time
3767 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3770 note: previous borrow ends here
3779 This error comes in three parts. Let's go over each in turn.
3782 error: cannot borrow `x` as mutable more than once at a time
3787 This error states the restriction: you cannot lend out something mutable more
3788 than once at the same time. The borrow checker knows the rules!
3791 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3796 Some compiler errors come with notes to help you fix the error. This error comes
3797 with two notes, and this is the first. This note informs us of exactly where
3798 the first mutable borrow occurred. The error showed us the second. So now we
3799 see both parts of the problem. It also alludes to rule #3, by reminding us that
3800 we can't change `x` until the borrow is over.
3803 note: previous borrow ends here
3812 Here's the second note, which lets us know where the first borrow would be over.
3813 This is useful, because if we wait to try to borrow `x` after this borrow is
3814 over, then everything will work.
3816 These rules are very simple, but that doesn't mean that they're easy. For more
3817 advanced patterns, please consult the [Lifetime Guide](guide-lifetimes.html).
3818 You'll also learn what this type signature with the `'a` syntax is:
3821 pub fn as_maybe_owned(&self) -> MaybeOwned<'a> { ... }
3826 All of our references so far have been to variables we've created on the stack.
3827 In Rust, the simplest way to allocate heap variables is using a *box*. To
3828 create a box, use the `box` keyword:
3834 This allocates an integer `5` on the heap, and creates a binding `x` that
3835 refers to it.. The great thing about boxed pointers is that we don't have to
3836 manually free this allocation! If we write
3845 then Rust will automatically free `x` at the end of the block. This isn't
3846 because Rust has a garbage collector -- it doesn't. Instead, when `x` goes out
3847 of scope, Rust `free`s `x`. This Rust code will do the same thing as the
3852 int *x = (int *)malloc(sizeof(int));
3858 This means we get the benefits of manual memory management, but the compiler
3859 ensures that we don't do something wrong. We can't forget to `free` our memory.
3861 Boxes are the sole owner of their contents, so you cannot take a mutable
3862 reference to them and then use the original box:
3868 *x; // you might expect 5, but this is actually an error
3871 This gives us this error:
3874 8:7 error: cannot use `*x` because it was mutably borrowed
3877 6:19 note: borrow of `x` occurs here
3882 As long as `y` is borrowing the contents, we cannot use `x`. After `y` is
3883 done borrowing the value, we can use it again. This works fine:
3890 } // y goes out of scope at the end of the block
3897 Sometimes, you need to allocate something on the heap, but give out multiple
3898 references to the memory. Rust's `Rc<T>` (pronounced 'arr cee tee') and
3899 `Arc<T>` types (again, the `T` is for generics, we'll learn more later) provide
3900 you with this ability. **Rc** stands for 'reference counted,' and **Arc** for
3901 'atomically reference counted.' This is how Rust keeps track of the multiple
3902 owners: every time we make a new reference to the `Rc<T>`, we add one to its
3903 internal 'reference count.' Every time a reference goes out of scope, we
3904 subtract one from the count. When the count is zero, the `Rc<T>` can be safely
3905 deallocated. `Arc<T>` is almost identical to `Rc<T>`, except for one thing: The
3906 'atomically' in 'Arc' means that increasing and decreasing the count uses a
3907 thread-safe mechanism to do so. Why two types? `Rc<T>` is faster, so if you're
3908 not in a multi-threaded scenario, you can have that advantage. Since we haven't
3909 talked about threading yet in Rust, we'll show you `Rc<T>` for the rest of this
3912 To create an `Rc<T>`, use `Rc::new()`:
3917 let x = Rc::new(5i);
3920 To create a second reference, use the `.clone()` method:
3925 let x = Rc::new(5i);
3929 The `Rc<T>` will live as long as any of its references are alive. After they
3930 all go out of scope, the memory will be `free`d.
3932 If you use `Rc<T>` or `Arc<T>`, you have to be careful about introducing
3933 cycles. If you have two `Rc<T>`s that point to each other, the reference counts
3934 will never drop to zero, and you'll have a memory leak. To learn more, check
3935 out [the section on `Rc<T>` and `Arc<T>` in the pointers
3936 guide](http://doc.rust-lang.org/guide-pointers.html#rc-and-arc).
3940 We've made use of patterns a few times in the guide: first with `let` bindings,
3941 then with `match` statements. Let's go on a whirlwind tour of all of the things
3944 A quick refresher: you can match against literals directly, and `_` acts as an
3951 1 => println!("one"),
3952 2 => println!("two"),
3953 3 => println!("three"),
3954 _ => println!("anything"),
3958 You can match multiple patterns with `|`:
3964 1 | 2 => println!("one or two"),
3965 3 => println!("three"),
3966 _ => println!("anything"),
3970 You can match a range of values with `..`:
3976 1 .. 5 => println!("one through five"),
3977 _ => println!("anything"),
3981 Ranges are mostly used with integers and single characters.
3983 If you're matching multiple things, via a `|` or a `..`, you can bind
3984 the value to a name with `@`:
3990 x @ 1 .. 5 => println!("got {}", x),
3991 _ => println!("anything"),
3995 If you're matching on an enum which has variants, you can use `..` to
3996 ignore the value in the variant:
4007 Value(..) => println!("Got an int!"),
4008 Missing => println!("No such luck."),
4012 You can introduce **match guards** with `if`:
4023 Value(x) if x > 5 => println!("Got an int bigger than five!"),
4024 Value(..) => println!("Got an int!"),
4025 Missing => println!("No such luck."),
4029 If you're matching on a pointer, you can use the same syntax as you declared it
4036 &x => println!("Got a value: {}", x),
4040 Here, the `x` inside the `match` has type `int`. In other words, the left hand
4041 side of the pattern destructures the value. If we have `&5i`, then in `&x`, `x`
4044 If you want to get a reference, use the `ref` keyword:
4050 ref x => println!("Got a reference to {}", x),
4054 Here, the `x` inside the `match` has the type `&int`. In other words, the `ref`
4055 keyword _creates_ a reference, for use in the pattern. If you need a mutable
4056 reference, `ref mut` will work in the same way:
4062 ref mut x => println!("Got a mutable reference to {}", x),
4066 If you have a struct, you can desugar it inside of a pattern:
4074 let origin = Point { x: 0i, y: 0i };
4077 Point { x: x, y: y } => println!("({},{})", x, y),
4081 If we only care about some of the values, we don't have to give them all names:
4089 let origin = Point { x: 0i, y: 0i };
4092 Point { x: x, .. } => println!("x is {}", x),
4096 Whew! That's a lot of different ways to match things, and they can all be
4097 mixed and matched, depending on what you're doing:
4101 Foo { x: Some(ref name), y: None } => ...
4105 Patterns are very powerful. Make good use of them.
4109 Functions are great, but if you want to call a bunch of them on some data, it
4110 can be awkward. Consider this code:
4116 We would read this left-to right, and so we see 'baz bar foo.' But this isn't the
4117 order that the functions would get called in, that's inside-out: 'foo bar baz.'
4118 Wouldn't it be nice if we could do this instead?
4121 x.foo().bar().baz();
4124 Luckily, as you may have guessed with the leading question, you can! Rust provides
4125 the ability to use this **method call syntax** via the `impl` keyword.
4127 Here's how it works:
4137 fn area(&self) -> f64 {
4138 std::f64::consts::PI * (self.radius * self.radius)
4143 let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
4144 println!("{}", c.area());
4148 This will print `12.566371`.
4150 We've made a struct that represents a circle. We then write an `impl` block,
4151 and inside it, define a method, `area`. Methods take a special first
4152 parameter, `&self`. There are three variants: `self`, `&self`, and `&mut self`.
4153 You can think of this first parameter as being the `x` in `x.foo()`. The three
4154 variants correspond to the three kinds of thing `x` could be: `self` if it's
4155 just a value on the stack, `&self` if it's a reference, and `&mut self` if it's
4156 a mutable reference. We should default to using `&self`, as it's the most
4159 Finally, as you may remember, the value of the area of a circle is `π*r²`.
4160 Because we took the `&self` parameter to `area`, we can use it just like any
4161 other parameter. Because we know it's a `Circle`, we can access the `radius`
4162 just like we would with any other struct. An import of π and some
4163 multiplications later, and we have our area.
4165 You can also define methods that do not take a `self` parameter. Here's a
4166 pattern that's very common in Rust code:
4176 fn new(x: f64, y: f64, radius: f64) -> Circle {
4186 let c = Circle::new(0.0, 0.0, 2.0);
4190 This **static method** builds a new `Circle` for us. Note that static methods
4191 are called with the `Struct::method()` syntax, rather than the `ref.method()`
4196 So far, we've made lots of functions in Rust. But we've given them all names.
4197 Rust also allows us to create anonymous functions too. Rust's anonymous
4198 functions are called **closure**s. By themselves, closures aren't all that
4199 interesting, but when you combine them with functions that take closures as
4200 arguments, really powerful things are possible.
4202 Let's make a closure:
4205 let add_one = |x| { 1i + x };
4207 println!("The 5 plus 1 is {}.", add_one(5i));
4210 We create a closure using the `|...| { ... }` syntax, and then we create a
4211 binding so we can use it later. Note that we call the function using the
4212 binding name and two parentheses, just like we would for a named function.
4214 Let's compare syntax. The two are pretty close:
4217 let add_one = |x: int| -> int { 1i + x };
4218 fn add_one (x: int) -> int { 1i + x }
4221 As you may have noticed, closures infer their argument and return types, so you
4222 don't need to declare one. This is different from named functions, which
4223 default to returning unit (`()`).
4225 There's one big difference between a closure and named functions, and it's in
4226 the name: a function "closes over its environment." What's that mean? It means
4233 let printer = || { println!("x is: {}", x); };
4235 printer(); // prints "x is: 5"
4239 The `||` syntax means this is an anonymous closure that takes no arguments.
4240 Without it, we'd just have a block of code in `{}`s.
4242 In other words, a closure has access to variables in the scope that it's
4243 defined. The closure borrows any variables that it uses. This will error:
4249 let printer = || { println!("x is: {}", x); };
4251 x = 6i; // error: cannot assign to `x` because it is borrowed
4257 Rust has a second type of closure, called a **proc**. Procs are created
4258 with the `proc` keyword:
4263 let p = proc() { x * x };
4264 println!("{}", p()); // prints 25
4267 Procs have a big difference from closures: they may only be called once. This
4268 will error when we try to compile:
4273 let p = proc() { x * x };
4274 println!("{}", p());
4275 println!("{}", p()); // error: use of moved value `p`
4278 This restriction is important. Procs are allowed to consume values that they
4279 capture, and thus have to be restricted to being called once for soundness
4280 reasons: any value consumed would be invalid on a second call.
4282 Procs are most useful with Rust's concurrency features, and so we'll just leave
4283 it at this for now. We'll talk about them more in the "Tasks" section of the
4286 ## Accepting closures as arguments
4288 Closures are most useful as an argument to another function. Here's an example:
4291 fn twice(x: int, f: |int| -> int) -> int {
4296 let square = |x: int| { x * x };
4298 twice(5i, square); // evaluates to 50
4302 Let's break example down, starting with `main`:
4305 let square = |x: int| { x * x };
4308 We've seen this before. We make a closure that takes an integer, and returns
4312 twice(5i, square); // evaluates to 50
4315 This line is more interesting. Here, we call our function, `twice`, and we pass
4316 it two arguments: an integer, `5`, and our closure, `square`. This is just like
4317 passing any other two variable bindings to a function, but if you've never
4318 worked with closures before, it can seem a little complex. Just think: "I'm
4319 passing two variables, one is an int, and one is a function."
4321 Next, let's look at how `twice` is defined:
4324 fn twice(x: int, f: |int| -> int) -> int {
4327 `twice` takes two arguments, `x` and `f`. That's why we called it with two
4328 arguments. `x` is an `int`, we've done that a ton of times. `f` is a function,
4329 though, and that function takes an `int` and returns an `int`. Notice
4330 how the `|int| -> int` syntax looks a lot like our definition of `square`
4331 above, if we added the return type in:
4334 let square = |x: int| -> int { x * x };
4338 This function takes an `int` and returns an `int`.
4340 This is the most complicated function signature we've seen yet! Give it a read
4341 a few times until you can see how it works. It takes a teeny bit of practice, and
4344 Finally, `twice` returns an `int` as well.
4346 Okay, let's look at the body of `twice`:
4349 fn twice(x: int, f: |int| -> int) -> int {
4354 Since our closure is named `f`, we can call it just like we called our closures
4355 before. And we pass in our `x` argument to each one. Hence 'twice.'
4357 If you do the math, `(5 * 5) + (5 * 5) == 50`, so that's the output we get.
4359 Play around with this concept until you're comfortable with it. Rust's standard
4360 library uses lots of closures, where appropriate, so you'll be using
4361 this technique a lot.
4363 If we didn't want to give `square` a name, we could also just define it inline.
4364 This example is the same as the previous one:
4367 fn twice(x: int, f: |int| -> int) -> int {
4372 twice(5i, |x: int| { x * x }); // evaluates to 50
4376 A named function's name can be used wherever you'd use a closure. Another
4377 way of writing the previous example:
4380 fn twice(x: int, f: |int| -> int) -> int {
4384 fn square(x: int) -> int { x * x }
4387 twice(5i, square); // evaluates to 50
4391 Doing this is not particularly common, but every once in a while, it's useful.
4393 That's all you need to get the hang of closures! Closures are a little bit
4394 strange at first, but once you're used to using them, you'll miss them in any
4395 language that doesn't have them. Passing functions to other functions is
4396 incredibly powerful. Next, let's look at one of those things: iterators.
4400 Let's talk about loops.
4402 Remember Rust's `for` loop? Here's an example:
4405 for x in range(0i, 10i) {
4406 println!("{:d}", x);
4410 Now that you know more Rust, we can talk in detail about how this works. The
4411 `range` function returns an **iterator**. An iterator is something that we can
4412 call the `.next()` method on repeatedly, and it gives us a sequence of things.
4417 let mut range = range(0i, 10i);
4420 match range.next() {
4429 We make a mutable binding to the return value of `range`, which is our iterator.
4430 We then `loop`, with an inner `match`. This `match` is used on the result of
4431 `range.next()`, which gives us a reference to the next value of the iterator.
4432 `next` returns an `Option<int>`, in this case, which will be `Some(int)` when
4433 we have a value and `None` once we run out. If we get `Some(int)`, we print it
4434 out, and if we get `None`, we `break` out of the loop.
4436 This code sample is basically the same as our `for` loop version. The `for`
4437 loop is just a handy way to write this `loop`/`match`/`break` construct.
4439 `for` loops aren't the only thing that uses iterators, however. Writing your
4440 own iterator involves implementing the `Iterator` trait. While doing that is
4441 outside of the scope of this guide, Rust provides a number of useful iterators
4442 to accomplish various tasks. Before we talk about those, we should talk about a
4443 Rust anti-pattern. And that's `range`.
4445 Yes, we just talked about how `range` is cool. But `range` is also very
4446 primitive. For example, if you needed to iterate over the contents of
4447 a vector, you may be tempted to write this:
4450 let nums = vec![1i, 2i, 3i];
4452 for i in range(0u, nums.len()) {
4453 println!("{}", nums[i]);
4457 This is strictly worse than using an actual iterator. The `.iter()` method on
4458 vectors returns an iterator which iterates through a reference to each element
4459 of the vector in turn. So write this:
4462 let nums = vec![1i, 2i, 3i];
4464 for num in nums.iter() {
4465 println!("{}", num);
4469 There are two reasons for this. First, this is more semantic. We iterate
4470 through the entire vector, rather than iterating through indexes, and then
4471 indexing the vector. Second, this version is more efficient: the first version
4472 will have extra bounds checking because it used indexing, `nums[i]`. But since
4473 we yield a reference to each element of the vector in turn with the iterator,
4474 there's no bounds checking in the second example. This is very common with
4475 iterators: we can ignore unnecessary bounds checks, but still know that we're
4478 There's another detail here that's not 100% clear because of how `println!`
4479 works. `num` is actually of type `&int`, that is, it's a reference to an `int`,
4480 not an `int` itself. `println!` handles the dereferencing for us, so we don't
4481 see it. This code works fine too:
4484 let nums = vec![1i, 2i, 3i];
4486 for num in nums.iter() {
4487 println!("{}", *num);
4491 Now we're explicitly dereferencing `num`. Why does `iter()` give us references?
4492 Well, if it gave us the data itself, we would have to be its owner, which would
4493 involve making a copy of the data and giving us the copy. With references,
4494 we're just borrowing a reference to the data, and so it's just passing
4495 a reference, without needing to do the copy.
4497 So, now that we've established that `range` is often not what you want, let's
4498 talk about what you do want instead.
4500 There are three broad classes of things that are relevant here: iterators,
4501 **iterator adapters**, and **consumers**. Here's some definitions:
4503 * 'iterators' give you a sequence of values.
4504 * 'iterator adapters' operate on an iterator, producing a new iterator with a
4505 different output sequence.
4506 * 'consumers' operate on an iterator, producing some final set of values.
4508 Let's talk about consumers first, since you've already seen an iterator,
4513 A 'consumer' operates on an iterator, returning some kind of value or values.
4514 The most common consumer is `collect()`. This code doesn't quite compile,
4515 but it shows the intention:
4518 let one_to_one_hundred = range(0i, 100i).collect();
4521 As you can see, we call `collect()` on our iterator. `collect()` takes
4522 as many values as the iterator will give it, and returns a collection
4523 of the results. So why won't this compile? Rust can't determine what
4524 type of things you want to collect, and so you need to let it know.
4525 Here's the version that does compile:
4528 let one_to_one_hundred = range(0i, 100i).collect::<Vec<int>>();
4531 If you remember, the `::<>` syntax allows us to give a type hint,
4532 and so we tell it that we want a vector of integers.
4534 `collect()` is the most common consumer, but there are others too. `find()`
4538 let one_to_one_hundred = range(0i, 100i);
4540 let greater_than_forty_two = range(0i, 100i)
4541 .find(|x| *x >= 42);
4543 match greater_than_forty_two {
4544 Some(_) => println!("We got some numbers!"),
4545 None => println!("No numbers found :("),
4549 `find` takes a closure, and works on a reference to each element of an
4550 iterator. This closure returns `true` if the element is the element we're
4551 looking for, and `false` otherwise. Because we might not find a matching
4552 element, `find` returns an `Option` rather than the element itself.
4554 Another important consumer is `fold`. Here's what it looks like:
4557 let sum = range(1i, 100i)
4558 .fold(0i, |sum, x| sum + x);
4561 `fold()` is a consumer that looks like this:
4562 `fold(base, |accumulator, element| ...)`. It takes two arguments: the first
4563 is an element called the "base". The second is a closure that itself takes two
4564 arguments: the first is called the "accumulator," and the second is an
4565 "element." Upon each iteration, the closure is called, and the result is the
4566 value of the accumulator on the next iteration. On the first iteration, the
4567 base is the value of the accumulator.
4569 Okay, that's a bit confusing. Let's examine the values of all of these things
4572 | base | accumulator | element | closure result |
4573 |------|-------------|---------|----------------|
4574 | 0i | 0i | 1i | 1i |
4575 | 0i | 1i | 2i | 3i |
4576 | 0i | 3i | 3i | 6i |
4578 We called `fold()` with these arguments:
4582 .fold(0i, |sum, x| sum + x);
4585 So, `0i` is our base, `sum` is our accumulator, and `x` is our element. On the
4586 first iteration, we set `sum` to `0i`, and `x` is the first element of `nums`,
4587 `1i`. We then add `sum` and `x`, which gives us `0i + 1i = 1i`. On the second
4588 iteration, that value becomes our accumulator, `sum`, and the element is
4589 the second element of the array, `2i`. `1i + 2i = 3i`, and so that becomes
4590 the value of the accumulator for the last iteration. On that iteration,
4591 `x` is the last element, `3i`, and `3i + 3i = 6i`, which is our final
4592 result for our sum. `1 + 2 + 3 = 6`, and that's the result we got.
4594 Whew. `fold` can be a bit strange the first few times you see it, but once it
4595 clicks, you can use it all over the place. Any time you have a list of things,
4596 and you want a single result, `fold` is appropriate.
4598 Consumers are important due to one additional property of iterators we haven't
4599 talked about yet: laziness. Let's talk some more about iterators, and you'll
4600 see why consumers matter.
4604 As we've said before, an iterator is something that we can call the `.next()`
4605 method on repeatedly, and it gives us a sequence of things. Because you need
4606 to call the method, this means that iterators are **lazy**. This code, for
4607 example, does not actually generate the numbers `1-100`, and just creates a
4608 value that represents the sequence:
4611 let nums = range(1i, 100i);
4614 Since we didn't do anything with the range, it didn't generate the sequence.
4615 Once we add the consumer:
4618 let nums = range(1i, 100i).collect::<Vec<int>>();
4621 Now, `collect()` will require that `range()` give it some numbers, and so
4622 it will do the work of generating the sequence.
4624 `range` is one of two basic iterators that you'll see. The other is `iter()`,
4625 which you've used before. `iter()` can turn a vector into a simple iterator
4626 that gives you each element in turn:
4629 let nums = [1i, 2i, 3i];
4631 for num in nums.iter() {
4632 println!("{}", num);
4636 These two basic iterators should serve you well. There are some more
4637 advanced iterators, including ones that are infinite. Like `count`:
4640 std::iter::count(1i, 5i);
4643 This iterator counts up from one, adding five each time. It will give
4644 you a new integer every time, forever. Well, technically, until the
4645 maximum number that an `int` can represent. But since iterators are lazy,
4646 that's okay! You probably don't want to use `collect()` on it, though...
4648 That's enough about iterators. Iterator adapters are the last concept
4649 we need to talk about with regards to iterators. Let's get to it!
4651 ## Iterator adapters
4653 "Iterator adapters" take an iterator and modify it somehow, producing
4654 a new iterator. The simplest one is called `map`:
4657 range(1i, 100i).map(|x| x + 1i);
4660 `map` is called upon another iterator, and produces a new iterator where each
4661 element reference has the closure it's been given as an argument called on it.
4662 So this would give us the numbers from `2-101`. Well, almost! If you
4663 compile the example, you'll get a warning:
4666 2:37 warning: unused result which must be used: iterator adaptors are lazy and
4667 do nothing unless consumed, #[warn(unused_must_use)] on by default
4668 range(1i, 100i).map(|x| x + 1i);
4669 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4672 Laziness strikes again! That closure will never execute. This example
4673 doesn't print any numbers:
4676 range(1i, 100i).map(|x| println!("{}", x));
4679 If you are trying to execute a closure on an iterator for its side effects,
4680 just use `for` instead.
4682 There are tons of interesting iterator adapters. `take(n)` will get the
4683 first `n` items out of an iterator, and return them as a list. Let's
4684 try it out with our infinite iterator from before, `count()`:
4687 for i in std::iter::count(1i, 5i).take(5) {
4702 `filter()` is an adapter that takes a closure as an argument. This closure
4703 returns `true` or `false`. The new iterator `filter()` produces returns
4704 only the elements that that closure returned `true` for:
4707 for i in range(1i, 100i).filter(|x| x % 2 == 0) {
4712 This will print all of the even numbers between one and a hundred.
4714 You can chain all three things together: start with an iterator, adapt it
4715 a few times, and then consume the result. Check it out:
4719 .filter(|x| x % 2 == 0)
4720 .filter(|x| x % 3 == 0)
4722 .collect::<Vec<int>>();
4725 This will give you a vector containing `6`, `12`, `18`, `24`, and `30`.
4727 This is just a small taste of what iterators, iterator adapters, and consumers
4728 can help you with. There are a number of really useful iterators, and you can
4729 write your own as well. Iterators provide a safe, efficient way to manipulate
4730 all kinds of lists. They're a little unusual at first, but if you play with
4731 them, you'll get hooked. For a full list of the different iterators and
4732 consumers, check out the [iterator module documentation](std/iter/index.html).
4736 Sometimes, when writing a function or data type, we may want it to work for
4737 multiple types of arguments. For example, remember our `OptionalInt` type?
4746 If we wanted to also have an `OptionalFloat64`, we would need a new enum:
4749 enum OptionalFloat64 {
4755 This is really unfortunate. Luckily, Rust has a feature that gives us a better
4756 way: generics. Generics are called **parametric polymorphism** in type theory,
4757 which means that they are types or functions that have multiple forms ("poly"
4758 is multiple, "morph" is form) over a given parameter ("parametric").
4760 Anyway, enough with type theory declarations, let's check out the generic form
4761 of `OptionalInt`. It is actually provided by Rust itself, and looks like this:
4770 The `<T>` part, which you've seen a few times before, indicates that this is
4771 a generic data type. Inside the declaration of our enum, wherever we see a `T`,
4772 we substitute that type for the same type used in the generic. Here's an
4773 example of using `Option<T>`, with some extra type annotations:
4776 let x: Option<int> = Some(5i);
4779 In the type declaration, we say `Option<int>`. Note how similar this looks to
4780 `Option<T>`. So, in this particular `Option`, `T` has the value of `int`. On
4781 the right hand side of the binding, we do make a `Some(T)`, where `T` is `5i`.
4782 Since that's an `int`, the two sides match, and Rust is happy. If they didn't
4783 match, we'd get an error:
4786 let x: Option<f64> = Some(5i);
4787 // error: mismatched types: expected `core::option::Option<f64>`
4788 // but found `core::option::Option<int>` (expected f64 but found int)
4791 That doesn't mean we can't make `Option<T>`s that hold an `f64`! They just have to
4795 let x: Option<int> = Some(5i);
4796 let y: Option<f64> = Some(5.0f64);
4799 This is just fine. One definition, multiple uses.
4801 Generics don't have to only be generic over one type. Consider Rust's built-in
4802 `Result<T, E>` type:
4811 This type is generic over _two_ types: `T` and `E`. By the way, the capital letters
4812 can be any letter you'd like. We could define `Result<T, E>` as:
4821 if we wanted to. Convention says that the first generic parameter should be
4822 `T`, for 'type,' and that we use `E` for 'error.' Rust doesn't care, however.
4824 The `Result<T, E>` type is intended to
4825 be used to return the result of a computation, and to have the ability to
4826 return an error if it didn't work out. Here's an example:
4829 let x: Result<f64, String> = Ok(2.3f64);
4830 let y: Result<f64, String> = Err("There was an error.".to_string());
4833 This particular Result will return an `f64` if there's a success, and a
4834 `String` if there's a failure. Let's write a function that uses `Result<T, E>`:
4837 fn inverse(x: f64) -> Result<f64, String> {
4838 if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4844 We don't want to take the inverse of zero, so we check to make sure that we
4845 weren't passed zero. If we were, then we return an `Err`, with a message. If
4846 it's okay, we return an `Ok`, with the answer.
4848 Why does this matter? Well, remember how `match` does exhaustive matches?
4849 Here's how this function gets used:
4852 # fn inverse(x: f64) -> Result<f64, String> {
4853 # if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4856 let x = inverse(25.0f64);
4859 Ok(x) => println!("The inverse of 25 is {}", x),
4860 Err(msg) => println!("Error: {}", msg),
4864 The `match` enforces that we handle the `Err` case. In addition, because the
4865 answer is wrapped up in an `Ok`, we can't just use the result without doing
4869 let x = inverse(25.0f64);
4870 println!("{}", x + 2.0f64); // error: binary operation `+` cannot be applied
4871 // to type `core::result::Result<f64,collections::string::String>`
4874 This function is great, but there's one other problem: it only works for 64 bit
4875 floating point values. What if we wanted to handle 32 bit floating point as
4876 well? We'd have to write this:
4879 fn inverse32(x: f32) -> Result<f32, String> {
4880 if x == 0.0f32 { return Err("x cannot be zero!".to_string()); }
4886 Bummer. What we need is a **generic function**. Luckily, we can write one!
4887 However, it won't _quite_ work yet. Before we get into that, let's talk syntax.
4888 A generic version of `inverse` would look something like this:
4891 fn inverse<T>(x: T) -> Result<T, String> {
4892 if x == 0.0 { return Err("x cannot be zero!".to_string()); }
4898 Just like how we had `Option<T>`, we use a similar syntax for `inverse<T>`.
4899 We can then use `T` inside the rest of the signature: `x` has type `T`, and half
4900 of the `Result` has type `T`. However, if we try to compile that example, we'll get
4904 error: binary operation `==` cannot be applied to type `T`
4907 Because `T` can be _any_ type, it may be a type that doesn't implement `==`,
4908 and therefore, the first line would be wrong. What do we do?
4910 To fix this example, we need to learn about another Rust feature: traits.
4914 Do you remember the `impl` keyword, used to call a function with method
4925 fn area(&self) -> f64 {
4926 std::f64::consts::PI * (self.radius * self.radius)
4931 Traits are similar, except that we define a trait with just the method
4932 signature, then implement the trait for that struct. Like this:
4942 fn area(&self) -> f64;
4945 impl HasArea for Circle {
4946 fn area(&self) -> f64 {
4947 std::f64::consts::PI * (self.radius * self.radius)
4952 As you can see, the `trait` block looks very similar to the `impl` block,
4953 but we don't define a body, just a type signature. When we `impl` a trait,
4954 we use `impl Trait for Item`, rather than just `impl Item`.
4956 So what's the big deal? Remember the error we were getting with our generic
4960 error: binary operation `==` cannot be applied to type `T`
4963 We can use traits to constrain our generics. Consider this function, which
4964 does not compile, and gives us a similar error:
4967 fn print_area<T>(shape: T) {
4968 println!("This shape has an area of {}", shape.area());
4975 error: type `T` does not implement any method in scope named `area`
4978 Because `T` can be any type, we can't be sure that it implements the `area`
4979 method. But we can add a **trait constraint** to our generic `T`, ensuring
4984 # fn area(&self) -> f64;
4986 fn print_area<T: HasArea>(shape: T) {
4987 println!("This shape has an area of {}", shape.area());
4991 The syntax `<T: HasArea>` means `any type that implements the HasArea trait`.
4992 Because traits define function type signatures, we can be sure that any type
4993 which implements `HasArea` will have an `.area()` method.
4995 Here's an extended example of how this works:
4999 fn area(&self) -> f64;
5008 impl HasArea for Circle {
5009 fn area(&self) -> f64 {
5010 std::f64::consts::PI * (self.radius * self.radius)
5020 impl HasArea for Square {
5021 fn area(&self) -> f64 {
5022 self.side * self.side
5026 fn print_area<T: HasArea>(shape: T) {
5027 println!("This shape has an area of {}", shape.area());
5048 This program outputs:
5051 This shape has an area of 3.141593
5052 This shape has an area of 1
5055 As you can see, `print_area` is now generic, but also ensures that we
5056 have passed in the correct types. If we pass in an incorrect type:
5062 We get a compile-time error:
5065 error: failed to find an implementation of trait main::HasArea for int
5068 So far, we've only added trait implementations to structs, but you can
5069 implement a trait for any type. So technically, we _could_ implement
5070 `HasArea` for `int`:
5074 fn area(&self) -> f64;
5077 impl HasArea for int {
5078 fn area(&self) -> f64 {
5079 println!("this is silly");
5088 It is considered poor style to implement methods on such primitive types, even
5089 though it is possible.
5091 This may seem like the Wild West, but there are two other restrictions around
5092 implementing traits that prevent this from getting out of hand. First, traits
5093 must be `use`d in any scope where you wish to use the trait's method. So for
5094 example, this does not work:
5098 use std::f64::consts;
5101 fn area(&self) -> f64;
5110 impl HasArea for Circle {
5111 fn area(&self) -> f64 {
5112 consts::PI * (self.radius * self.radius)
5118 let c = shapes::Circle {
5124 println!("{}", c.area());
5128 Now that we've moved the structs and traits into their own module, we get an
5132 error: type `shapes::Circle` does not implement any method in scope named `area`
5135 If we add a `use` line right above `main` and make the right things public,
5139 use shapes::HasArea;
5142 use std::f64::consts;
5145 fn area(&self) -> f64;
5154 impl HasArea for Circle {
5155 fn area(&self) -> f64 {
5156 consts::PI * (self.radius * self.radius)
5163 let c = shapes::Circle {
5169 println!("{}", c.area());
5173 This means that even if someone does something bad like add methods to `int`,
5174 it won't affect you, unless you `use` that trait.
5176 There's one more restriction on implementing traits. Either the trait or the
5177 type you're writing the `impl` for must be inside your crate. So, we could
5178 implement the `HasArea` type for `int`, because `HasArea` is in our crate. But
5179 if we tried to implement `Float`, a trait provided by Rust, for `int`, we could
5180 not, because both the trait and the type aren't in our crate.
5182 One last thing about traits: generic functions with a trait bound use
5183 **monomorphization** ("mono": one, "morph": form), so they are statically
5184 dispatched. What's that mean? Well, let's take a look at `print_area` again:
5187 fn print_area<T: HasArea>(shape: T) {
5188 println!("This shape has an area of {}", shape.area());
5192 let c = Circle { ... };
5194 let s = Square { ... };
5201 When we use this trait with `Circle` and `Square`, Rust ends up generating
5202 two different functions with the concrete type, and replacing the call sites with
5203 calls to the concrete implementations. In other words, you get something like
5207 fn __print_area_circle(shape: Circle) {
5208 println!("This shape has an area of {}", shape.area());
5211 fn __print_area_square(shape: Square) {
5212 println!("This shape has an area of {}", shape.area());
5216 let c = Circle { ... };
5218 let s = Square { ... };
5220 __print_area_circle(c);
5221 __print_area_square(s);
5225 The names don't actually change to this, it's just for illustration. But
5226 as you can see, there's no overhead of deciding which version to call here,
5227 hence 'statically dispatched.' The downside is that we have two copies of
5228 the same function, so our binary is a little bit larger.
5232 Concurrency and parallelism are topics that are of increasing interest to a
5233 broad subsection of software developers. Modern computers are often multi-core,
5234 to the point that even embedded devices like cell phones have more than one
5235 processor. Rust's semantics lend themselves very nicely to solving a number of
5236 issues that programmers have with concurrency. Many concurrency errors that are
5237 runtime errors in other languages are compile-time errors in Rust.
5239 Rust's concurrency primitive is called a **task**. Tasks are lightweight, and
5240 do not share memory in an unsafe manner, preferring message passing to
5241 communicate. It's worth noting that tasks are implemented as a library, and
5242 not part of the language. This means that in the future, other concurrency
5243 libraries can be written for Rust to help in specific scenarios. Here's an
5244 example of creating a task:
5248 println!("Hello from a task!");
5252 The `spawn` function takes a proc as an argument, and runs that proc in a new
5253 task. A proc takes ownership of its entire environment, and so any variables
5254 that you use inside the proc will not be usable afterward:
5257 let mut x = vec![1i, 2i, 3i];
5260 println!("The value of x[0] is: {}", x[0]);
5263 println!("The value of x[0] is: {}", x[0]); // error: use of moved value: `x`
5266 `x` is now owned by the proc, and so we can't use it anymore. Many other
5267 languages would let us do this, but it's not safe to do so. Rust's type system
5270 If tasks were only able to capture these values, they wouldn't be very useful.
5271 Luckily, tasks can communicate with each other through **channel**s. Channels
5275 let (tx, rx) = channel();
5278 tx.send("Hello from a task!".to_string());
5281 let message = rx.recv();
5282 println!("{}", message);
5285 The `channel()` function returns two endpoints: a `Receiver<T>` and a
5286 `Sender<T>`. You can use the `.send()` method on the `Sender<T>` end, and
5287 receive the message on the `Receiver<T>` side with the `recv()` method. This
5288 method blocks until it gets a message. There's a similar method, `.try_recv()`,
5289 which returns an `Option<T>` and does not block.
5291 If you want to send messages to the task as well, create two channels!
5294 let (tx1, rx1) = channel();
5295 let (tx2, rx2) = channel();
5298 tx1.send("Hello from a task!".to_string());
5299 let message = rx2.recv();
5300 println!("{}", message);
5303 let message = rx1.recv();
5304 println!("{}", message);
5306 tx2.send("Goodbye from main!".to_string());
5309 The proc has one sending end and one receiving end, and the main task has one
5310 of each as well. Now they can talk back and forth in whatever way they wish.
5312 Notice as well that because `Sender` and `Receiver` are generic, while you can
5313 pass any kind of information through the channel, the ends are strongly typed.
5314 If you try to pass a string, and then an integer, Rust will complain.
5318 With these basic primitives, many different concurrency patterns can be
5319 developed. Rust includes some of these types in its standard library. For
5320 example, if you wish to compute some value in the background, `Future` is
5321 a useful thing to use:
5324 use std::sync::Future;
5326 let mut delayed_value = Future::spawn(proc() {
5327 // just return anything for examples' sake
5331 println!("value = {}", delayed_value.get());
5334 Calling `Future::spawn` works just like `spawn()`: it takes a proc. In this
5335 case, though, you don't need to mess with the channel: just have the proc
5338 `Future::spawn` will return a value which we can bind with `let`. It needs
5339 to be mutable, because once the value is computed, it saves a copy of the
5340 value, and if it were immutable, it couldn't update itself.
5342 The proc will go on processing in the background, and when we need the final
5343 value, we can call `get()` on it. This will block until the result is done,
5344 but if it's finished computing in the background, we'll just get the value
5347 ## Success and failure
5349 Tasks don't always succeed, they can also fail. A task that wishes to fail
5350 can call the `fail!` macro, passing a message:
5358 If a task fails, it is not possible for it to recover. However, it can
5359 notify other tasks that it has failed. We can do this with `task::try`:
5365 let result = task::try(proc() {
5374 This task will randomly fail or succeed. `task::try` returns a `Result`
5375 type, so we can handle the response like any other computation that may
5380 One of Rust's most advanced features is its system of **macro**s. While
5381 functions allow you to provide abstractions over values and operations, macros
5382 allow you to provide abstractions over syntax. Do you wish Rust had the ability
5383 to do something that it can't currently do? You may be able to write a macro
5384 to extend Rust's capabilities.
5386 You've already used one macro extensively: `println!`. When we invoke
5387 a Rust macro, we need to use the exclamation mark (`!`). There's two reasons
5388 that this is true: the first is that it makes it clear when you're using a
5389 macro. The second is that macros allow for flexible syntax, and so Rust must
5390 be able to tell where a macro starts and ends. The `!(...)` helps with this.
5392 Let's talk some more about `println!`. We could have implemented `println!` as
5393 a function, but it would be worse. Why? Well, what macros allow you to do
5394 is write code that generates more code. So when we call `println!` like this:
5398 println!("x is: {}", x);
5401 The `println!` macro does a few things:
5403 1. It parses the string to find any `{}`s
5404 2. It checks that the number of `{}`s matches the number of other arguments.
5405 3. It generates a bunch of Rust code, taking this in mind.
5407 What this means is that you get type checking at compile time, because
5408 Rust will generate code that takes all of the types into account. If
5409 `println!` was a function, it could still do this type checking, but it
5410 would happen at run time rather than compile time.
5412 We can check this out using a special flag to `rustc`. This code, in a file
5418 println!("x is: {:s}", x);
5422 Can have its macros expanded like this: `rustc print.rs --pretty=expanded`, will
5423 give us this huge result:
5429 #[phase(plugin, link)]
5430 extern crate std = "std";
5431 extern crate rt = "native";
5432 use std::prelude::*;
5439 static __STATIC_FMTSTR: [::std::fmt::rt::Piece<'static>, ..2u] =
5440 [::std::fmt::rt::String("x is: "),
5441 ::std::fmt::rt::Argument(::std::fmt::rt::Argument{position:
5442 ::std::fmt::rt::ArgumentNext,
5444 ::std::fmt::rt::FormatSpec{fill:
5447 ::std::fmt::rt::AlignUnknown,
5451 ::std::fmt::rt::CountImplied,
5453 ::std::fmt::rt::CountImplied,},})];
5455 &[::std::fmt::argument(::std::fmt::secret_string, __arg0)];
5458 ::std::fmt::Arguments::new(__STATIC_FMTSTR, __args_vec)
5460 ::std::io::stdio::println_args(&__args)
5466 Intense. Here's a trimmed down version that's a bit easier to read:
5473 static __STATIC_FMTSTR: =
5476 position: ArgumentNext,
5477 format: FormatSpec {
5479 align: AlignUnknown,
5481 precision: CountImplied,
5482 width: CountImplied,
5486 let __args_vec = &[argument(secret_string, __arg0)];
5487 let __args = unsafe { Arguments::new(__STATIC_FMTSTR, __args_vec) };
5489 println_args(&__args)
5495 Whew! This isn't too terrible. You can see that we still `let x = 5i`,
5496 but then things get a little bit hairy. Three more bindings get set: a
5497 static format string, an argument vector, and the aruments. We then
5498 invoke the `println_args` function with the generated arguments.
5500 This is the code (well, the full version) that Rust actually compiles. You can
5501 see all of the extra information that's here. We get all of the type safety and
5502 options that it provides, but at compile time, and without needing to type all
5503 of this out. This is how macros are powerful. Without them, you would need to
5504 type all of this by hand to get a type checked `println`.
5506 For more on macros, please consult [the Macros Guide](guide-macros.html).
5507 Macros are a very advanced and still slightly experimental feature, but don't
5508 require a deep understanding to call, since they look just like functions. The
5509 Guide can help you if you want to write your own.
5513 Finally, there's one more Rust concept that you should be aware of: `unsafe`.
5514 There are two circumstances where Rust's safety provisions don't work well.
5515 The first is when interfacing with C code, and the second is when building
5516 certain kinds of abstractions.
5518 Rust has support for FFI (which you can read about in the [FFI
5519 Guide](guide-ffi.html)), but can't guarantee that the C code will be safe.
5520 Therefore, Rust marks such functions with the `unsafe`
5521 keyword, which indicates that the function may not behave properly.
5523 Second, if you'd like to create some sort of shared-memory data structure, Rust
5524 won't allow it, because memory must be owned by a single owner. However, if
5525 you're planning on making access to that shared memory safe, such as with a
5526 mutex, _you_ know that it's safe, but Rust can't know. Writing an `unsafe`
5527 block allows you to ask the compiler to trust you. In this case, the _internal_
5528 implementation of the mutex is considered unsafe, but the _external_ interface
5529 we present is safe. This allows it to be effectively used in normal Rust, while
5530 being able to implement functionality that the compiler can't double check for
5533 Doesn't an escape hatch undermine the safety of the entire system? Well, if
5534 Rust code segfaults, it _must_ be because of unsafe code somewhere. By
5535 annotating exactly where that is, you have a significantly smaller area to
5538 We haven't even talked about any examples here, and that's because I want to
5539 emphasize that you should not be writing unsafe code unless you know exactly
5540 what you're doing. The vast majority of Rust developers will only interact with
5541 it when doing FFI, and advanced library authors may use it to build certain
5542 kinds of abstraction.
5546 We covered a lot of ground here. When you've mastered everything in this Guide,
5547 you will have a firm grasp of basic Rust development. There's a whole lot more
5548 out there, we've just covered the surface. There's tons of topics that you can
5549 dig deeper into, and we've built specialized guides for many of them. To learn
5550 more, dig into the [full documentation
5551 index](http://doc.rust-lang.org/index.html).