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, license 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. Next, let's
524 talk about this stuff we've added to `println!`.
526 If you include two curly braces (`{}`, some call them moustaches...) in your
527 string to print, Rust will interpret this as a request to interpolate some sort
528 of value. **String interpolation** is a computer science term that means "stick
529 in the middle of a string." We add a comma, and then `x`, to indicate that we
530 want `x` to be the value we're interpolating. The comma is used to separate
531 arguments we pass to functions and macros, if you're passing more than one.
533 When you just use the curly braces, Rust will attempt to display the
534 value in a meaningful way by checking out its type. If you want to specify the
535 format in a more detailed manner, there are a [wide number of options
536 available](std/fmt/index.html). For now, we'll just stick to the default:
537 integers aren't very complicated to print.
541 Rust's take on `if` is not particularly complex, but it's much more like the
542 `if` you'll find in a dynamically typed language than in a more traditional
543 systems language. So let's talk about it, to make sure you grasp the nuances.
545 `if` is a specific form of a more general concept, the 'branch.' The name comes
546 from a branch in a tree: a decision point, where depending on a choice,
547 multiple paths can be taken.
549 In the case of `if`, there is one choice that leads down two paths:
555 println!("x is five!");
559 If we changed the value of `x` to something else, this line would not print.
560 More specifically, if the expression after the `if` evaluates to `true`, then
561 the block is executed. If it's `false`, then it is not.
563 If you want something to happen in the `false` case, use an `else`:
569 println!("x is five!");
571 println!("x is not five :(");
575 This is all pretty standard. However, you can also do this:
588 Which we can (and probably should) write like this:
593 let y = if x == 5i { 10i } else { 15i };
596 This reveals two interesting things about Rust: it is an expression-based
597 language, and semicolons are different than in other 'curly brace and
598 semicolon'-based languages. These two things are related.
600 ## Expressions vs. Statements
602 Rust is primarily an expression based language. There are only two kinds of
603 statements, and everything else is an expression.
605 So what's the difference? Expressions return a value, and statements do not.
606 In many languages, `if` is a statement, and therefore, `let x = if ...` would
607 make no sense. But in Rust, `if` is an expression, which means that it returns
608 a value. We can then use this value to initialize the binding.
610 Speaking of which, bindings are a kind of the first of Rust's two statements.
611 The proper name is a **declaration statement**. So far, `let` is the only kind
612 of declaration statement we've seen. Let's talk about that some more.
614 In some languages, variable bindings can be written as expressions, not just
615 statements. Like Ruby:
621 In Rust, however, using `let` to introduce a binding is _not_ an expression. The
622 following will produce a compile-time error:
625 let x = (let y = 5i); // expected identifier, found keyword `let`
628 The compiler is telling us here that it was expecting to see the beginning of
629 an expression, and a `let` can only begin a statement, not an expression.
631 Note that assigning to an already-bound variable (e.g. `y = 5i`) is still an
632 expression, although its value is not particularly useful. Unlike C, where an
633 assignment evaluates to the assigned value (e.g. `5i` in the previous example),
634 in Rust the value of an assignment is the unit type `()` (which we'll cover later).
636 The second kind of statement in Rust is the **expression statement**. Its
637 purpose is to turn any expression into a statement. In practical terms, Rust's
638 grammar expects statements to follow other statements. This means that you use
639 semicolons to separate expressions from each other. This means that Rust
640 looks a lot like most other languages that require you to use semicolons
641 at the end of every line, and you will see semicolons at the end of almost
642 every line of Rust code you see.
644 What is this exception that makes us say 'almost?' You saw it already, in this
650 let y: int = if x == 5i { 10i } else { 15i };
653 Note that I've added the type annotation to `y`, to specify explicitly that I
654 want `y` to be an integer.
656 This is not the same as this, which won't compile:
661 let y: int = if x == 5i { 10i; } else { 15i; };
664 Note the semicolons after the 10 and 15. Rust will give us the following error:
667 error: mismatched types: expected `int` but found `()` (expected int but found ())
670 We expected an integer, but we got `()`. `()` is pronounced 'unit', and is a
671 special type in Rust's type system. `()` is different than `null` in other
672 languages, because `()` is distinct from other types. For example, in C, `null`
673 is a valid value for a variable of type `int`. In Rust, `()` is _not_ a valid
674 value for a variable of type `int`. It's only a valid value for variables of
675 the type `()`, which aren't very useful. Remember how we said statements don't
676 return a value? Well, that's the purpose of unit in this case. The semicolon
677 turns any expression into a statement by throwing away its value and returning
680 There's one more time in which you won't see a semicolon at the end of a line
681 of Rust code. For that, we'll need our next concept: functions.
685 You've already seen one function so far, the `main` function:
692 This is the simplest possible function declaration. As we mentioned before,
693 `fn` says 'this is a function,' followed by the name, some parenthesis because
694 this function takes no arguments, and then some curly braces to indicate the
695 body. Here's a function named `foo`:
702 So, what about taking arguments? Here's a function that prints a number:
705 fn print_number(x: int) {
706 println!("x is: {}", x);
710 Here's a complete program that uses `print_number`:
717 fn print_number(x: int) {
718 println!("x is: {}", x);
722 As you can see, function arguments work very similar to `let` declarations:
723 you add a type to the argument name, after a colon.
725 Here's a complete program that adds two numbers together and prints them:
732 fn print_sum(x: int, y: int) {
733 println!("sum is: {}", x + y);
737 You separate arguments with a comma, both when you call the function, as well
738 as when you declare it.
740 Unlike `let`, you _must_ declare the types of function arguments. This does
744 fn print_number(x, y) {
745 println!("x is: {}", x + y);
752 hello.rs:5:18: 5:19 error: expected `:` but found `,`
753 hello.rs:5 fn print_number(x, y) {
756 This is a deliberate design decision. While full-program inference is possible,
757 languages which have it, like Haskell, often suggest that documenting your
758 types explicitly is a best-practice. We agree that forcing functions to declare
759 types while allowing for inference inside of function bodies is a wonderful
760 sweet spot between full inference and no inference.
762 What about returning a value? Here's a function that adds one to an integer:
765 fn add_one(x: int) -> int {
770 Rust functions return exactly one value, and you declare the type after an
771 'arrow', which is a dash (`-`) followed by a greater-than sign (`>`).
773 You'll note the lack of a semicolon here. If we added it in:
776 fn add_one(x: int) -> int {
781 We would get an error:
784 error: not all control paths return a value
785 fn add_one(x: int) -> int {
789 note: consider removing this semicolon:
794 Remember our earlier discussions about semicolons and `()`? Our function claims
795 to return an `int`, but with a semicolon, it would return `()` instead. Rust
796 realizes this probably isn't what we want, and suggests removing the semicolon.
798 This is very much like our `if` statement before: the result of the block
799 (`{}`) is the value of the expression. Other expression-oriented languages,
800 such as Ruby, work like this, but it's a bit unusual in the systems programming
801 world. When people first learn about this, they usually assume that it
802 introduces bugs. But because Rust's type system is so strong, and because unit
803 is its own unique type, we have never seen an issue where adding or removing a
804 semicolon in a return position would cause a bug.
806 But what about early returns? Rust does have a keyword for that, `return`:
809 fn foo(x: int) -> int {
810 if x < 5 { return x; }
816 Using a `return` as the last line of a function works, but is considered poor
820 fn foo(x: int) -> int {
821 if x < 5 { return x; }
827 There are some additional ways to define functions, but they involve features
828 that we haven't learned about yet, so let's just leave it at that for now.
833 Now that we have some functions, it's a good idea to learn about comments.
834 Comments are notes that you leave to other programmers to help explain things
835 about your code. The compiler mostly ignores them.
837 Rust has two kinds of comments that you should care about: **line comment**s
838 and **doc comment**s.
841 // Line comments are anything after '//' and extend to the end of the line.
843 let x = 5i; // this is also a line comment.
845 // If you have a long explanation for something, you can put line comments next
846 // to each other. Put a space between the // and your comment so that it's
850 The other kind of comment is a doc comment. Doc comments use `///` instead of
851 `//`, and support Markdown notation inside:
854 /// `hello` is a function that prints a greeting that is personalized based on
859 /// * `name` - The name of the person you'd like to greet.
864 /// let name = "Steve";
865 /// hello(name); // prints "Hello, Steve!"
867 fn hello(name: &str) {
868 println!("Hello, {}!", name);
872 When writing doc comments, adding sections for any arguments, return values,
873 and providing some examples of usage is very, very helpful.
875 You can use the `rustdoc` tool to generate HTML documentation from these doc
876 comments. We will talk more about `rustdoc` when we get to modules, as
877 generally, you want to export documentation for a full module.
879 # Compound Data Types
881 Rust, like many programming languages, has a number of different data types
882 that are built-in. You've already done some simple work with integers and
883 strings, but next, let's talk about some more complicated ways of storing data.
887 The first compound data type we're going to talk about are called **tuple**s.
888 Tuples are an ordered list of a fixed size. Like this:
891 let x = (1i, "hello");
894 The parenthesis and commas form this two-length tuple. Here's the same code, but
895 with the type annotated:
898 let x: (int, &str) = (1, "hello");
901 As you can see, the type of a tuple looks just like the tuple, but with each
902 position having a type name rather than the value. Careful readers will also
903 note that tuples are heterogeneous: we have an `int` and a `&str` in this tuple.
904 You haven't seen `&str` as a type before, and we'll discuss the details of
905 strings later. In systems programming languages, strings are a bit more complex
906 than in other languages. For now, just read `&str` as "a string slice," and
907 we'll learn more soon.
909 You can access the fields in a tuple through a **destructuring let**. Here's
913 let (x, y, z) = (1i, 2i, 3i);
915 println!("x is {}", x);
918 Remember before when I said the left hand side of a `let` statement was more
919 powerful than just assigning a binding? Here we are. We can put a pattern on
920 the left hand side of the `let`, and if it matches up to the right hand side,
921 we can assign multiple bindings at once. In this case, `let` 'destructures,'
922 or 'breaks up,' the tuple, and assigns the bits to three bindings.
924 This pattern is very powerful, and we'll see it repeated more later.
926 The last thing to say about tuples is that they are only equivalent if
927 the arity, types, and values are all identical.
930 let x = (1i, 2i, 3i);
931 let y = (2i, 3i, 4i);
940 This will print `no`, as the values aren't equal.
942 One other use of tuples is to return multiple values from a function:
945 fn next_two(x: int) -> (int, int) { (x + 1i, x + 2i) }
948 let (x, y) = next_two(5i);
949 println!("x, y = {}, {}", x, y);
953 Even though Rust functions can only return one value, a tuple _is_ one value,
954 that happens to be made up of two. You can also see in this example how you
955 can destructure a pattern returned by a function, as well.
957 Tuples are a very simple data structure, and so are not often what you want.
958 Let's move on to their bigger sibling, structs.
962 A struct is another form of a 'record type,' just like a tuple. There's a
963 difference: structs give each element that they contain a name, called a
964 'field' or a 'member.' Check it out:
973 let origin = Point { x: 0i, y: 0i };
975 println!("The origin is at ({}, {})", origin.x, origin.y);
979 There's a lot going on here, so let's break it down. We declare a struct with
980 the `struct` keyword, and then with a name. By convention, structs begin with a
981 capital letter and are also camel cased: `PointInSpace`, not `Point_In_Space`.
983 We can create an instance of our struct via `let`, as usual, but we use a `key:
984 value` style syntax to set each field. The order doesn't need to be the same as
985 in the original declaration.
987 Finally, because fields have names, we can access the field through dot
988 notation: `origin.x`.
990 The values in structs are immutable, like other bindings in Rust. However, you
991 can use `mut` to make them mutable:
1000 let mut point = Point { x: 0i, y: 0i };
1004 println!("The point is at ({}, {})", point.x, point.y);
1008 This will print `The point is at (5, 0)`.
1010 ## Tuple Structs and Newtypes
1012 Rust has another data type that's like a hybrid between a tuple and a struct,
1013 called a **tuple struct**. Tuple structs do have a name, but their fields
1018 struct Color(int, int, int);
1019 struct Point(int, int, int);
1022 These two will not be equal, even if they have the same values:
1025 let black = Color(0, 0, 0);
1026 let origin = Point(0, 0, 0);
1029 It is almost always better to use a struct than a tuple struct. We would write
1030 `Color` and `Point` like this instead:
1046 Now, we have actual names, rather than positions. Good names are important,
1047 and with a struct, we have actual names.
1049 There _is_ one case when a tuple struct is very useful, though, and that's a
1050 tuple struct with only one element. We call this a 'newtype,' because it lets
1051 you create a new type that's a synonym for another one:
1056 let length = Inches(10);
1058 let Inches(integer_length) = length;
1059 println!("length is {} inches", integer_length);
1062 As you can see here, you can extract the inner integer type through a
1063 destructuring `let`.
1067 Finally, Rust has a "sum type", an **enum**. Enums are an incredibly useful
1068 feature of Rust, and are used throughout the standard library. This is an enum
1069 that is provided by the Rust standard library:
1079 An `Ordering` can only be _one_ of `Less`, `Equal`, or `Greater` at any given
1080 time. Here's an example:
1083 fn cmp(a: int, b: int) -> Ordering {
1085 else if a > b { Greater }
1093 let ordering = cmp(x, y);
1095 if ordering == Less {
1097 } else if ordering == Greater {
1098 println!("greater");
1099 } else if ordering == Equal {
1105 `cmp` is a function that compares two things, and returns an `Ordering`. We
1106 return either `Less`, `Greater`, or `Equal`, depending on if the two values
1107 are greater, less, or equal.
1109 The `ordering` variable has the type `Ordering`, and so contains one of the
1110 three values. We can then do a bunch of `if`/`else` comparisons to check
1113 However, repeated `if`/`else` comparisons get quite tedious. Rust has a feature
1114 that not only makes them nicer to read, but also makes sure that you never
1115 miss a case. Before we get to that, though, let's talk about another kind of
1116 enum: one with values.
1118 This enum has two variants, one of which has a value:
1131 Value(n) => println!("x is {:d}", n),
1132 Missing => println!("x is missing!"),
1136 Value(n) => println!("y is {:d}", n),
1137 Missing => println!("y is missing!"),
1142 This enum represents an `int` that we may or may not have. In the `Missing`
1143 case, we have no value, but in the `Value` case, we do. This enum is specific
1144 to `int`s, though. We can make it usable by any type, but we haven't quite
1147 You can have any number of values in an enum:
1150 enum OptionalColor {
1151 Color(int, int, int),
1156 Enums with values are quite useful, but as I mentioned, they're even more
1157 useful when they're generic across types. But before we get to generics, let's
1158 talk about how to fix this big `if`/`else` statements we've been writing. We'll
1159 do that with `match`.
1163 Often, a simple `if`/`else` isn't enough, because you have more than two
1164 possible options. And `else` conditions can get incredibly complicated. So
1165 what's the solution?
1167 Rust has a keyword, `match`, that allows you to replace complicated `if`/`else`
1168 groupings with something more powerful. Check it out:
1174 1 => println!("one"),
1175 2 => println!("two"),
1176 3 => println!("three"),
1177 4 => println!("four"),
1178 5 => println!("five"),
1179 _ => println!("something else"),
1183 `match` takes an expression, and then branches based on its value. Each 'arm' of
1184 the branch is of the form `val => expression`. When the value matches, that arm's
1185 expression will be evaluated. It's called `match` because of the term 'pattern
1186 matching,' which `match` is an implementation of.
1188 So what's the big advantage here? Well, there are a few. First of all, `match`
1189 does 'exhaustiveness checking.' Do you see that last arm, the one with the
1190 underscore (`_`)? If we remove that arm, Rust will give us an error:
1193 error: non-exhaustive patterns: `_` not covered
1196 In other words, Rust is trying to tell us we forgot a value. Because `x` is an
1197 integer, Rust knows that it can have a number of different values. For example,
1198 `6i`. But without the `_`, there is no arm that could match, and so Rust refuses
1199 to compile. `_` is sort of like a catch-all arm. If none of the other arms match,
1200 the arm with `_` will. And since we have this catch-all arm, we now have an arm
1201 for every possible value of `x`, and so our program will now compile.
1203 `match` statements also destructure enums, as well. Remember this code from the
1207 fn cmp(a: int, b: int) -> Ordering {
1209 else if a > b { Greater }
1217 let ordering = cmp(x, y);
1219 if ordering == Less {
1221 } else if ordering == Greater {
1222 println!("greater");
1223 } else if ordering == Equal {
1229 We can re-write this as a `match`:
1232 fn cmp(a: int, b: int) -> Ordering {
1234 else if a > b { Greater }
1243 Less => println!("less"),
1244 Greater => println!("greater"),
1245 Equal => println!("equal"),
1250 This version has way less noise, and it also checks exhaustively to make sure
1251 that we have covered all possible variants of `Ordering`. With our `if`/`else`
1252 version, if we had forgotten the `Greater` case, for example, our program would
1253 have happily compiled. If we forget in the `match`, it will not. Rust helps us
1254 make sure to cover all of our bases.
1256 `match` is also an expression, which means we can use it on the right hand side
1257 of a `let` binding. We could also implement the previous line like this:
1260 fn cmp(a: int, b: int) -> Ordering {
1262 else if a > b { Greater }
1270 let result = match cmp(x, y) {
1272 Greater => "greater",
1276 println!("{}", result);
1280 In this case, it doesn't make a lot of sense, as we are just making a temporary
1281 string where we don't need to, but sometimes, it's a nice pattern.
1285 Looping is the last basic construct that we haven't learned yet in Rust. Rust has
1286 two main looping constructs: `for` and `while`.
1290 The `for` loop is used to loop a particular number of times. Rust's `for` loops
1291 work a bit differently than in other systems languages, however. Rust's `for`
1292 loop doesn't look like this C `for` loop:
1295 for (x = 0; x < 10; x++) {
1296 printf( "%d\n", x );
1303 for x in range(0i, 10i) {
1304 println!("{:d}", x);
1308 In slightly more abstract terms,
1311 for var in expression {
1316 The expression is an iterator, which we will discuss in more depth later in the
1317 guide. The iterator gives back a series of elements. Each element is one
1318 iteration of the loop. That value is then bound to the name `var`, which is
1319 valid for the loop body. Once the body is over, the next value is fetched from
1320 the iterator, and we loop another time. When there are no more values, the
1323 In our example, the `range` function is a function, provided by Rust, that
1324 takes a start and an end position, and gives an iterator over those values. The
1325 upper bound is exclusive, though, so our loop will print `0` through `9`, not
1328 Rust does not have the "C style" `for` loop on purpose. Manually controlling
1329 each element of the loop is complicated and error prone, even for experienced C
1330 developers. There's an old joke that goes, "There are two hard problems in
1331 computer science: naming things, cache invalidation, and off-by-one errors."
1332 The joke, of course, being that the setup says "two hard problems" but then
1333 lists three things. This happens quite a bit with "C style" `for` loops.
1335 We'll talk more about `for` when we cover **iterator**s, later in the Guide.
1339 The other kind of looping construct in Rust is the `while` loop. It looks like
1344 let mut done = false;
1349 if x % 5 == 0 { done = true; }
1353 `while` loops are the correct choice when you're not sure how many times
1356 If you need an infinite loop, you may be tempted to write this:
1362 Rust has a dedicated keyword, `loop`, to handle this case:
1368 Rust's control-flow analysis treats this construct differently than a
1369 `while true`, since we know that it will always loop. The details of what
1370 that _means_ aren't super important to understand at this stage, but in
1371 general, the more information we can give to the compiler, the better it
1372 can do with safety and code generation. So you should always prefer
1373 `loop` when you plan to loop infinitely.
1375 ## Ending iteration early
1377 Let's take a look at that `while` loop we had earlier:
1381 let mut done = false;
1386 if x % 5 == 0 { done = true; }
1390 We had to keep a dedicated `mut` boolean variable binding, `done`, to know
1391 when we should skip out of the loop. Rust has two keywords to help us with
1392 modifying iteration: `break` and `continue`.
1394 In this case, we can write the loop in a better way with `break`:
1402 if x % 5 == 0 { break; }
1406 We now loop forever with `loop`, and use `break` to break out early.
1408 `continue` is similar, but instead of ending the loop, goes to the next
1409 iteration: This will only print the odd numbers:
1412 for x in range(0i, 10i) {
1413 if x % 2 == 0 { continue; }
1415 println!("{:d}", x);
1419 Both `continue` and `break` are valid in both kinds of loops.
1423 Strings are an important concept for any programmer to master. Rust's string
1424 handling system is a bit different than in other languages, due to its systems
1425 focus. Any time you have a data structure of variable size, things can get
1426 tricky, and strings are a re-sizable data structure. That said, Rust's strings
1427 also work differently than in some other systems languages, such as C.
1429 Let's dig into the details. A **string** is a sequence of unicode scalar values
1430 encoded as a stream of UTF-8 bytes. All strings are guaranteed to be
1431 validly-encoded UTF-8 sequences. Additionally, strings are not null-terminated
1432 and can contain null bytes.
1434 Rust has two main types of strings: `&str` and `String`.
1436 The first kind is a `&str`. This is pronounced a 'string slice.' String literals
1437 are of the type `&str`:
1440 let string = "Hello there.";
1443 This string is statically allocated, meaning that it's saved inside our
1444 compiled program, and exists for the entire duration it runs. The `string`
1445 binding is a reference to this statically allocated string. String slices
1446 have a fixed size, and cannot be mutated.
1448 A `String`, on the other hand, is an in-memory string. This string is
1449 growable, and is also guaranteed to be UTF-8.
1452 let mut s = "Hello".to_string();
1455 s.push_str(", world.");
1459 You can coerce a `String` into a `&str` with the `as_slice()` method:
1462 fn takes_slice(slice: &str) {
1463 println!("Got: {}", slice);
1467 let s = "Hello".to_string();
1468 takes_slice(s.as_slice());
1472 To compare a String to a constant string, prefer `as_slice()`...
1475 fn compare(string: String) {
1476 if string.as_slice() == "Hello" {
1482 ... over `to_string()`:
1485 fn compare(string: String) {
1486 if string == "Hello".to_string() {
1492 Converting a `String` to a `&str` is cheap, but converting the `&str` to a
1493 `String` involves allocating memory. No reason to do that unless you have to!
1495 That's the basics of strings in Rust! They're probably a bit more complicated
1496 than you are used to, if you come from a scripting language, but when the
1497 low-level details matter, they really matter. Just remember that `String`s
1498 allocate memory and control their data, while `&str`s are a reference to
1499 another string, and you'll be all set.
1503 Like many programming languages, Rust has a list type for when you want a list
1504 of things. But similar to strings, Rust has different types to represent this
1505 idea: `Vec<T>` (a 'vector'), `[T, .. N]` (an 'array'), and `&[T]` (a 'slice').
1508 Vectors are similar to `String`s: they have a dynamic length, and they
1509 allocate enough memory to fit. You can create a vector with the `vec!` macro:
1512 let nums = vec![1i, 2i, 3i];
1515 Notice that unlike the `println!` macro we've used in the past, we use square
1516 brackets (`[]`) with `vec!`. Rust allows you to use either in either situation,
1517 this is just convention.
1519 You can create an array with just square brackets:
1522 let nums = [1i, 2i, 3i];
1525 So what's the difference? An array has a fixed size, so you can't add or
1529 let mut nums = vec![1i, 2i, 3i];
1530 nums.push(4i); // works
1532 let mut nums = [1i, 2i, 3i];
1533 nums.push(4i); // error: type `[int, .. 3]` does not implement any method
1534 // in scope named `push`
1537 The `push()` method lets you append a value to the end of the vector. But
1538 since arrays have fixed sizes, adding an element doesn't make any sense.
1539 You can see how it has the exact type in the error message: `[int, .. 3]`.
1540 An array of `int`s, with length 3.
1542 Similar to `&str`, a slice is a reference to another array. We can get a
1543 slice from a vector by using the `as_slice()` method:
1546 let vec = vec![1i, 2i, 3i];
1547 let slice = vec.as_slice();
1550 All three types implement an `iter()` method, which returns an iterator. We'll
1551 talk more about the details of iterators later, but for now, the `iter()` method
1552 allows you to write a `for` loop that prints out the contents of a vector, array,
1556 let vec = vec![1i, 2i, 3i];
1558 for i in vec.iter() {
1563 This code will print each number in order, on its own line.
1565 You can access a particular element of a vector, array, or slice by using
1566 **subscript notation**:
1569 let names = ["Graydon", "Brian", "Niko"];
1571 println!("The second name is: {}", names[1]);
1574 These subscripts start at zero, like in most programming languages, so the
1575 first name is `names[0]` and the second name is `names[1]`. The above example
1576 prints `The second name is Brian`.
1578 There's a whole lot more to vectors, but that's enough to get started. We have
1579 now learned all of the most basic Rust concepts. We're ready to start building
1580 our guessing game, but we need to know how to do one last thing first: get
1581 input from the keyboard. You can't have a guessing game without the ability to
1586 Getting input from the keyboard is pretty easy, but uses some things
1587 we haven't seen before. Here's a simple program that reads some input,
1588 and then prints it back out:
1594 println!("Type something!");
1596 let input = std::io::stdin().read_line().ok().expect("Failed to read line");
1598 println!("{}", input);
1602 Let's go over these chunks, one by one:
1608 This calls a function, `stdin()`, that lives inside the `std::io` module. As
1609 you can imagine, everything in `std` is provided by Rust, the 'standard
1610 library.' We'll talk more about the module system later.
1612 Since writing the fully qualified name all the time is annoying, we can use
1613 the `use` statement to import it in:
1621 However, it's considered better practice to not import individual functions, but
1622 to import the module, and only use one level of qualification:
1630 Let's update our example to use this style:
1636 println!("Type something!");
1638 let input = io::stdin().read_line().ok().expect("Failed to read line");
1640 println!("{}", input);
1650 The `read_line()` method can be called on the result of `stdin()` to return
1651 a full line of input. Nice and easy.
1654 .ok().expect("Failed to read line");
1657 Do you remember this code?
1670 Value(n) => println!("x is {:d}", n),
1671 Missing => println!("x is missing!"),
1675 Value(n) => println!("y is {:d}", n),
1676 Missing => println!("y is missing!"),
1681 We had to match each time, to see if we had a value or not. In this case,
1682 though, we _know_ that `x` has a `Value`. But `match` forces us to handle
1683 the `missing` case. This is what we want 99% of the time, but sometimes, we
1684 know better than the compiler.
1686 Likewise, `read_line()` does not return a line of input. It _might_ return a
1687 line of input. It might also fail to do so. This could happen if our program
1688 isn't running in a terminal, but as part of a cron job, or some other context
1689 where there's no standard input. Because of this, `read_line` returns a type
1690 very similar to our `OptionalInt`: an `IoResult<T>`. We haven't talked about
1691 `IoResult<T>` yet because it is the **generic** form of our `OptionalInt`.
1692 Until then, you can think of it as being the same thing, just for any type, not
1695 Rust provides a method on these `IoResult<T>`s called `ok()`, which does the
1696 same thing as our `match` statement, but assuming that we have a valid value.
1697 If we don't, it will terminate our program. In this case, if we can't get
1698 input, our program doesn't work, so we're okay with that. In most cases, we
1699 would want to handle the error case explicitly. The result of `ok()` has a
1700 method, `expect()`, which allows us to give an error message if this crash
1703 We will cover the exact details of how all of this works later in the Guide.
1704 For now, this gives you enough of a basic understanding to work with.
1706 Back to the code we were working on! Here's a refresher:
1712 println!("Type something!");
1714 let input = io::stdin().read_line().ok().expect("Failed to read line");
1716 println!("{}", input);
1720 With long lines like this, Rust gives you some flexibility with the whitespace.
1721 We _could_ write the example like this:
1727 println!("Type something!");
1729 let input = io::stdin()
1732 .expect("Failed to read line");
1734 println!("{}", input);
1738 Sometimes, this makes things more readable. Sometimes, less. Use your judgment
1741 That's all you need to get basic input from the standard input! It's not too
1742 complicated, but there are a number of small parts.
1746 Okay! We've got the basics of Rust down. Let's write a bigger program.
1748 For our first project, we'll implement a classic beginner programming problem:
1749 the guessing game. Here's how it works: Our program will generate a random
1750 integer between one and a hundred. It will then prompt us to enter a guess.
1751 Upon entering our guess, it will tell us if we're too low or too high. Once we
1752 guess correctly, it will congratulate us, and print the number of guesses we've
1753 taken to the screen. Sound good?
1757 Let's set up a new project. Go to your projects directory. Remember how we
1758 had to create our directory structure and a `Cargo.toml` for `hello_world`? Cargo
1759 has a command that does that for us. Let's give it a shot:
1763 $ cargo new guessing_game --bin
1767 We pass the name of our project to `cargo new`, and then the `--bin` flag,
1768 since we're making a binary, rather than a library.
1770 Check out the generated `Cargo.toml`:
1775 name = "guessing_game"
1777 authors = ["Your Name <you@example.com>"]
1780 Cargo gets this information from your environment. If it's not correct, go ahead
1783 Finally, Cargo generated a hello, world for us. Check out `src/main.rs`:
1787 println!("Hello, world!");
1791 Let's try compiling what Cargo gave us:
1795 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1798 Excellent! Open up your `src/main.rs` again. We'll be writing all of
1799 our code in this file. We'll talk about multiple-file projects later on in the
1802 Before we move on, let me show you one more Cargo command: `run`. `cargo run`
1803 is kind of like `cargo build`, but it also then runs the produced executable.
1808 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1809 Running `target/guessing_game`
1813 Great! The `run` command comes in handy when you need to rapidly iterate on a project.
1814 Our game is just such a project, we need to quickly test each iteration before moving on to the next one.
1816 ## Processing a Guess
1818 Let's get to it! The first thing we need to do for our guessing game is
1819 allow our player to input a guess. Put this in your `src/main.rs`:
1825 println!("Guess the number!");
1827 println!("Please input your guess.");
1829 let input = io::stdin().read_line()
1831 .expect("Failed to read line");
1833 println!("You guessed: {}", input);
1837 You've seen this code before, when we talked about standard input. We
1838 import the `std::io` module with `use`, and then our `main` function contains
1839 our program's logic. We print a little message announcing the game, ask the
1840 user to input a guess, get their input, and then print it out.
1842 Because we talked about this in the section on standard I/O, I won't go into
1843 more details here. If you need a refresher, go re-read that section.
1845 ## Generating a secret number
1847 Next, we need to generate a secret number. To do that, we need to use Rust's
1848 random number generation, which we haven't talked about yet. Rust includes a
1849 bunch of interesting functions in its standard library. If you need a bit of
1850 code, it's possible that it's already been written for you! In this case,
1851 we do know that Rust has random number generation, but we don't know how to
1854 Enter the docs. Rust has a page specifically to document the standard library.
1855 You can find that page [here](std/index.html). There's a lot of information on
1856 that page, but the best part is the search bar. Right up at the top, there's
1857 a box that you can enter in a search term. The search is pretty primitive
1858 right now, but is getting better all the time. If you type 'random' in that
1859 box, the page will update to [this
1860 one](http://doc.rust-lang.org/std/index.html?search=random). The very first
1862 [std::rand::random](http://doc.rust-lang.org/std/rand/fn.random.html). If we
1863 click on that result, we'll be taken to its documentation page.
1865 This page shows us a few things: the type signature of the function, some
1866 explanatory text, and then an example. Let's modify our code to add in the
1874 println!("Guess the number!");
1876 let secret_number = (rand::random() % 100i) + 1i;
1878 println!("The secret number is: {}", secret_number);
1880 println!("Please input your guess.");
1882 let input = io::stdin().read_line()
1884 .expect("Failed to read line");
1887 println!("You guessed: {}", input);
1891 The first thing we changed was to `use std::rand`, as the docs
1892 explained. We then added in a `let` expression to create a variable binding
1893 named `secret_number`, and we printed out its result.
1895 Also, you may wonder why we are using `%` on the result of `rand::random()`.
1896 This operator is called 'modulo', and it returns the remainder of a division.
1897 By taking the modulo of the result of `rand::random()`, we're limiting the
1898 values to be between 0 and 99. Then, we add one to the result, making it from 1
1899 to 100. Using modulo can give you a very, very small bias in the result, but
1900 for this example, it is not important.
1902 Let's try to compile this using `cargo build`:
1906 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1907 src/main.rs:7:26: 7:34 error: the type of this value must be known in this context
1908 src/main.rs:7 let secret_number = (rand::random() % 100i) + 1i;
1910 error: aborting due to previous error
1913 It didn't work! Rust says "the type of this value must be known in this
1914 context." What's up with that? Well, as it turns out, `rand::random()` can
1915 generate many kinds of random values, not just integers. And in this case, Rust
1916 isn't sure what kind of value `random()` should generate. So we have to help
1917 it. With number literals, we just add an `i` onto the end to tell Rust they're
1918 integers, but that does not work with functions. There's a different syntax,
1919 and it looks like this:
1922 rand::random::<int>();
1925 This says "please give me a random `int` value." We can change our code to use
1933 println!("Guess the number!");
1935 let secret_number = (rand::random::<int>() % 100i) + 1i;
1937 println!("The secret number is: {}", secret_number);
1939 println!("Please input your guess.");
1941 let input = io::stdin().read_line()
1943 .expect("Failed to read line");
1946 println!("You guessed: {}", input);
1950 Try running our new program a few times:
1954 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1955 Running `target/guessing_game`
1957 The secret number is: 7
1958 Please input your guess.
1961 $ ./target/guessing_game
1963 The secret number is: 83
1964 Please input your guess.
1967 $ ./target/guessing_game
1969 The secret number is: -29
1970 Please input your guess.
1975 Wait. Negative 29? We wanted a number between one and a hundred! We have two
1976 options here: we can either ask `random()` to generate an unsigned integer, which
1977 can only be positive, or we can use the `abs()` function. Let's go with the
1978 unsigned integer approach. If we want a random positive number, we should ask for
1979 a random positive number. Our code looks like this now:
1986 println!("Guess the number!");
1988 let secret_number = (rand::random::<uint>() % 100u) + 1u;
1990 println!("The secret number is: {}", secret_number);
1992 println!("Please input your guess.");
1994 let input = io::stdin().read_line()
1996 .expect("Failed to read line");
1999 println!("You guessed: {}", input);
2007 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2008 Running `target/guessing_game`
2010 The secret number is: 57
2011 Please input your guess.
2016 Great! Next up: let's compare our guess to the secret guess.
2018 ## Comparing guesses
2020 If you remember, earlier in the tutorial, we made a `cmp` function that compared
2021 two numbers. Let's add that in, along with a `match` statement to compare the
2022 guess to the secret guess:
2029 println!("Guess the number!");
2031 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2033 println!("The secret number is: {}", secret_number);
2035 println!("Please input your guess.");
2037 let input = io::stdin().read_line()
2039 .expect("Failed to read line");
2042 println!("You guessed: {}", input);
2044 match cmp(input, secret_number) {
2045 Less => println!("Too small!"),
2046 Greater => println!("Too big!"),
2047 Equal => { println!("You win!"); },
2051 fn cmp(a: int, b: int) -> Ordering {
2053 else if a > b { Greater }
2058 If we try to compile, we'll get some errors:
2062 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2063 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)
2064 src/main.rs:20 match cmp(input, secret_number) {
2066 src/main.rs:20:22: 20:35 error: mismatched types: expected `int` but found `uint` (expected int but found uint)
2067 src/main.rs:20 match cmp(input, secret_number) {
2069 error: aborting due to 2 previous errors
2072 This often happens when writing Rust programs, and is one of Rust's greatest
2073 strengths. You try out some code, see if it compiles, and Rust tells you that
2074 you've done something wrong. In this case, our `cmp` function works on integers,
2075 but we've given it unsigned integers. In this case, the fix is easy, because
2076 we wrote the `cmp` function! Let's change it to take `uint`s:
2083 println!("Guess the number!");
2085 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2087 println!("The secret number is: {}", secret_number);
2089 println!("Please input your guess.");
2091 let input = io::stdin().read_line()
2093 .expect("Failed to read line");
2096 println!("You guessed: {}", input);
2098 match cmp(input, secret_number) {
2099 Less => println!("Too small!"),
2100 Greater => println!("Too big!"),
2101 Equal => { println!("You win!"); },
2105 fn cmp(a: uint, b: uint) -> Ordering {
2107 else if a > b { Greater }
2112 And try compiling again:
2116 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2117 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)
2118 src/main.rs:20 match cmp(input, secret_number) {
2120 error: aborting due to previous error
2123 This error is similar to the last one: we expected to get a `uint`, but we got
2124 a `String` instead! That's because our `input` variable is coming from the
2125 standard input, and you can guess anything. Try it:
2128 $ ./target/guessing_game
2130 The secret number is: 73
2131 Please input your guess.
2136 Oops! Also, you'll note that we just ran our program even though it didn't compile.
2137 This works because the older version we did successfully compile was still lying
2138 around. Gotta be careful!
2140 Anyway, we have a `String`, but we need a `uint`. What to do? Well, there's
2141 a function for that:
2144 let input = io::stdin().read_line()
2146 .expect("Failed to read line");
2147 let input_num: Option<uint> = from_str(input.as_slice());
2150 The `from_str` function takes in a `&str` value and converts it into something.
2151 We tell it what kind of something with a type hint. Remember our type hint with
2152 `random()`? It looked like this:
2155 rand::random::<uint>();
2158 There's an alternate way of providing a hint too, and that's declaring the type
2162 let x: uint = rand::random();
2165 In this case, we say `x` is a `uint` explicitly, so Rust is able to properly
2166 tell `random()` what to generate. In a similar fashion, both of these work:
2169 let input_num = from_str::<Option<uint>>("5");
2170 let input_num: Option<uint> = from_str("5");
2173 In this case, I happen to prefer the latter, and in the `random()` case, I prefer
2174 the former. I think the nested `<>`s make the first option especially ugly and
2175 a bit harder to read.
2177 Anyway, with us now converting our input to a number, our code looks like this:
2184 println!("Guess the number!");
2186 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2188 println!("The secret number is: {}", secret_number);
2190 println!("Please input your guess.");
2192 let input = io::stdin().read_line()
2194 .expect("Failed to read line");
2195 let input_num: Option<uint> = from_str(input.as_slice());
2199 println!("You guessed: {}", input_num);
2201 match cmp(input_num, secret_number) {
2202 Less => println!("Too small!"),
2203 Greater => println!("Too big!"),
2204 Equal => { println!("You win!"); },
2208 fn cmp(a: uint, b: uint) -> Ordering {
2210 else if a > b { Greater }
2219 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2220 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)
2221 src/main.rs:22 match cmp(input_num, secret_number) {
2223 error: aborting due to previous error
2226 Oh yeah! Our `input_num` has the type `Option<uint>`, rather than `uint`. We
2227 need to unwrap the Option. If you remember from before, `match` is a great way
2228 to do that. Try this code:
2235 println!("Guess the number!");
2237 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2239 println!("The secret number is: {}", secret_number);
2241 println!("Please input your guess.");
2243 let input = io::stdin().read_line()
2245 .expect("Failed to read line");
2246 let input_num: Option<uint> = from_str(input.as_slice());
2248 let num = match input_num {
2251 println!("Please input a number!");
2257 println!("You guessed: {}", num);
2259 match cmp(num, secret_number) {
2260 Less => println!("Too small!"),
2261 Greater => println!("Too big!"),
2262 Equal => { println!("You win!"); },
2266 fn cmp(a: uint, b: uint) -> Ordering {
2268 else if a > b { Greater }
2273 We use a `match` to either give us the `uint` inside of the `Option`, or we
2274 print an error message and return. Let's give this a shot:
2278 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2279 Running `target/guessing_game`
2281 The secret number is: 17
2282 Please input your guess.
2284 Please input a number!
2287 Uh, what? But we did!
2289 ... actually, we didn't. See, when you get a line of input from `stdin()`,
2290 you get all the input. Including the `\n` character from you pressing Enter.
2291 So, `from_str()` sees the string `"5\n"` and says "nope, that's not a number,
2292 there's non-number stuff in there!" Luckily for us, `&str`s have an easy
2293 method we can use defined on them: `trim()`. One small modification, and our
2294 code looks like this:
2301 println!("Guess the number!");
2303 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2305 println!("The secret number is: {}", secret_number);
2307 println!("Please input your guess.");
2309 let input = io::stdin().read_line()
2311 .expect("Failed to read line");
2312 let input_num: Option<uint> = from_str(input.as_slice().trim());
2314 let num = match input_num {
2317 println!("Please input a number!");
2323 println!("You guessed: {}", num);
2325 match cmp(num, secret_number) {
2326 Less => println!("Too small!"),
2327 Greater => println!("Too big!"),
2328 Equal => { println!("You win!"); },
2332 fn cmp(a: uint, b: uint) -> Ordering {
2334 else if a > b { Greater }
2343 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2344 Running `target/guessing_game`
2346 The secret number is: 58
2347 Please input your guess.
2353 Nice! You can see I even added spaces before my guess, and it still figured
2354 out that I guessed 76. Run the program a few times, and verify that guessing
2355 the number works, as well as guessing a number too small.
2357 The Rust compiler helped us out quite a bit there! This technique is called
2358 "lean on the compiler," and it's often useful when working on some code. Let
2359 the error messages help guide you towards the correct types.
2361 Now we've got most of the game working, but we can only make one guess. Let's
2362 change that by adding loops!
2366 As we already discussed, the `loop` keyword gives us an infinite loop. So
2374 println!("Guess the number!");
2376 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2378 println!("The secret number is: {}", secret_number);
2382 println!("Please input your guess.");
2384 let input = io::stdin().read_line()
2386 .expect("Failed to read line");
2387 let input_num: Option<uint> = from_str(input.as_slice().trim());
2389 let num = match input_num {
2392 println!("Please input a number!");
2398 println!("You guessed: {}", num);
2400 match cmp(num, secret_number) {
2401 Less => println!("Too small!"),
2402 Greater => println!("Too big!"),
2403 Equal => { println!("You win!"); },
2408 fn cmp(a: uint, b: uint) -> Ordering {
2410 else if a > b { Greater }
2415 And try it out. But wait, didn't we just add an infinite loop? Yup. Remember
2416 that `return`? If we give a non-number answer, we'll `return` and quit. Observe:
2420 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2421 Running `target/guessing_game`
2423 The secret number is: 59
2424 Please input your guess.
2428 Please input your guess.
2432 Please input your guess.
2436 Please input your guess.
2438 Please input a number!
2441 Ha! `quit` actually quits. As does any other non-number input. Well, this is
2442 suboptimal to say the least. First, let's actually quit when you win the game:
2449 println!("Guess the number!");
2451 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2453 println!("The secret number is: {}", secret_number);
2457 println!("Please input your guess.");
2459 let input = io::stdin().read_line()
2461 .expect("Failed to read line");
2462 let input_num: Option<uint> = from_str(input.as_slice().trim());
2464 let num = match input_num {
2467 println!("Please input a number!");
2473 println!("You guessed: {}", num);
2475 match cmp(num, secret_number) {
2476 Less => println!("Too small!"),
2477 Greater => println!("Too big!"),
2479 println!("You win!");
2486 fn cmp(a: uint, b: uint) -> Ordering {
2488 else if a > b { Greater }
2493 By adding the `return` line after the `You win!`, we'll exit the program when
2494 we win. We have just one more tweak to make: when someone inputs a non-number,
2495 we don't want to quit, we just want to ignore it. Change that `return` to
2504 println!("Guess the number!");
2506 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2508 println!("The secret number is: {}", secret_number);
2512 println!("Please input your guess.");
2514 let input = io::stdin().read_line()
2516 .expect("Failed to read line");
2517 let input_num: Option<uint> = from_str(input.as_slice().trim());
2519 let num = match input_num {
2522 println!("Please input a number!");
2528 println!("You guessed: {}", num);
2530 match cmp(num, secret_number) {
2531 Less => println!("Too small!"),
2532 Greater => println!("Too big!"),
2534 println!("You win!");
2541 fn cmp(a: uint, b: uint) -> Ordering {
2543 else if a > b { Greater }
2548 Now we should be good! Let's try:
2552 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2553 Running `target/guessing_game`
2555 The secret number is: 61
2556 Please input your guess.
2560 Please input your guess.
2564 Please input your guess.
2566 Please input a number!
2567 Please input your guess.
2573 Awesome! With one tiny last tweak, we have finished the guessing game. Can you
2574 think of what it is? That's right, we don't want to print out the secret number.
2575 It was good for testing, but it kind of ruins the game. Here's our final source:
2582 println!("Guess the number!");
2584 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2588 println!("Please input your guess.");
2590 let input = io::stdin().read_line()
2592 .expect("Failed to read line");
2593 let input_num: Option<uint> = from_str(input.as_slice().trim());
2595 let num = match input_num {
2598 println!("Please input a number!");
2604 println!("You guessed: {}", num);
2606 match cmp(num, secret_number) {
2607 Less => println!("Too small!"),
2608 Greater => println!("Too big!"),
2610 println!("You win!");
2617 fn cmp(a: uint, b: uint) -> Ordering {
2619 else if a > b { Greater }
2626 At this point, you have successfully built the Guessing Game! Congratulations!
2628 You've now learned the basic syntax of Rust. All of this is relatively close to
2629 various other programming languages you have used in the past. These
2630 fundamental syntactical and semantic elements will form the foundation for the
2631 rest of your Rust education.
2633 Now that you're an expert at the basics, it's time to learn about some of
2634 Rust's more unique features.
2636 # Crates and Modules
2638 Rust features a strong module system, but it works a bit differently than in
2639 other programming languages. Rust's module system has two main components:
2640 **crate**s, and **module**s.
2642 A crate is Rust's unit of independent compilation. Rust always compiles one
2643 crate at a time, producing either a library or an executable. However, executables
2644 usually depend on libraries, and many libraries depend on other libraries as well.
2645 To support this, crates can depend on other crates.
2647 Each crate contains a hierarchy of modules. This tree starts off with a single
2648 module, called the **crate root**. Within the crate root, we can declare other
2649 modules, which can contain other modules, as deeply as you'd like.
2651 Note that we haven't mentioned anything about files yet. Rust does not impose a
2652 particular relationship between your filesystem structure and your module
2653 structure. That said, there is a conventional approach to how Rust looks for
2654 modules on the file system, but it's also overridable.
2656 Enough talk, let's build something! Let's make a new project called `modules`.
2660 $ cargo new modules --bin
2663 Let's double check our work by compiling:
2667 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2668 Running `target/modules`
2672 Excellent! So, we already have a single crate here: our `src/main.rs` is a crate.
2673 Everything in that file is in the crate root. A crate that generates an executable
2674 defines a `main` function inside its root, as we've done here.
2676 Let's define a new module inside our crate. Edit `src/main.rs` to look
2681 println!("Hello, world!");
2686 println!("Hello, world!");
2691 We now have a module named `hello` inside of our crate root. Modules use
2692 `snake_case` naming, like functions and variable bindings.
2694 Inside the `hello` module, we've defined a `print_hello` function. This will
2695 also print out our hello world message. Modules allow you to split up your
2696 program into nice neat boxes of functionality, grouping common things together,
2697 and keeping different things apart. It's kinda like having a set of shelves:
2698 a place for everything and everything in its place.
2700 To call our `print_hello` function, we use the double colon (`::`):
2703 hello::print_hello();
2706 You've seen this before, with `io::stdin()` and `rand::random()`. Now you know
2707 how to make your own. However, crates and modules have rules about
2708 **visibility**, which controls who exactly may use the functions defined in a
2709 given module. By default, everything in a module is private, which means that
2710 it can only be used by other functions in the same module. This will not
2715 hello::print_hello();
2720 println!("Hello, world!");
2728 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2729 src/main.rs:2:5: 2:23 error: function `print_hello` is private
2730 src/main.rs:2 hello::print_hello();
2734 To make it public, we use the `pub` keyword:
2738 hello::print_hello();
2742 pub fn print_hello() {
2743 println!("Hello, world!");
2752 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2753 Running `target/modules`
2759 There's a common pattern when you're building an executable: you build both an
2760 executable and a library, and put most of your logic in the library. That way,
2761 other programs can use that library to build their own functionality.
2763 Let's do that with our project. If you remember, libraries and executables
2764 are both crates, so while our project has one crate now, let's make a second:
2765 one for the library, and one for the executable.
2767 To make the second crate, open up `src/lib.rs` and put this code in it:
2771 pub fn print_hello() {
2772 println!("Hello, world!");
2777 And change your `src/main.rs` to look like this:
2780 extern crate modules;
2783 modules::hello::print_hello();
2787 There's been a few changes. First, we moved our `hello` module into its own
2788 file, `src/lib.rs`. This is the file that Cargo expects a library crate to
2789 be named, by convention.
2791 Next, we added an `extern crate modules` to the top of our `src/main.rs`. This,
2792 as you can guess, lets Rust know that our crate relies on another, external
2793 crate. We also had to modify our call to `print_hello`: now that it's in
2794 another crate, we need to specify that crate first.
2796 This doesn't _quite_ work yet. Try it:
2800 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2801 /home/you/projects/modules/src/lib.rs:2:5: 4:6 warning: code is never used: `print_hello`, #[warn(dead_code)] on by default
2802 /home/you/projects/modules/src/lib.rs:2 pub fn print_hello() {
2803 /home/you/projects/modules/src/lib.rs:3 println!("Hello, world!");
2804 /home/you/projects/modules/src/lib.rs:4 }
2805 /home/you/projects/modules/src/main.rs:4:5: 4:32 error: function `print_hello` is private
2806 /home/you/projects/modules/src/main.rs:4 modules::hello::print_hello();
2807 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
2808 error: aborting due to previous error
2809 Could not compile `modules`.
2812 First, we get a warning that some code is never used. Odd. Next, we get an error:
2813 `print_hello` is private, so we can't call it. Notice that the first error came
2814 from `src/lib.rs`, and the second came from `src/main.rs`: cargo is smart enough
2815 to build it all with one command. Also, after seeing the second error, the warning
2816 makes sense: we never actually call `hello_world`, because we're not allowed to!
2818 Just like modules, crates also have private visibility by default. Any modules
2819 inside of a crate can only be used by other modules in the crate, unless they
2820 use `pub`. In `src/lib.rs`, change this line:
2832 And everything should work:
2836 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2837 Running `target/modules`
2841 Let's do one more thing: add a `goodbye` module as well. Imagine a `src/lib.rs`
2842 that looks like this:
2846 pub fn print_hello() {
2847 println!("Hello, world!");
2852 pub fn print_goodbye() {
2853 println!("Goodbye for now!");
2858 Now, these two modules are pretty small, but imagine we've written a real, large
2859 program: they could both be huge. So maybe we want to move them into their own
2860 files. We can do that pretty easily, and there are two different conventions
2861 for doing it. Let's give each a try. First, make `src/lib.rs` look like this:
2868 This tells Rust that this crate has two public modules: `hello` and `goodbye`.
2870 Next, make a `src/hello.rs` that contains this:
2873 pub fn print_hello() {
2874 println!("Hello, world!");
2878 When we include a module like this, we don't need to make the `mod` declaration
2879 in `hello.rs`, because it's already been declared in `lib.rs`. `hello.rs` just
2880 contains the body of the module which is defined (by the `pub mod hello`) in
2881 `lib.rs`. This helps prevent 'rightward drift': when you end up indenting so
2882 many times that your code is hard to read.
2884 Finally, make a new directory, `src/goodbye`, and make a new file in it,
2885 `src/goodbye/mod.rs`:
2888 pub fn print_goodbye() {
2889 println!("Bye for now!");
2893 Same deal, but we can make a folder with a `mod.rs` instead of `mod_name.rs` in
2894 the same directory. If you have a lot of modules, nested folders can make
2895 sense. For example, if the `goodbye` module had its _own_ modules inside of
2896 it, putting all of that in a folder helps keep our directory structure tidy.
2897 And in fact, if you place the modules in separate files, they're required to be
2898 in separate folders.
2900 This should all compile as usual:
2904 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2907 We've seen how the `::` operator can be used to call into modules, but when
2908 we have deep nesting like `modules::hello::say_hello`, it can get tedious.
2909 That's why we have the `use` keyword.
2911 `use` allows us to bring certain names into another scope. For example, here's
2915 extern crate modules;
2918 modules::hello::print_hello();
2922 We could instead write this:
2925 extern crate modules;
2927 use modules::hello::print_hello;
2934 By bringing `print_hello` into scope, we don't need to qualify it anymore. However,
2935 it's considered proper style to do write this code like like this:
2938 extern crate modules;
2943 hello::print_hello();
2947 By just bringing the module into scope, we can keep one level of namespacing.
2951 Traditionally, testing has not been a strong suit of most systems programming
2952 languages. Rust, however, has very basic testing built into the language
2953 itself. While automated testing cannot prove that your code is bug-free, it is
2954 useful for verifying that certain behaviors work as intended.
2956 Here's a very basic test:
2960 fn is_one_equal_to_one() {
2965 You may notice something new: that `#[test]`. Before we get into the mechanics
2966 of testing, let's talk about attributes.
2970 Rust's testing system uses **attribute**s to mark which functions are tests.
2971 Attributes can be placed on any Rust **item**. Remember how most things in
2972 Rust are an expression, but `let` is not? Item declarations are also not
2973 expressions. Here's a list of things that qualify as an item:
2984 You haven't learned about all of these things yet, but that's the list. As
2985 you can see, functions are at the top of it.
2987 Attributes can appear in three ways:
2989 1. A single identifier, the attribute name. `#[test]` is an example of this.
2990 2. An identifier followed by an equals sign (`=`) and a literal. `#[cfg=test]`
2991 is an example of this.
2992 3. An identifier followed by a parenthesized list of sub-attribute arguments.
2993 `#[cfg(unix, target_word_size = "32")]` is an example of this, where one of
2994 the sub-arguments is of the second kind.
2996 There are a number of different kinds of attributes, enough that we won't go
2997 over them all here. Before we talk about the testing-specific attributes, I
2998 want to call out one of the most important kinds of attributes: stability
3001 ## Stability attributes
3003 Rust provides six attributes to indicate the stability level of various
3004 parts of your library. The six levels are:
3006 * deprecated: This item should no longer be used. No guarantee of backwards
3008 * experimental: This item was only recently introduced or is otherwise in a
3009 state of flux. It may change significantly, or even be removed. No guarantee
3010 of backwards-compatibility.
3011 * unstable: This item is still under development, but requires more testing to
3012 be considered stable. No guarantee of backwards-compatibility.
3013 * stable: This item is considered stable, and will not change significantly.
3014 Guarantee of backwards-compatibility.
3015 * frozen: This item is very stable, and is unlikely to change. Guarantee of
3016 backwards-compatibility.
3017 * locked: This item will never change unless a serious bug is found. Guarantee
3018 of backwards-compatibility.
3020 All of Rust's standard library uses these attribute markers to communicate
3021 their relative stability, and you should use them in your code, as well.
3022 There's an associated attribute, `warn`, that allows you to warn when you
3023 import an item marked with certain levels: deprecated, experimental and
3024 unstable. For now, only deprecated warns by default, but this will change once
3025 the standard library has been stabilized.
3027 You can use the `warn` attribute like this:
3033 And later, when you import a crate:
3036 extern crate some_crate;
3039 You'll get a warning if you use something marked unstable.
3041 You may have noticed an exclamation point in the `warn` attribute declaration.
3042 The `!` in this attribute means that this attribute applies to the enclosing
3043 item, rather than to the item that follows the attribute. So this `warn`
3044 attribute declaration applies to the enclosing crate itself, rather than
3045 to whatever item statement follows it:
3048 // applies to the crate we're in
3051 extern crate some_crate;
3053 // applies to the following `fn`.
3062 Let's write a very simple crate in a test-driven manner. You know the drill by
3063 now: make a new project:
3067 $ cargo new testing --bin
3075 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3076 Running `target/testing`
3080 Great. Rust's infrastructure supports tests in two sorts of places, and they're
3081 for two kinds of tests: you include **unit test**s inside of the crate itself,
3082 and you place **integration test**s inside a `tests` directory. "Unit tests"
3083 are small tests that test one focused unit, "integration tests" tests multiple
3084 units in integration. That said, this is a social convention, they're no different
3085 in syntax. Let's make a `tests` directory:
3091 Next, let's create an integration test in `tests/lib.rs`:
3100 It doesn't matter what you name your test functions, though it's nice if
3101 you give them descriptive names. You'll see why in a moment. We then use a
3102 macro, `assert!`, to assert that something is true. In this case, we're giving
3103 it `false`, so this test should fail. Let's try it!
3107 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3108 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3109 /home/you/projects/testing/src/main.rs:1 fn main() {
3110 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3111 /home/you/projects/testing/src/main.rs:3 }
3115 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3123 ---- foo stdout ----
3124 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
3131 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
3133 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
3136 Lots of output! Let's break this down:
3140 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3143 You can run all of your tests with `cargo test`. This runs both your tests in
3144 `tests`, as well as the tests you put inside of your crate.
3147 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3148 /home/you/projects/testing/src/main.rs:1 fn main() {
3149 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3150 /home/you/projects/testing/src/main.rs:3 }
3153 Rust has a **lint** called 'warn on dead code' used by default. A lint is a
3154 bit of code that checks your code, and can tell you things about it. In this
3155 case, Rust is warning us that we've written some code that's never used: our
3156 `main` function. Of course, since we're running tests, we don't use `main`.
3157 We'll turn this lint off for just this function soon. For now, just ignore this
3163 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3166 Wait a minute, zero tests? Didn't we define one? Yup. This output is from
3167 attempting to run the tests in our crate, of which we don't have any.
3168 You'll note that Rust reports on several kinds of tests: passed, failed,
3169 ignored, and measured. The 'measured' tests refer to benchmark tests, which
3170 we'll cover soon enough!
3177 Now we're getting somewhere. Remember when we talked about naming our tests
3178 with good names? This is why. Here, it says 'test foo' because we called our
3179 test 'foo.' If we had given it a good name, it'd be more clear which test
3180 failed, especially as we accumulate more tests.
3185 ---- foo stdout ----
3186 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
3193 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
3195 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
3198 After all the tests run, Rust will show us any output from our failed tests.
3199 In this instance, Rust tells us that our assertion failed, with false. This was
3202 Whew! Let's fix our test:
3211 And then try to run our tests again:
3215 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3216 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3217 /home/you/projects/testing/src/main.rs:1 fn main() {
3218 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3219 /home/you/projects/testing/src/main.rs:3 }
3223 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3229 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3232 Nice! Our test passes, as we expected. Let's get rid of that warning for our `main`
3233 function. Change your `src/main.rs` to look like this:
3238 println!("Hello, world");
3242 This attribute combines two things: `cfg` and `not`. The `cfg` attribute allows
3243 you to conditionally compile code based on something. The following item will
3244 only be compiled if the configuration says it's true. And when Cargo compiles
3245 our tests, it sets things up so that `cfg(test)` is true. But we want to only
3246 include `main` when it's _not_ true. So we use `not` to negate things:
3247 `cfg(not(test))` will only compile our code when the `cfg(test)` is false.
3249 With this attribute, we won't get the warning:
3253 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3257 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3263 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3266 Nice. Okay, let's write a real test now. Change your `tests/lib.rs`
3271 fn math_checks_out() {
3272 let result = add_three_times_four(5i);
3274 assert_eq!(32i, result);
3278 And try to run the test:
3282 Compiling testing v0.0.1 (file:///home/youg/projects/testing)
3283 /home/youg/projects/testing/tests/lib.rs:3:18: 3:38 error: unresolved name `add_three_times_four`.
3284 /home/youg/projects/testing/tests/lib.rs:3 let result = add_three_times_four(5i);
3285 ^~~~~~~~~~~~~~~~~~~~
3286 error: aborting due to previous error
3287 Build failed, waiting for other jobs to finish...
3288 Could not compile `testing`.
3290 To learn more, run the command again with --verbose.
3293 Rust can't find this function. That makes sense, as we didn't write it yet!
3295 In order to share this code with our tests, we'll need to make a library crate.
3296 This is also just good software design: as we mentioned before, it's a good idea
3297 to put most of your functionality into a library crate, and have your executable
3298 crate use that library. This allows for code re-use.
3300 To do that, we'll need to make a new module. Make a new file, `src/lib.rs`,
3305 pub fn add_three_times_four(x: int) -> int {
3310 We're calling this file `lib.rs` because it has the same name as our project,
3311 and so it's named this, by convention.
3313 We'll then need to use this crate in our `src/main.rs`:
3316 extern crate testing;
3320 println!("Hello, world");
3324 Finally, let's import this function in our `tests/lib.rs`:
3327 extern crate testing;
3328 use testing::add_three_times_four;
3331 fn math_checks_out() {
3332 let result = add_three_times_four(5i);
3334 assert_eq!(32i, result);
3338 Let's give it a run:
3342 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3346 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3351 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3355 test math_checks_out ... ok
3357 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3360 Great! One test passed. We've got an integration test showing that our public
3361 method works, but maybe we want to test some of the internal logic as well.
3362 While this function is simple, if it were more complicated, you can imagine
3363 we'd need more tests. So let's break it up into two helper functions, and
3364 write some unit tests to test those.
3366 Change your `src/lib.rs` to look like this:
3369 pub fn add_three_times_four(x: int) -> int {
3370 times_four(add_three(x))
3373 fn add_three(x: int) -> int { x + 3 }
3375 fn times_four(x: int) -> int { x * 4 }
3378 If you run `cargo test`, you should get the same output:
3382 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3386 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3391 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3395 test math_checks_out ... ok
3397 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3400 If we tried to write a test for these two new functions, it wouldn't
3404 extern crate testing;
3405 use testing::add_three_times_four;
3406 use testing::add_three;
3409 fn math_checks_out() {
3410 let result = add_three_times_four(5i);
3412 assert_eq!(32i, result);
3416 fn test_add_three() {
3417 let result = add_three(5i);
3419 assert_eq!(8i, result);
3423 We'd get this error:
3426 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3427 /home/you/projects/testing/tests/lib.rs:3:5: 3:24 error: function `add_three` is private
3428 /home/you/projects/testing/tests/lib.rs:3 use testing::add_three;
3432 Right. It's private. So external, integration tests won't work. We need a
3433 unit test. Open up your `src/lib.rs` and add this:
3436 pub fn add_three_times_four(x: int) -> int {
3437 times_four(add_three(x))
3440 fn add_three(x: int) -> int { x + 3 }
3442 fn times_four(x: int) -> int { x * 4 }
3446 use super::add_three;
3447 use super::times_four;
3450 fn test_add_three() {
3451 let result = add_three(5i);
3453 assert_eq!(8i, result);
3457 fn test_times_four() {
3458 let result = times_four(5i);
3460 assert_eq!(20i, result);
3465 Let's give it a shot:
3469 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3472 test test::test_times_four ... ok
3473 test test::test_add_three ... ok
3475 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3480 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3484 test math_checks_out ... ok
3486 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3489 Cool! We now have two tests of our internal functions. You'll note that there
3490 are three sets of output now: one for `src/main.rs`, one for `src/lib.rs`, and
3491 one for `tests/lib.rs`. There's one interesting thing that we haven't talked
3492 about yet, and that's these lines:
3495 use super::add_three;
3496 use super::times_four;
3499 Because we've made a nested module, we can import functions from the parent
3500 module by using `super`. Sub-modules are allowed to 'see' private functions in
3501 the parent. We sometimes call this usage of `use` a 're-export,' because we're
3502 exporting the name again, somewhere else.
3504 We've now covered the basics of testing. Rust's tools are primitive, but they
3505 work well in the simple cases. There are some Rustaceans working on building
3506 more complicated frameworks on top of all of this, but they're just starting
3511 In systems programming, pointers are an incredibly important topic. Rust has a
3512 very rich set of pointers, and they operate differently than in many other
3513 languages. They are important enough that we have a specific [Pointer
3514 Guide](guide-pointers.html) that goes into pointers in much detail. In fact,
3515 while you're currently reading this guide, which covers the language in broad
3516 overview, there are a number of other guides that put a specific topic under a
3517 microscope. You can find the list of guides on the [documentation index
3518 page](index.html#guides).
3520 In this section, we'll assume that you're familiar with pointers as a general
3521 concept. If you aren't, please read the [introduction to
3522 pointers](guide-pointers.html#an-introduction) section of the Pointer Guide,
3523 and then come back here. We'll wait.
3525 Got the gist? Great. Let's talk about pointers in Rust.
3529 The most primitive form of pointer in Rust is called a **reference**.
3530 References are created using the ampersand (`&`). Here's a simple
3538 `y` is a reference to `x`. To dereference (get the value being referred to
3539 rather than the reference itself) `y`, we use the asterisk (`*`):
3548 Like any `let` binding, references are immutable by default.
3550 You can declare that functions take a reference:
3553 fn add_one(x: &int) -> int { *x + 1 }
3556 assert_eq!(6, add_one(&5));
3560 As you can see, we can make a reference from a literal by applying `&` as well.
3561 Of course, in this simple function, there's not a lot of reason to take `x` by
3562 reference. It's just an example of the syntax.
3564 Because references are immutable, you can have multiple references that
3565 **alias** (point to the same place):
3573 We can make a mutable reference by using `&mut` instead of `&`:
3580 Note that `x` must also be mutable. If it isn't, like this:
3590 6:19 error: cannot borrow immutable local variable `x` as mutable
3595 We don't want a mutable reference to immutable data! This error message uses a
3596 term we haven't talked about yet, 'borrow.' We'll get to that in just a moment.
3598 This simple example actually illustrates a lot of Rust's power: Rust has
3599 prevented us, at compile time, from breaking our own rules. Because Rust's
3600 references check these kinds of rules entirely at compile time, there's no
3601 runtime overhead for this safety. At runtime, these are the same as a raw
3602 machine pointer, like in C or C++. We've just double-checked ahead of time
3603 that we haven't done anything dangerous.
3605 Rust will also prevent us from creating two mutable references that alias.
3614 It gives us this error:
3617 error: cannot borrow `x` as mutable more than once at a time
3620 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3623 note: previous borrow ends here
3632 This is a big error message. Let's dig into it for a moment. There are three
3633 parts: the error and two notes. The error says what we expected, we cannot have
3634 two pointers that point to the same memory.
3636 The two notes give some extra context. Rust's error messages often contain this
3637 kind of extra information when the error is complex. Rust is telling us two
3638 things: first, that the reason we cannot **borrow** `x` as `z` is that we
3639 previously borrowed `x` as `y`. The second note shows where `y`'s borrowing
3644 In order to truly understand this error, we have to learn a few new concepts:
3645 **ownership**, **borrowing**, and **lifetimes**.
3647 ## Ownership, borrowing, and lifetimes
3649 Whenever a resource of some kind is created, something must be responsible
3650 for destroying that resource as well. Given that we're discussing pointers
3651 right now, let's discuss this in the context of memory allocation, though
3652 it applies to other resources as well.
3654 When you allocate heap memory, you need a mechanism to free that memory. Many
3655 languages let the programmer control the allocation, and then use a garbage
3656 collector to handle the deallocation. This is a valid, time-tested strategy,
3657 but it's not without its drawbacks. Because the programmer does not have to
3658 think as much about deallocation, allocation becomes something commonplace,
3659 because it's easy. And if you need precise control over when something is
3660 deallocated, leaving it up to your runtime can make this difficult.
3662 Rust chooses a different path, and that path is called **ownership**. Any
3663 binding that creates a resource is the **owner** of that resource.
3665 Being an owner affords you some privileges:
3667 1. You control when that resource is deallocated.
3668 2. You may lend that resource, immutably, to as many borrowers as you'd like.
3669 3. You may lend that resource, mutably, to a single borrower.
3671 But it also comes with some restrictions:
3673 1. If someone is borrowing your resource (either mutably or immutably), you may
3674 not mutate the resource or mutably lend it to someone.
3675 2. If someone is mutably borrowing your resource, you may not lend it out at
3676 all (mutably or immutably) or access it in any way.
3678 What's up with all this 'lending' and 'borrowing'? When you allocate memory,
3679 you get a pointer to that memory. This pointer allows you to manipulate said
3680 memory. If you are the owner of a pointer, then you may allow another
3681 binding to temporarily borrow that pointer, and then they can manipulate the
3682 memory. The length of time that the borrower is borrowing the pointer
3683 from you is called a **lifetime**.
3685 If two distinct bindings share a pointer, and the memory that pointer points to
3686 is immutable, then there are no problems. But if it's mutable, both pointers
3687 can attempt to write to the memory at the same time, causing a **race
3688 condition**. Therefore, if someone wants to mutate something that they've
3689 borrowed from you, you must not have lent out that pointer to anyone else.
3691 Rust has a sophisticated system called the **borrow checker** to make sure that
3692 everyone plays by these rules. At compile time, it verifies that none of these
3693 rules are broken. If there's no problem, our program compiles successfully, and
3694 there is no runtime overhead for any of this. The borrow checker works only at
3695 compile time. If the borrow checker did find a problem, it will report a
3696 **lifetime error**, and your program will refuse to compile.
3698 That's a lot to take in. It's also one of the _most_ important concepts in
3699 all of Rust. Let's see this syntax in action:
3703 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3705 // other code here...
3707 } // privilege 1: when x goes out of scope, this memory is deallocated
3709 /// this function borrows an integer. It's given back automatically when the
3710 /// function returns.
3711 fn foo(x: &int) -> &int { x }
3714 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3716 // privilege 2: you may lend that resource, to as many borrowers as you'd like
3720 foo(&x); // functions can borrow too!
3722 let a = &x; // we can do this alllllll day!
3726 let mut x = 5i; // x is the owner of this integer, which is memory on the stack.
3728 let y = &mut x; // privilege 3: you may lend that resource to a single borrower,
3733 If you are a borrower, you get a few privileges as well, but must also obey a
3736 1. If the borrow is immutable, you may read the data the pointer points to.
3737 2. If the borrow is mutable, you may read and write the data the pointer points to.
3738 3. You may lend the pointer to someone else in an immutable fashion, **BUT**
3739 4. When you do so, they must return it to you before you must give your own
3742 This last requirement can seem odd, but it also makes sense. If you have to
3743 return something, and you've lent it to someone, they need to give it back to
3744 you for you to give it back! If we didn't, then the owner could deallocate
3745 the memory, and the person we've loaned it out to would have a pointer to
3746 invalid memory. This is called a 'dangling pointer.'
3748 Let's re-examine the error that led us to talk about all of this, which was a
3749 violation of the restrictions placed on owners who lend something out mutably.
3761 error: cannot borrow `x` as mutable more than once at a time
3764 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3767 note: previous borrow ends here
3776 This error comes in three parts. Let's go over each in turn.
3779 error: cannot borrow `x` as mutable more than once at a time
3784 This error states the restriction: you cannot lend out something mutable more
3785 than once at the same time. The borrow checker knows the rules!
3788 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3793 Some compiler errors come with notes to help you fix the error. This error comes
3794 with two notes, and this is the first. This note informs us of exactly where
3795 the first mutable borrow occurred. The error showed us the second. So now we
3796 see both parts of the problem. It also alludes to rule #3, by reminding us that
3797 we can't change `x` until the borrow is over.
3800 note: previous borrow ends here
3809 Here's the second note, which lets us know where the first borrow would be over.
3810 This is useful, because if we wait to try to borrow `x` after this borrow is
3811 over, then everything will work.
3813 These rules are very simple, but that doesn't mean that they're easy. For more
3814 advanced patterns, please consult the [Lifetime Guide](guide-lifetimes.html).
3815 You'll also learn what this type signature with the `'a` syntax is:
3818 pub fn as_maybe_owned(&self) -> MaybeOwned<'a> { ... }
3823 All of our references so far have been to variables we've created on the stack.
3824 In Rust, the simplest way to allocate heap variables is using a *box*. To
3825 create a box, use the `box` keyword:
3831 This allocates an integer `5` on the heap, and creates a binding `x` that
3832 refers to it.. The great thing about boxed pointers is that we don't have to
3833 manually free this allocation! If we write
3842 then Rust will automatically free `x` at the end of the block. This isn't
3843 because Rust has a garbage collector -- it doesn't. Instead, when `x` goes out
3844 of scope, Rust `free`s `x`. This Rust code will do the same thing as the
3849 int *x = (int *)malloc(sizeof(int));
3855 This means we get the benefits of manual memory management, but the compiler
3856 ensures that we don't do something wrong. We can't forget to `free` our memory.
3858 Boxes are the sole owner of their contents, so you cannot take a mutable
3859 reference to them and then use the original box:
3865 *x; // you might expect 5, but this is actually an error
3868 This gives us this error:
3871 8:7 error: cannot use `*x` because it was mutably borrowed
3874 6:19 note: borrow of `x` occurs here
3879 As long as `y` is borrowing the contents, we cannot use `x`. After `y` is
3880 done borrowing the value, we can use it again. This works fine:
3887 } // y goes out of scope at the end of the block
3894 Sometimes, you need to allocate something on the heap, but give out multiple
3895 references to the memory. Rust's `Rc<T>` (pronounced 'arr cee tee') and
3896 `Arc<T>` types (again, the `T` is for generics, we'll learn more later) provide
3897 you with this ability. **Rc** stands for 'reference counted,' and **Arc** for
3898 'atomically reference counted.' This is how Rust keeps track of the multiple
3899 owners: every time we make a new reference to the `Rc<T>`, we add one to its
3900 internal 'reference count.' Every time a reference goes out of scope, we
3901 subtract one from the count. When the count is zero, the `Rc<T>` can be safely
3902 deallocated. `Arc<T>` is almost identical to `Rc<T>`, except for one thing: The
3903 'atomically' in 'Arc' means that increasing and decreasing the count uses a
3904 thread-safe mechanism to do so. Why two types? `Rc<T>` is faster, so if you're
3905 not in a multi-threaded scenario, you can have that advantage. Since we haven't
3906 talked about threading yet in Rust, we'll show you `Rc<T>` for the rest of this
3909 To create an `Rc<T>`, use `Rc::new()`:
3914 let x = Rc::new(5i);
3917 To create a second reference, use the `.clone()` method:
3922 let x = Rc::new(5i);
3926 The `Rc<T>` will live as long as any of its references are alive. After they
3927 all go out of scope, the memory will be `free`d.
3929 If you use `Rc<T>` or `Arc<T>`, you have to be careful about introducing
3930 cycles. If you have two `Rc<T>`s that point to each other, the reference counts
3931 will never drop to zero, and you'll have a memory leak. To learn more, check
3932 out [the section on `Rc<T>` and `Arc<T>` in the pointers
3933 guide](http://doc.rust-lang.org/guide-pointers.html#rc-and-arc).
3937 We've made use of patterns a few times in the guide: first with `let` bindings,
3938 then with `match` statements. Let's go on a whirlwind tour of all of the things
3941 A quick refresher: you can match against literals directly, and `_` acts as an
3948 1 => println!("one"),
3949 2 => println!("two"),
3950 3 => println!("three"),
3951 _ => println!("anything"),
3955 You can match multiple patterns with `|`:
3961 1 | 2 => println!("one or two"),
3962 3 => println!("three"),
3963 _ => println!("anything"),
3967 You can match a range of values with `..`:
3973 1 .. 5 => println!("one through five"),
3974 _ => println!("anything"),
3978 Ranges are mostly used with integers and single characters.
3980 If you're matching multiple things, via a `|` or a `..`, you can bind
3981 the value to a name with `@`:
3987 x @ 1 .. 5 => println!("got {}", x),
3988 _ => println!("anything"),
3992 If you're matching on an enum which has variants, you can use `..` to
3993 ignore the value in the variant:
4004 Value(..) => println!("Got an int!"),
4005 Missing => println!("No such luck."),
4009 You can introduce **match guards** with `if`:
4020 Value(x) if x > 5 => println!("Got an int bigger than five!"),
4021 Value(..) => println!("Got an int!"),
4022 Missing => println!("No such luck."),
4026 If you're matching on a pointer, you can use the same syntax as you declared it
4033 &x => println!("Got a value: {}", x),
4037 Here, the `x` inside the `match` has type `int`. In other words, the left hand
4038 side of the pattern destructures the value. If we have `&5i`, then in `&x`, `x`
4041 If you want to get a reference, use the `ref` keyword:
4047 ref x => println!("Got a reference to {}", x),
4051 Here, the `x` inside the `match` has the type `&int`. In other words, the `ref`
4052 keyword _creates_ a reference, for use in the pattern. If you need a mutable
4053 reference, `ref mut` will work in the same way:
4059 ref mut x => println!("Got a mutable reference to {}", x),
4063 If you have a struct, you can destructure it inside of a pattern:
4071 let origin = Point { x: 0i, y: 0i };
4074 Point { x: x, y: y } => println!("({},{})", x, y),
4078 If we only care about some of the values, we don't have to give them all names:
4086 let origin = Point { x: 0i, y: 0i };
4089 Point { x: x, .. } => println!("x is {}", x),
4093 Whew! That's a lot of different ways to match things, and they can all be
4094 mixed and matched, depending on what you're doing:
4098 Foo { x: Some(ref name), y: None } => ...
4102 Patterns are very powerful. Make good use of them.
4106 Functions are great, but if you want to call a bunch of them on some data, it
4107 can be awkward. Consider this code:
4113 We would read this left-to right, and so we see 'baz bar foo.' But this isn't the
4114 order that the functions would get called in, that's inside-out: 'foo bar baz.'
4115 Wouldn't it be nice if we could do this instead?
4118 x.foo().bar().baz();
4121 Luckily, as you may have guessed with the leading question, you can! Rust provides
4122 the ability to use this **method call syntax** via the `impl` keyword.
4124 Here's how it works:
4134 fn area(&self) -> f64 {
4135 std::f64::consts::PI * (self.radius * self.radius)
4140 let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
4141 println!("{}", c.area());
4145 This will print `12.566371`.
4147 We've made a struct that represents a circle. We then write an `impl` block,
4148 and inside it, define a method, `area`. Methods take a special first
4149 parameter, `&self`. There are three variants: `self`, `&self`, and `&mut self`.
4150 You can think of this first parameter as being the `x` in `x.foo()`. The three
4151 variants correspond to the three kinds of thing `x` could be: `self` if it's
4152 just a value on the stack, `&self` if it's a reference, and `&mut self` if it's
4153 a mutable reference. We should default to using `&self`, as it's the most
4156 Finally, as you may remember, the value of the area of a circle is `π*r²`.
4157 Because we took the `&self` parameter to `area`, we can use it just like any
4158 other parameter. Because we know it's a `Circle`, we can access the `radius`
4159 just like we would with any other struct. An import of π and some
4160 multiplications later, and we have our area.
4162 You can also define methods that do not take a `self` parameter. Here's a
4163 pattern that's very common in Rust code:
4173 fn new(x: f64, y: f64, radius: f64) -> Circle {
4183 let c = Circle::new(0.0, 0.0, 2.0);
4187 This **static method** builds a new `Circle` for us. Note that static methods
4188 are called with the `Struct::method()` syntax, rather than the `ref.method()`
4193 So far, we've made lots of functions in Rust. But we've given them all names.
4194 Rust also allows us to create anonymous functions too. Rust's anonymous
4195 functions are called **closure**s. By themselves, closures aren't all that
4196 interesting, but when you combine them with functions that take closures as
4197 arguments, really powerful things are possible.
4199 Let's make a closure:
4202 let add_one = |x| { 1i + x };
4204 println!("The 5 plus 1 is {}.", add_one(5i));
4207 We create a closure using the `|...| { ... }` syntax, and then we create a
4208 binding so we can use it later. Note that we call the function using the
4209 binding name and two parentheses, just like we would for a named function.
4211 Let's compare syntax. The two are pretty close:
4214 let add_one = |x: int| -> int { 1i + x };
4215 fn add_one (x: int) -> int { 1i + x }
4218 As you may have noticed, closures infer their argument and return types, so you
4219 don't need to declare one. This is different from named functions, which
4220 default to returning unit (`()`).
4222 There's one big difference between a closure and named functions, and it's in
4223 the name: a closure "closes over its environment." What's that mean? It means
4230 let printer = || { println!("x is: {}", x); };
4232 printer(); // prints "x is: 5"
4236 The `||` syntax means this is an anonymous closure that takes no arguments.
4237 Without it, we'd just have a block of code in `{}`s.
4239 In other words, a closure has access to variables in the scope that it's
4240 defined. The closure borrows any variables that it uses. This will error:
4246 let printer = || { println!("x is: {}", x); };
4248 x = 6i; // error: cannot assign to `x` because it is borrowed
4254 Rust has a second type of closure, called a **proc**. Procs are created
4255 with the `proc` keyword:
4260 let p = proc() { x * x };
4261 println!("{}", p()); // prints 25
4264 Procs have a big difference from closures: they may only be called once. This
4265 will error when we try to compile:
4270 let p = proc() { x * x };
4271 println!("{}", p());
4272 println!("{}", p()); // error: use of moved value `p`
4275 This restriction is important. Procs are allowed to consume values that they
4276 capture, and thus have to be restricted to being called once for soundness
4277 reasons: any value consumed would be invalid on a second call.
4279 Procs are most useful with Rust's concurrency features, and so we'll just leave
4280 it at this for now. We'll talk about them more in the "Tasks" section of the
4283 ## Accepting closures as arguments
4285 Closures are most useful as an argument to another function. Here's an example:
4288 fn twice(x: int, f: |int| -> int) -> int {
4293 let square = |x: int| { x * x };
4295 twice(5i, square); // evaluates to 50
4299 Let's break example down, starting with `main`:
4302 let square = |x: int| { x * x };
4305 We've seen this before. We make a closure that takes an integer, and returns
4309 twice(5i, square); // evaluates to 50
4312 This line is more interesting. Here, we call our function, `twice`, and we pass
4313 it two arguments: an integer, `5`, and our closure, `square`. This is just like
4314 passing any other two variable bindings to a function, but if you've never
4315 worked with closures before, it can seem a little complex. Just think: "I'm
4316 passing two variables, one is an int, and one is a function."
4318 Next, let's look at how `twice` is defined:
4321 fn twice(x: int, f: |int| -> int) -> int {
4324 `twice` takes two arguments, `x` and `f`. That's why we called it with two
4325 arguments. `x` is an `int`, we've done that a ton of times. `f` is a function,
4326 though, and that function takes an `int` and returns an `int`. Notice
4327 how the `|int| -> int` syntax looks a lot like our definition of `square`
4328 above, if we added the return type in:
4331 let square = |x: int| -> int { x * x };
4335 This function takes an `int` and returns an `int`.
4337 This is the most complicated function signature we've seen yet! Give it a read
4338 a few times until you can see how it works. It takes a teeny bit of practice, and
4341 Finally, `twice` returns an `int` as well.
4343 Okay, let's look at the body of `twice`:
4346 fn twice(x: int, f: |int| -> int) -> int {
4351 Since our closure is named `f`, we can call it just like we called our closures
4352 before. And we pass in our `x` argument to each one. Hence 'twice.'
4354 If you do the math, `(5 * 5) + (5 * 5) == 50`, so that's the output we get.
4356 Play around with this concept until you're comfortable with it. Rust's standard
4357 library uses lots of closures, where appropriate, so you'll be using
4358 this technique a lot.
4360 If we didn't want to give `square` a name, we could also just define it inline.
4361 This example is the same as the previous one:
4364 fn twice(x: int, f: |int| -> int) -> int {
4369 twice(5i, |x: int| { x * x }); // evaluates to 50
4373 A named function's name can be used wherever you'd use a closure. Another
4374 way of writing the previous example:
4377 fn twice(x: int, f: |int| -> int) -> int {
4381 fn square(x: int) -> int { x * x }
4384 twice(5i, square); // evaluates to 50
4388 Doing this is not particularly common, but every once in a while, it's useful.
4390 That's all you need to get the hang of closures! Closures are a little bit
4391 strange at first, but once you're used to using them, you'll miss them in any
4392 language that doesn't have them. Passing functions to other functions is
4393 incredibly powerful. Next, let's look at one of those things: iterators.
4397 Let's talk about loops.
4399 Remember Rust's `for` loop? Here's an example:
4402 for x in range(0i, 10i) {
4403 println!("{:d}", x);
4407 Now that you know more Rust, we can talk in detail about how this works. The
4408 `range` function returns an **iterator**. An iterator is something that we can
4409 call the `.next()` method on repeatedly, and it gives us a sequence of things.
4414 let mut range = range(0i, 10i);
4417 match range.next() {
4426 We make a mutable binding to the return value of `range`, which is our iterator.
4427 We then `loop`, with an inner `match`. This `match` is used on the result of
4428 `range.next()`, which gives us a reference to the next value of the iterator.
4429 `next` returns an `Option<int>`, in this case, which will be `Some(int)` when
4430 we have a value and `None` once we run out. If we get `Some(int)`, we print it
4431 out, and if we get `None`, we `break` out of the loop.
4433 This code sample is basically the same as our `for` loop version. The `for`
4434 loop is just a handy way to write this `loop`/`match`/`break` construct.
4436 `for` loops aren't the only thing that uses iterators, however. Writing your
4437 own iterator involves implementing the `Iterator` trait. While doing that is
4438 outside of the scope of this guide, Rust provides a number of useful iterators
4439 to accomplish various tasks. Before we talk about those, we should talk about a
4440 Rust anti-pattern. And that's `range`.
4442 Yes, we just talked about how `range` is cool. But `range` is also very
4443 primitive. For example, if you needed to iterate over the contents of
4444 a vector, you may be tempted to write this:
4447 let nums = vec![1i, 2i, 3i];
4449 for i in range(0u, nums.len()) {
4450 println!("{}", nums[i]);
4454 This is strictly worse than using an actual iterator. The `.iter()` method on
4455 vectors returns an iterator which iterates through a reference to each element
4456 of the vector in turn. So write this:
4459 let nums = vec![1i, 2i, 3i];
4461 for num in nums.iter() {
4462 println!("{}", num);
4466 There are two reasons for this. First, this is more semantic. We iterate
4467 through the entire vector, rather than iterating through indexes, and then
4468 indexing the vector. Second, this version is more efficient: the first version
4469 will have extra bounds checking because it used indexing, `nums[i]`. But since
4470 we yield a reference to each element of the vector in turn with the iterator,
4471 there's no bounds checking in the second example. This is very common with
4472 iterators: we can ignore unnecessary bounds checks, but still know that we're
4475 There's another detail here that's not 100% clear because of how `println!`
4476 works. `num` is actually of type `&int`, that is, it's a reference to an `int`,
4477 not an `int` itself. `println!` handles the dereferencing for us, so we don't
4478 see it. This code works fine too:
4481 let nums = vec![1i, 2i, 3i];
4483 for num in nums.iter() {
4484 println!("{}", *num);
4488 Now we're explicitly dereferencing `num`. Why does `iter()` give us references?
4489 Well, if it gave us the data itself, we would have to be its owner, which would
4490 involve making a copy of the data and giving us the copy. With references,
4491 we're just borrowing a reference to the data, and so it's just passing
4492 a reference, without needing to do the copy.
4494 So, now that we've established that `range` is often not what you want, let's
4495 talk about what you do want instead.
4497 There are three broad classes of things that are relevant here: iterators,
4498 **iterator adapters**, and **consumers**. Here's some definitions:
4500 * 'iterators' give you a sequence of values.
4501 * 'iterator adapters' operate on an iterator, producing a new iterator with a
4502 different output sequence.
4503 * 'consumers' operate on an iterator, producing some final set of values.
4505 Let's talk about consumers first, since you've already seen an iterator,
4510 A 'consumer' operates on an iterator, returning some kind of value or values.
4511 The most common consumer is `collect()`. This code doesn't quite compile,
4512 but it shows the intention:
4515 let one_to_one_hundred = range(0i, 100i).collect();
4518 As you can see, we call `collect()` on our iterator. `collect()` takes
4519 as many values as the iterator will give it, and returns a collection
4520 of the results. So why won't this compile? Rust can't determine what
4521 type of things you want to collect, and so you need to let it know.
4522 Here's the version that does compile:
4525 let one_to_one_hundred = range(0i, 100i).collect::<Vec<int>>();
4528 If you remember, the `::<>` syntax allows us to give a type hint,
4529 and so we tell it that we want a vector of integers.
4531 `collect()` is the most common consumer, but there are others too. `find()`
4535 let one_to_one_hundred = range(0i, 100i);
4537 let greater_than_forty_two = range(0i, 100i)
4538 .find(|x| *x >= 42);
4540 match greater_than_forty_two {
4541 Some(_) => println!("We got some numbers!"),
4542 None => println!("No numbers found :("),
4546 `find` takes a closure, and works on a reference to each element of an
4547 iterator. This closure returns `true` if the element is the element we're
4548 looking for, and `false` otherwise. Because we might not find a matching
4549 element, `find` returns an `Option` rather than the element itself.
4551 Another important consumer is `fold`. Here's what it looks like:
4554 let sum = range(1i, 100i)
4555 .fold(0i, |sum, x| sum + x);
4558 `fold()` is a consumer that looks like this:
4559 `fold(base, |accumulator, element| ...)`. It takes two arguments: the first
4560 is an element called the "base". The second is a closure that itself takes two
4561 arguments: the first is called the "accumulator," and the second is an
4562 "element." Upon each iteration, the closure is called, and the result is the
4563 value of the accumulator on the next iteration. On the first iteration, the
4564 base is the value of the accumulator.
4566 Okay, that's a bit confusing. Let's examine the values of all of these things
4569 | base | accumulator | element | closure result |
4570 |------|-------------|---------|----------------|
4571 | 0i | 0i | 1i | 1i |
4572 | 0i | 1i | 2i | 3i |
4573 | 0i | 3i | 3i | 6i |
4575 We called `fold()` with these arguments:
4579 .fold(0i, |sum, x| sum + x);
4582 So, `0i` is our base, `sum` is our accumulator, and `x` is our element. On the
4583 first iteration, we set `sum` to `0i`, and `x` is the first element of `nums`,
4584 `1i`. We then add `sum` and `x`, which gives us `0i + 1i = 1i`. On the second
4585 iteration, that value becomes our accumulator, `sum`, and the element is
4586 the second element of the array, `2i`. `1i + 2i = 3i`, and so that becomes
4587 the value of the accumulator for the last iteration. On that iteration,
4588 `x` is the last element, `3i`, and `3i + 3i = 6i`, which is our final
4589 result for our sum. `1 + 2 + 3 = 6`, and that's the result we got.
4591 Whew. `fold` can be a bit strange the first few times you see it, but once it
4592 clicks, you can use it all over the place. Any time you have a list of things,
4593 and you want a single result, `fold` is appropriate.
4595 Consumers are important due to one additional property of iterators we haven't
4596 talked about yet: laziness. Let's talk some more about iterators, and you'll
4597 see why consumers matter.
4601 As we've said before, an iterator is something that we can call the `.next()`
4602 method on repeatedly, and it gives us a sequence of things. Because you need
4603 to call the method, this means that iterators are **lazy**. This code, for
4604 example, does not actually generate the numbers `1-100`, and just creates a
4605 value that represents the sequence:
4608 let nums = range(1i, 100i);
4611 Since we didn't do anything with the range, it didn't generate the sequence.
4612 Once we add the consumer:
4615 let nums = range(1i, 100i).collect::<Vec<int>>();
4618 Now, `collect()` will require that `range()` give it some numbers, and so
4619 it will do the work of generating the sequence.
4621 `range` is one of two basic iterators that you'll see. The other is `iter()`,
4622 which you've used before. `iter()` can turn a vector into a simple iterator
4623 that gives you each element in turn:
4626 let nums = [1i, 2i, 3i];
4628 for num in nums.iter() {
4629 println!("{}", num);
4633 These two basic iterators should serve you well. There are some more
4634 advanced iterators, including ones that are infinite. Like `count`:
4637 std::iter::count(1i, 5i);
4640 This iterator counts up from one, adding five each time. It will give
4641 you a new integer every time, forever. Well, technically, until the
4642 maximum number that an `int` can represent. But since iterators are lazy,
4643 that's okay! You probably don't want to use `collect()` on it, though...
4645 That's enough about iterators. Iterator adapters are the last concept
4646 we need to talk about with regards to iterators. Let's get to it!
4648 ## Iterator adapters
4650 "Iterator adapters" take an iterator and modify it somehow, producing
4651 a new iterator. The simplest one is called `map`:
4654 range(1i, 100i).map(|x| x + 1i);
4657 `map` is called upon another iterator, and produces a new iterator where each
4658 element reference has the closure it's been given as an argument called on it.
4659 So this would give us the numbers from `2-101`. Well, almost! If you
4660 compile the example, you'll get a warning:
4663 2:37 warning: unused result which must be used: iterator adaptors are lazy and
4664 do nothing unless consumed, #[warn(unused_must_use)] on by default
4665 range(1i, 100i).map(|x| x + 1i);
4666 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4669 Laziness strikes again! That closure will never execute. This example
4670 doesn't print any numbers:
4673 range(1i, 100i).map(|x| println!("{}", x));
4676 If you are trying to execute a closure on an iterator for its side effects,
4677 just use `for` instead.
4679 There are tons of interesting iterator adapters. `take(n)` will get the
4680 first `n` items out of an iterator, and return them as a list. Let's
4681 try it out with our infinite iterator from before, `count()`:
4684 for i in std::iter::count(1i, 5i).take(5) {
4699 `filter()` is an adapter that takes a closure as an argument. This closure
4700 returns `true` or `false`. The new iterator `filter()` produces returns
4701 only the elements that that closure returned `true` for:
4704 for i in range(1i, 100i).filter(|x| x % 2 == 0) {
4709 This will print all of the even numbers between one and a hundred.
4711 You can chain all three things together: start with an iterator, adapt it
4712 a few times, and then consume the result. Check it out:
4716 .filter(|x| x % 2 == 0)
4717 .filter(|x| x % 3 == 0)
4719 .collect::<Vec<int>>();
4722 This will give you a vector containing `6`, `12`, `18`, `24`, and `30`.
4724 This is just a small taste of what iterators, iterator adapters, and consumers
4725 can help you with. There are a number of really useful iterators, and you can
4726 write your own as well. Iterators provide a safe, efficient way to manipulate
4727 all kinds of lists. They're a little unusual at first, but if you play with
4728 them, you'll get hooked. For a full list of the different iterators and
4729 consumers, check out the [iterator module documentation](std/iter/index.html).
4733 Sometimes, when writing a function or data type, we may want it to work for
4734 multiple types of arguments. For example, remember our `OptionalInt` type?
4743 If we wanted to also have an `OptionalFloat64`, we would need a new enum:
4746 enum OptionalFloat64 {
4752 This is really unfortunate. Luckily, Rust has a feature that gives us a better
4753 way: generics. Generics are called **parametric polymorphism** in type theory,
4754 which means that they are types or functions that have multiple forms ("poly"
4755 is multiple, "morph" is form) over a given parameter ("parametric").
4757 Anyway, enough with type theory declarations, let's check out the generic form
4758 of `OptionalInt`. It is actually provided by Rust itself, and looks like this:
4767 The `<T>` part, which you've seen a few times before, indicates that this is
4768 a generic data type. Inside the declaration of our enum, wherever we see a `T`,
4769 we substitute that type for the same type used in the generic. Here's an
4770 example of using `Option<T>`, with some extra type annotations:
4773 let x: Option<int> = Some(5i);
4776 In the type declaration, we say `Option<int>`. Note how similar this looks to
4777 `Option<T>`. So, in this particular `Option`, `T` has the value of `int`. On
4778 the right hand side of the binding, we do make a `Some(T)`, where `T` is `5i`.
4779 Since that's an `int`, the two sides match, and Rust is happy. If they didn't
4780 match, we'd get an error:
4783 let x: Option<f64> = Some(5i);
4784 // error: mismatched types: expected `core::option::Option<f64>`
4785 // but found `core::option::Option<int>` (expected f64 but found int)
4788 That doesn't mean we can't make `Option<T>`s that hold an `f64`! They just have to
4792 let x: Option<int> = Some(5i);
4793 let y: Option<f64> = Some(5.0f64);
4796 This is just fine. One definition, multiple uses.
4798 Generics don't have to only be generic over one type. Consider Rust's built-in
4799 `Result<T, E>` type:
4808 This type is generic over _two_ types: `T` and `E`. By the way, the capital letters
4809 can be any letter you'd like. We could define `Result<T, E>` as:
4818 if we wanted to. Convention says that the first generic parameter should be
4819 `T`, for 'type,' and that we use `E` for 'error.' Rust doesn't care, however.
4821 The `Result<T, E>` type is intended to
4822 be used to return the result of a computation, and to have the ability to
4823 return an error if it didn't work out. Here's an example:
4826 let x: Result<f64, String> = Ok(2.3f64);
4827 let y: Result<f64, String> = Err("There was an error.".to_string());
4830 This particular Result will return an `f64` if there's a success, and a
4831 `String` if there's a failure. Let's write a function that uses `Result<T, E>`:
4834 fn inverse(x: f64) -> Result<f64, String> {
4835 if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4841 We don't want to take the inverse of zero, so we check to make sure that we
4842 weren't passed zero. If we were, then we return an `Err`, with a message. If
4843 it's okay, we return an `Ok`, with the answer.
4845 Why does this matter? Well, remember how `match` does exhaustive matches?
4846 Here's how this function gets used:
4849 # fn inverse(x: f64) -> Result<f64, String> {
4850 # if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4853 let x = inverse(25.0f64);
4856 Ok(x) => println!("The inverse of 25 is {}", x),
4857 Err(msg) => println!("Error: {}", msg),
4861 The `match` enforces that we handle the `Err` case. In addition, because the
4862 answer is wrapped up in an `Ok`, we can't just use the result without doing
4866 let x = inverse(25.0f64);
4867 println!("{}", x + 2.0f64); // error: binary operation `+` cannot be applied
4868 // to type `core::result::Result<f64,collections::string::String>`
4871 This function is great, but there's one other problem: it only works for 64 bit
4872 floating point values. What if we wanted to handle 32 bit floating point as
4873 well? We'd have to write this:
4876 fn inverse32(x: f32) -> Result<f32, String> {
4877 if x == 0.0f32 { return Err("x cannot be zero!".to_string()); }
4883 Bummer. What we need is a **generic function**. Luckily, we can write one!
4884 However, it won't _quite_ work yet. Before we get into that, let's talk syntax.
4885 A generic version of `inverse` would look something like this:
4888 fn inverse<T>(x: T) -> Result<T, String> {
4889 if x == 0.0 { return Err("x cannot be zero!".to_string()); }
4895 Just like how we had `Option<T>`, we use a similar syntax for `inverse<T>`.
4896 We can then use `T` inside the rest of the signature: `x` has type `T`, and half
4897 of the `Result` has type `T`. However, if we try to compile that example, we'll get
4901 error: binary operation `==` cannot be applied to type `T`
4904 Because `T` can be _any_ type, it may be a type that doesn't implement `==`,
4905 and therefore, the first line would be wrong. What do we do?
4907 To fix this example, we need to learn about another Rust feature: traits.
4911 Do you remember the `impl` keyword, used to call a function with method
4922 fn area(&self) -> f64 {
4923 std::f64::consts::PI * (self.radius * self.radius)
4928 Traits are similar, except that we define a trait with just the method
4929 signature, then implement the trait for that struct. Like this:
4939 fn area(&self) -> f64;
4942 impl HasArea for Circle {
4943 fn area(&self) -> f64 {
4944 std::f64::consts::PI * (self.radius * self.radius)
4949 As you can see, the `trait` block looks very similar to the `impl` block,
4950 but we don't define a body, just a type signature. When we `impl` a trait,
4951 we use `impl Trait for Item`, rather than just `impl Item`.
4953 So what's the big deal? Remember the error we were getting with our generic
4957 error: binary operation `==` cannot be applied to type `T`
4960 We can use traits to constrain our generics. Consider this function, which
4961 does not compile, and gives us a similar error:
4964 fn print_area<T>(shape: T) {
4965 println!("This shape has an area of {}", shape.area());
4972 error: type `T` does not implement any method in scope named `area`
4975 Because `T` can be any type, we can't be sure that it implements the `area`
4976 method. But we can add a **trait constraint** to our generic `T`, ensuring
4981 # fn area(&self) -> f64;
4983 fn print_area<T: HasArea>(shape: T) {
4984 println!("This shape has an area of {}", shape.area());
4988 The syntax `<T: HasArea>` means `any type that implements the HasArea trait`.
4989 Because traits define function type signatures, we can be sure that any type
4990 which implements `HasArea` will have an `.area()` method.
4992 Here's an extended example of how this works:
4996 fn area(&self) -> f64;
5005 impl HasArea for Circle {
5006 fn area(&self) -> f64 {
5007 std::f64::consts::PI * (self.radius * self.radius)
5017 impl HasArea for Square {
5018 fn area(&self) -> f64 {
5019 self.side * self.side
5023 fn print_area<T: HasArea>(shape: T) {
5024 println!("This shape has an area of {}", shape.area());
5045 This program outputs:
5048 This shape has an area of 3.141593
5049 This shape has an area of 1
5052 As you can see, `print_area` is now generic, but also ensures that we
5053 have passed in the correct types. If we pass in an incorrect type:
5059 We get a compile-time error:
5062 error: failed to find an implementation of trait main::HasArea for int
5065 So far, we've only added trait implementations to structs, but you can
5066 implement a trait for any type. So technically, we _could_ implement
5067 `HasArea` for `int`:
5071 fn area(&self) -> f64;
5074 impl HasArea for int {
5075 fn area(&self) -> f64 {
5076 println!("this is silly");
5085 It is considered poor style to implement methods on such primitive types, even
5086 though it is possible.
5088 This may seem like the Wild West, but there are two other restrictions around
5089 implementing traits that prevent this from getting out of hand. First, traits
5090 must be `use`d in any scope where you wish to use the trait's method. So for
5091 example, this does not work:
5095 use std::f64::consts;
5098 fn area(&self) -> f64;
5107 impl HasArea for Circle {
5108 fn area(&self) -> f64 {
5109 consts::PI * (self.radius * self.radius)
5115 let c = shapes::Circle {
5121 println!("{}", c.area());
5125 Now that we've moved the structs and traits into their own module, we get an
5129 error: type `shapes::Circle` does not implement any method in scope named `area`
5132 If we add a `use` line right above `main` and make the right things public,
5136 use shapes::HasArea;
5139 use std::f64::consts;
5142 fn area(&self) -> f64;
5151 impl HasArea for Circle {
5152 fn area(&self) -> f64 {
5153 consts::PI * (self.radius * self.radius)
5160 let c = shapes::Circle {
5166 println!("{}", c.area());
5170 This means that even if someone does something bad like add methods to `int`,
5171 it won't affect you, unless you `use` that trait.
5173 There's one more restriction on implementing traits. Either the trait or the
5174 type you're writing the `impl` for must be inside your crate. So, we could
5175 implement the `HasArea` type for `int`, because `HasArea` is in our crate. But
5176 if we tried to implement `Float`, a trait provided by Rust, for `int`, we could
5177 not, because both the trait and the type aren't in our crate.
5179 One last thing about traits: generic functions with a trait bound use
5180 **monomorphization** ("mono": one, "morph": form), so they are statically
5181 dispatched. What's that mean? Well, let's take a look at `print_area` again:
5184 fn print_area<T: HasArea>(shape: T) {
5185 println!("This shape has an area of {}", shape.area());
5189 let c = Circle { ... };
5191 let s = Square { ... };
5198 When we use this trait with `Circle` and `Square`, Rust ends up generating
5199 two different functions with the concrete type, and replacing the call sites with
5200 calls to the concrete implementations. In other words, you get something like
5204 fn __print_area_circle(shape: Circle) {
5205 println!("This shape has an area of {}", shape.area());
5208 fn __print_area_square(shape: Square) {
5209 println!("This shape has an area of {}", shape.area());
5213 let c = Circle { ... };
5215 let s = Square { ... };
5217 __print_area_circle(c);
5218 __print_area_square(s);
5222 The names don't actually change to this, it's just for illustration. But
5223 as you can see, there's no overhead of deciding which version to call here,
5224 hence 'statically dispatched.' The downside is that we have two copies of
5225 the same function, so our binary is a little bit larger.
5229 Concurrency and parallelism are topics that are of increasing interest to a
5230 broad subsection of software developers. Modern computers are often multi-core,
5231 to the point that even embedded devices like cell phones have more than one
5232 processor. Rust's semantics lend themselves very nicely to solving a number of
5233 issues that programmers have with concurrency. Many concurrency errors that are
5234 runtime errors in other languages are compile-time errors in Rust.
5236 Rust's concurrency primitive is called a **task**. Tasks are lightweight, and
5237 do not share memory in an unsafe manner, preferring message passing to
5238 communicate. It's worth noting that tasks are implemented as a library, and
5239 not part of the language. This means that in the future, other concurrency
5240 libraries can be written for Rust to help in specific scenarios. Here's an
5241 example of creating a task:
5245 println!("Hello from a task!");
5249 The `spawn` function takes a proc as an argument, and runs that proc in a new
5250 task. A proc takes ownership of its entire environment, and so any variables
5251 that you use inside the proc will not be usable afterward:
5254 let mut x = vec![1i, 2i, 3i];
5257 println!("The value of x[0] is: {}", x[0]);
5260 println!("The value of x[0] is: {}", x[0]); // error: use of moved value: `x`
5263 `x` is now owned by the proc, and so we can't use it anymore. Many other
5264 languages would let us do this, but it's not safe to do so. Rust's type system
5267 If tasks were only able to capture these values, they wouldn't be very useful.
5268 Luckily, tasks can communicate with each other through **channel**s. Channels
5272 let (tx, rx) = channel();
5275 tx.send("Hello from a task!".to_string());
5278 let message = rx.recv();
5279 println!("{}", message);
5282 The `channel()` function returns two endpoints: a `Receiver<T>` and a
5283 `Sender<T>`. You can use the `.send()` method on the `Sender<T>` end, and
5284 receive the message on the `Receiver<T>` side with the `recv()` method. This
5285 method blocks until it gets a message. There's a similar method, `.try_recv()`,
5286 which returns an `Option<T>` and does not block.
5288 If you want to send messages to the task as well, create two channels!
5291 let (tx1, rx1) = channel();
5292 let (tx2, rx2) = channel();
5295 tx1.send("Hello from a task!".to_string());
5296 let message = rx2.recv();
5297 println!("{}", message);
5300 let message = rx1.recv();
5301 println!("{}", message);
5303 tx2.send("Goodbye from main!".to_string());
5306 The proc has one sending end and one receiving end, and the main task has one
5307 of each as well. Now they can talk back and forth in whatever way they wish.
5309 Notice as well that because `Sender` and `Receiver` are generic, while you can
5310 pass any kind of information through the channel, the ends are strongly typed.
5311 If you try to pass a string, and then an integer, Rust will complain.
5315 With these basic primitives, many different concurrency patterns can be
5316 developed. Rust includes some of these types in its standard library. For
5317 example, if you wish to compute some value in the background, `Future` is
5318 a useful thing to use:
5321 use std::sync::Future;
5323 let mut delayed_value = Future::spawn(proc() {
5324 // just return anything for examples' sake
5328 println!("value = {}", delayed_value.get());
5331 Calling `Future::spawn` works just like `spawn()`: it takes a proc. In this
5332 case, though, you don't need to mess with the channel: just have the proc
5335 `Future::spawn` will return a value which we can bind with `let`. It needs
5336 to be mutable, because once the value is computed, it saves a copy of the
5337 value, and if it were immutable, it couldn't update itself.
5339 The proc will go on processing in the background, and when we need the final
5340 value, we can call `get()` on it. This will block until the result is done,
5341 but if it's finished computing in the background, we'll just get the value
5344 ## Success and failure
5346 Tasks don't always succeed, they can also fail. A task that wishes to fail
5347 can call the `fail!` macro, passing a message:
5355 If a task fails, it is not possible for it to recover. However, it can
5356 notify other tasks that it has failed. We can do this with `task::try`:
5362 let result = task::try(proc() {
5371 This task will randomly fail or succeed. `task::try` returns a `Result`
5372 type, so we can handle the response like any other computation that may
5377 One of Rust's most advanced features is its system of **macro**s. While
5378 functions allow you to provide abstractions over values and operations, macros
5379 allow you to provide abstractions over syntax. Do you wish Rust had the ability
5380 to do something that it can't currently do? You may be able to write a macro
5381 to extend Rust's capabilities.
5383 You've already used one macro extensively: `println!`. When we invoke
5384 a Rust macro, we need to use the exclamation mark (`!`). There's two reasons
5385 that this is true: the first is that it makes it clear when you're using a
5386 macro. The second is that macros allow for flexible syntax, and so Rust must
5387 be able to tell where a macro starts and ends. The `!(...)` helps with this.
5389 Let's talk some more about `println!`. We could have implemented `println!` as
5390 a function, but it would be worse. Why? Well, what macros allow you to do
5391 is write code that generates more code. So when we call `println!` like this:
5395 println!("x is: {}", x);
5398 The `println!` macro does a few things:
5400 1. It parses the string to find any `{}`s
5401 2. It checks that the number of `{}`s matches the number of other arguments.
5402 3. It generates a bunch of Rust code, taking this in mind.
5404 What this means is that you get type checking at compile time, because
5405 Rust will generate code that takes all of the types into account. If
5406 `println!` was a function, it could still do this type checking, but it
5407 would happen at run time rather than compile time.
5409 We can check this out using a special flag to `rustc`. This code, in a file
5415 println!("x is: {:s}", x);
5419 Can have its macros expanded like this: `rustc print.rs --pretty=expanded`, will
5420 give us this huge result:
5426 #[phase(plugin, link)]
5427 extern crate std = "std";
5428 extern crate rt = "native";
5429 use std::prelude::*;
5436 static __STATIC_FMTSTR: [::std::fmt::rt::Piece<'static>, ..2u] =
5437 [::std::fmt::rt::String("x is: "),
5438 ::std::fmt::rt::Argument(::std::fmt::rt::Argument{position:
5439 ::std::fmt::rt::ArgumentNext,
5441 ::std::fmt::rt::FormatSpec{fill:
5444 ::std::fmt::rt::AlignUnknown,
5448 ::std::fmt::rt::CountImplied,
5450 ::std::fmt::rt::CountImplied,},})];
5452 &[::std::fmt::argument(::std::fmt::secret_string, __arg0)];
5455 ::std::fmt::Arguments::new(__STATIC_FMTSTR, __args_vec)
5457 ::std::io::stdio::println_args(&__args)
5463 Intense. Here's a trimmed down version that's a bit easier to read:
5470 static __STATIC_FMTSTR: =
5473 position: ArgumentNext,
5474 format: FormatSpec {
5476 align: AlignUnknown,
5478 precision: CountImplied,
5479 width: CountImplied,
5483 let __args_vec = &[argument(secret_string, __arg0)];
5484 let __args = unsafe { Arguments::new(__STATIC_FMTSTR, __args_vec) };
5486 println_args(&__args)
5492 Whew! This isn't too terrible. You can see that we still `let x = 5i`,
5493 but then things get a little bit hairy. Three more bindings get set: a
5494 static format string, an argument vector, and the arguments. We then
5495 invoke the `println_args` function with the generated arguments.
5497 This is the code (well, the full version) that Rust actually compiles. You can
5498 see all of the extra information that's here. We get all of the type safety and
5499 options that it provides, but at compile time, and without needing to type all
5500 of this out. This is how macros are powerful. Without them, you would need to
5501 type all of this by hand to get a type checked `println`.
5503 For more on macros, please consult [the Macros Guide](guide-macros.html).
5504 Macros are a very advanced and still slightly experimental feature, but don't
5505 require a deep understanding to call, since they look just like functions. The
5506 Guide can help you if you want to write your own.
5510 Finally, there's one more Rust concept that you should be aware of: `unsafe`.
5511 There are two circumstances where Rust's safety provisions don't work well.
5512 The first is when interfacing with C code, and the second is when building
5513 certain kinds of abstractions.
5515 Rust has support for FFI (which you can read about in the [FFI
5516 Guide](guide-ffi.html)), but can't guarantee that the C code will be safe.
5517 Therefore, Rust marks such functions with the `unsafe`
5518 keyword, which indicates that the function may not behave properly.
5520 Second, if you'd like to create some sort of shared-memory data structure, Rust
5521 won't allow it, because memory must be owned by a single owner. However, if
5522 you're planning on making access to that shared memory safe, such as with a
5523 mutex, _you_ know that it's safe, but Rust can't know. Writing an `unsafe`
5524 block allows you to ask the compiler to trust you. In this case, the _internal_
5525 implementation of the mutex is considered unsafe, but the _external_ interface
5526 we present is safe. This allows it to be effectively used in normal Rust, while
5527 being able to implement functionality that the compiler can't double check for
5530 Doesn't an escape hatch undermine the safety of the entire system? Well, if
5531 Rust code segfaults, it _must_ be because of unsafe code somewhere. By
5532 annotating exactly where that is, you have a significantly smaller area to
5535 We haven't even talked about any examples here, and that's because I want to
5536 emphasize that you should not be writing unsafe code unless you know exactly
5537 what you're doing. The vast majority of Rust developers will only interact with
5538 it when doing FFI, and advanced library authors may use it to build certain
5539 kinds of abstraction.
5543 We covered a lot of ground here. When you've mastered everything in this Guide,
5544 you will have a firm grasp of basic Rust development. There's a whole lot more
5545 out there, we've just covered the surface. There's tons of topics that you can
5546 dig deeper into, and we've built specialized guides for many of them. To learn
5547 more, dig into the [full documentation
5548 index](http://doc.rust-lang.org/index.html).