3 Hey there! Welcome to the Rust guide. This is the place to be if you'd like to
4 learn how to program in Rust. Rust is a systems programming language with a
5 focus on "high-level, bare-metal programming": the lowest level control a
6 programming language can give you, but with zero-cost, higher level
7 abstractions, because people aren't computers. We really think Rust is
8 something special, and we hope you do too.
10 To show you how to get going with Rust, we're going to write the traditional
11 "Hello, World!" program. Next, we'll introduce you to a tool that's useful for
12 writing real-world Rust programs and libraries: "Cargo." After that, we'll talk
13 about the basics of Rust, write a little program to try them out, and then learn
20 The first step to using Rust is to install it! There are a number of ways to
21 install Rust, but the easiest is to use the `rustup` script. If you're on
22 Linux or a Mac, all you need to do is this (note that you don't need to type
23 in the `$`s, they just indicate the start of each command):
26 $ curl -s https://static.rust-lang.org/rustup.sh | sudo sh
29 (If you're concerned about `curl | sudo sh`, please keep reading. Disclaimer
32 If you're on Windows, please [download this .exe and run
33 it](https://static.rust-lang.org/dist/rust-nightly-install.exe).
35 If you decide you don't want Rust anymore, we'll be a bit sad, but that's okay.
36 Not every programming language is great for everyone. Just pass an argument to
40 $ curl -s https://static.rust-lang.org/rustup.sh | sudo sh -s -- --uninstall
43 If you used the Windows installer, just re-run the `.exe` and it will give you
46 You can re-run this script any time you want to update Rust. Which, at this
47 point, is often. Rust is still pre-1.0, and so people assume that you're using
50 This brings me to one other point: some people, and somewhat rightfully so, get
51 very upset when we tell you to `curl | sudo sh`. And they should be! Basically,
52 when you do this, you are trusting that the good people who maintain Rust
53 aren't going to hack your computer and do bad things. That's a good instinct!
54 If you're one of those people, please check out the documentation on [building
55 Rust from Source](https://github.com/rust-lang/rust#building-from-source), or
56 [the official binary downloads](http://www.rust-lang.org/install.html). And we
57 promise that this method will not be the way to install Rust forever: it's just
58 the easiest way to keep people updated while Rust is in its alpha state.
60 Oh, we should also mention the officially supported platforms:
62 * Windows (7, 8, Server 2008 R2), x86 only
63 * Linux (2.6.18 or later, various distributions), x86 and x86-64
64 * OSX 10.7 (Lion) or greater, x86 and x86-64
66 We extensively test Rust on these platforms, and a few others, too, like
67 Android. But these are the ones most likely to work, as they have the most
70 Finally, a comment about Windows. Rust considers Windows to be a first-class
71 platform upon release, but if we're honest, the Windows experience isn't as
72 integrated as the Linux/OS X experience is. We're working on it! If anything
73 does not work, it is a bug. Please let us know if that happens. Each and every
74 commit is tested against Windows just like any other platform.
76 If you've got Rust installed, you can open up a shell, and type this:
82 You should see some output that looks something like this:
85 rustc 0.12.0-pre (443a1cd 2014-06-08 14:56:52 -0700)
88 If you did, Rust has been installed successfully! Congrats!
90 If not, there are a number of places where you can get help. The easiest is
91 [the #rust IRC channel on irc.mozilla.org](irc://irc.mozilla.org/#rust), which
92 you can access through
93 [Mibbit](http://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust). Click
94 that link, and you'll be chatting with other Rustaceans (a silly nickname we
95 call ourselves), and we can help you out. Other great resources include [our
96 mailing list](https://mail.mozilla.org/listinfo/rust-dev), [the /r/rust
97 subreddit](http://www.reddit.com/r/rust), and [Stack
98 Overflow](http://stackoverflow.com/questions/tagged/rust).
102 Now that you have Rust installed, let's write your first Rust program. It's
103 traditional to make your first program in any new language one that prints the
104 text "Hello, world!" to the screen. The nice thing about starting with such a
105 simple program is that you can verify that your compiler isn't just installed,
106 but also working properly. And printing information to the screen is a pretty
109 The first thing that we need to do is make a file to put our code in. I like
110 to make a `projects` directory in my home directory, and keep all my projects
111 there. Rust does not care where your code lives.
113 This actually leads to one other concern we should address: this guide will
114 assume that you have basic familiarity with the command line. Rust does not
115 require that you know a whole ton about the command line, but until the
116 language is in a more finished state, IDE support is spotty. Rust makes no
117 specific demands on your editing tooling, or where your code lives.
119 With that said, let's make a directory in our projects directory.
128 If you're on Windows and not using PowerShell, the `~` may not work. Consult
129 the documentation for your shell for more details.
131 Let's make a new source file next. I'm going to use the syntax `editor
132 filename` to represent editing a file in these examples, but you should use
133 whatever method you want. We'll call our file `main.rs`:
139 Rust files always end in a `.rs` extension. If you're using more than one word
140 in your file name, use an underscore. `hello_world.rs` rather than
143 Now that you've got your file open, type this in:
147 println!("Hello, world!");
151 Save the file, and then type this into your terminal window:
155 $ ./hello_world # or hello_world.exe on Windows
159 Success! Let's go over what just happened in detail.
167 These two lines define a **function** in Rust. The `main` function is special:
168 it's the beginning of every Rust program. The first line says "I'm declaring a
169 function named `main`, which takes no arguments and returns nothing." If there
170 were arguments, they would go inside the parentheses (`(` and `)`), and because
171 we aren't returning anything from this function, we've dropped that notation
172 entirely. We'll get to it later.
174 You'll also note that the function is wrapped in curly braces (`{` and `}`).
175 Rust requires these around all function bodies. It is also considered good
176 style to put the opening curly brace on the same line as the function
177 declaration, with one space in between.
179 Next up is this line:
182 println!("Hello, world!");
185 This line does all of the work in our little program. There are a number of
186 details that are important here. The first is that it's indented with four
187 spaces, not tabs. Please configure your editor of choice to insert four spaces
188 with the tab key. We provide some sample configurations for various editors
189 [here](https://github.com/rust-lang/rust/tree/master/src/etc).
191 The second point is the `println!()` part. This is calling a Rust **macro**,
192 which is how metaprogramming is done in Rust. If it were a function instead, it
193 would look like this: `println()`. For our purposes, we don't need to worry
194 about this difference. Just know that sometimes, you'll see a `!`, and that
195 means that you're calling a macro instead of a normal function. One last thing
196 to mention: Rust's macros are significantly different than C macros, if you've
197 used those. Don't be scared of using macros. We'll get to the details
198 eventually, you'll just have to trust us for now.
200 Next, `"Hello, world!"` is a **string**. Strings are a surprisingly complicated
201 topic in a systems programming language, and this is a **statically allocated**
202 string. We will talk more about different kinds of allocation later. We pass
203 this string as an argument to `println!`, which prints the string to the
206 Finally, the line ends with a semicolon (`;`). Rust is an **expression
207 oriented** language, which means that most things are expressions. The `;` is
208 used to indicate that this expression is over, and the next one is ready to
209 begin. Most lines of Rust code end with a `;`. We will cover this in-depth
212 Finally, actually **compiling** and **running** our program. We can compile
213 with our compiler, `rustc`, by passing it the name of our source file:
219 This is similar to `gcc` or `clang`, if you come from a C or C++ background. Rust
220 will output a binary executable. You can see it with `ls`:
234 There are now two files: our source code, with the `.rs` extension, and the
235 executable (`hello_world.exe` on Windows, `hello_world` everywhere else)
238 $ ./hello_world # or hello_world.exe on Windows
241 This prints out our `Hello, world!` text to our terminal.
243 If you come from a dynamically typed language like Ruby, Python, or JavaScript,
244 you may not be used to these two steps being separate. Rust is an
245 **ahead-of-time compiled language**, which means that you can compile a
246 program, give it to someone else, and they don't need to have Rust installed.
247 If you give someone a `.rb` or `.py` or `.js` file, they need to have
248 Ruby/Python/JavaScript installed, but you just need one command to both compile
249 and run your program. Everything is a tradeoff in language design, and Rust has
252 Congratulations! You have officially written a Rust program. That makes you a
253 Rust programmer! Welcome.
255 Next, I'd like to introduce you to another tool, Cargo, which is used to write
256 real-world Rust programs. Just using `rustc` is nice for simple things, but as
257 your project grows, you'll want something to help you manage all of the options
258 that it has, and to make it easy to share your code with other people and
263 [Cargo](http://crates.io) is a tool that Rustaceans use to help manage their
264 Rust projects. Cargo is currently in an alpha state, just like Rust, and so it
265 is still a work in progress. However, it is already good enough to use for many
266 Rust projects, and so it is assumed that Rust projects will use Cargo from the
269 Cargo manages three things: building your code, downloading the dependencies
270 your code needs, and building the dependencies your code needs. At first, your
271 program doesn't have any dependencies, so we'll only be using the first part of
272 its functionality. Eventually, we'll add more. Since we started off by using
273 Cargo, it'll be easy to add later.
275 Let's convert Hello World to Cargo. The first thing we need to do to begin
276 using Cargo is to install Cargo. Luckily for us, the script we ran to install
277 Rust includes Cargo by default. If you installed Rust some other way, you may
278 want to [check the Cargo
279 README](https://github.com/rust-lang/cargo#installing-cargo-from-nightlies)
280 for specific instructions about installing it.
282 To Cargo-ify our project, we need to do two things: Make a `Cargo.toml`
283 configuration file, and put our source file in the right place. Let's
288 $ mv main.rs src/main.rs
291 Cargo expects your source files to live inside a `src` directory. That leaves
292 the top level for other things, like READMEs, license information, and anything
293 not related to your code. Cargo helps us keep our projects nice and tidy. A
294 place for everything, and everything in its place.
296 Next, our configuration file:
302 Make sure to get this name right: you need the capital `C`!
311 authors = [ "Your name <you@example.com>" ]
318 This file is in the [TOML](https://github.com/toml-lang/toml) format. Let's let
319 it explain itself to you:
321 > TOML aims to be a minimal configuration file format that's easy to read due
322 > to obvious semantics. TOML is designed to map unambiguously to a hash table.
323 > TOML should be easy to parse into data structures in a wide variety of
326 TOML is very similar to INI, but with some extra goodies.
328 Anyway, there are two **table**s in this file: `package` and `bin`. The first
329 tells Cargo metadata about your package. The second tells Cargo that we're
330 interested in building a binary, not a library (though we could do both!), as
331 well as what it is named.
333 Once you have this file in place, we should be ready to build! Try this:
337 Compiling hello_world v0.0.1 (file:///home/yourname/projects/hello_world)
338 $ ./target/hello_world
342 Bam! We build our project with `cargo build`, and run it with
343 `./target/hello_world`. This hasn't bought us a whole lot over our simple use
344 of `rustc`, but think about the future: when our project has more than one
345 file, we would need to call `rustc` twice, and pass it a bunch of options to
346 tell it to build everything together. With Cargo, as our project grows, we can
347 just `cargo build` and it'll work the right way.
349 You'll also notice that Cargo has created a new file: `Cargo.lock`.
357 This file is used by Cargo to keep track of dependencies in your application.
358 Right now, we don't have any, so it's a bit sparse. You won't ever need
359 to touch this file yourself, just let Cargo handle it.
361 That's it! We've successfully built `hello_world` with Cargo. Even though our
362 program is simple, it's using much of the real tooling that you'll use for the
363 rest of your Rust career.
365 Now that you've got the tools down, let's actually learn more about the Rust
366 language itself. These are the basics that will serve you well through the rest
367 of your time with Rust.
371 The first thing we'll learn about are 'variable bindings.' They look like this:
377 In many languages, this is called a 'variable.' But Rust's variable bindings
378 have a few tricks up their sleeves. Rust has a very powerful feature called
379 'pattern matching' that we'll get into detail with later, but the left
380 hand side of a `let` expression is a full pattern, not just a variable name.
381 This means we can do things like:
384 let (x, y) = (1i, 2i);
387 After this expression is evaluated, `x` will be one, and `y` will be two.
388 Patterns are really powerful, but this is about all we can do with them so far.
389 So let's just keep this in the back of our minds as we go forward.
391 By the way, in these examples, `i` indicates that the number is an integer.
393 Rust is a statically typed language, which means that we specify our types up
394 front. So why does our first example compile? Well, Rust has this thing called
395 "[Hindley-Milner type
396 inference](http://en.wikipedia.org/wiki/Hindley%E2%80%93Milner_type_system)",
397 named after some really smart type theorists. If you clicked that link, don't
398 be scared: what this means for you is that Rust will attempt to infer the types
399 in your program, and it's pretty good at it. If it can infer the type, Rust
400 doesn't require you to actually type it out.
402 We can add the type if we want to. Types come after a colon (`:`):
408 If I asked you to read this out loud to the rest of the class, you'd say "`x`
409 is a binding with the type `int` and the value `five`."
411 By default, bindings are **immutable**. This code will not compile:
418 It will give you this error:
421 error: re-assignment of immutable variable `x`
426 If you want a binding to be mutable, you can use `mut`:
433 There is no single reason that bindings are immutable by default, but we can
434 think about it through one of Rust's primary focuses: safety. If you forget to
435 say `mut`, the compiler will catch it, and let you know that you have mutated
436 something you may not have cared to mutate. If bindings were mutable by
437 default, the compiler would not be able to tell you this. If you _did_ intend
438 mutation, then the solution is quite easy: add `mut`.
440 There are other good reasons to avoid mutable state when possible, but they're
441 out of the scope of this guide. In general, you can often avoid explicit
442 mutation, and so it is preferable in Rust. That said, sometimes, mutation is
443 what you need, so it's not verboten.
445 Let's get back to bindings. Rust variable bindings have one more aspect that
446 differs from other languages: bindings are required to be initialized with a
447 value before you're allowed to use them. If we try...
453 ...we'll get an error:
456 src/main.rs:2:9: 2:10 error: cannot determine a type for this local variable: unconstrained type
461 Giving it a type will compile, though:
467 Let's try it out. Change your `src/main.rs` file to look like this:
473 println!("Hello world!");
477 You can use `cargo build` on the command line to build it. You'll get a warning,
478 but it will still print "Hello, world!":
481 Compiling hello_world v0.0.1 (file:///home/you/projects/hello_world)
482 src/main.rs:2:9: 2:10 warning: unused variable: `x`, #[warn(unused_variable)] on by default
483 src/main.rs:2 let x: int;
487 Rust warns us that we never use the variable binding, but since we never use it,
488 no harm, no foul. Things change if we try to actually use this `x`, however. Let's
489 do that. Change your program to look like this:
495 println!("The value of x is: {}", x);
499 And try to build it. You'll get an error:
503 Compiling hello_world v0.0.1 (file:///home/you/projects/hello_world)
504 src/main.rs:4:39: 4:40 error: use of possibly uninitialized variable: `x`
505 src/main.rs:4 println!("The value of x is: {}", x);
507 note: in expansion of format_args!
508 <std macros>:2:23: 2:77 note: expansion site
509 <std macros>:1:1: 3:2 note: in expansion of println!
510 src/main.rs:4:5: 4:42 note: expansion site
511 error: aborting due to previous error
512 Could not compile `hello_world`.
515 Rust will not let us use a value that has not been initialized. Next, let's
516 talk about this stuff we've added to `println!`.
518 If you include two curly braces (`{}`, some call them moustaches...) in your
519 string to print, Rust will interpret this as a request to interpolate some sort
520 of value. **String interpolation** is a computer science term that means "stick
521 in the middle of a string." We add a comma, and then `x`, to indicate that we
522 want `x` to be the value we're interpolating. The comma is used to separate
523 arguments we pass to functions and macros, if you're passing more than one.
525 When you just use the curly braces, Rust will attempt to display the
526 value in a meaningful way by checking out its type. If you want to specify the
527 format in a more detailed manner, there are a [wide number of options
528 available](std/fmt/index.html). For now, we'll just stick to the default:
529 integers aren't very complicated to print.
533 Rust's take on `if` is not particularly complex, but it's much more like the
534 `if` you'll find in a dynamically typed language than in a more traditional
535 systems language. So let's talk about it, to make sure you grasp the nuances.
537 `if` is a specific form of a more general concept, the 'branch.' The name comes
538 from a branch in a tree: a decision point, where depending on a choice,
539 multiple paths can be taken.
541 In the case of `if`, there is one choice that leads down two paths:
547 println!("x is five!");
551 If we changed the value of `x` to something else, this line would not print.
552 More specifically, if the expression after the `if` evaluates to `true`, then
553 the block is executed. If it's `false`, then it is not.
555 If you want something to happen in the `false` case, use an `else`:
561 println!("x is five!");
563 println!("x is not five :(");
567 This is all pretty standard. However, you can also do this:
580 Which we can (and probably should) write like this:
585 let y = if x == 5i { 10i } else { 15i };
588 This reveals two interesting things about Rust: it is an expression-based
589 language, and semicolons are different than in other 'curly brace and
590 semicolon'-based languages. These two things are related.
592 ## Expressions vs. Statements
594 Rust is primarily an expression based language. There are only two kinds of
595 statements, and everything else is an expression.
597 So what's the difference? Expressions return a value, and statements do not.
598 In many languages, `if` is a statement, and therefore, `let x = if ...` would
599 make no sense. But in Rust, `if` is an expression, which means that it returns
600 a value. We can then use this value to initialize the binding.
602 Speaking of which, bindings are a kind of the first of Rust's two statements.
603 The proper name is a **declaration statement**. So far, `let` is the only kind
604 of declaration statement we've seen. Let's talk about that some more.
606 In some languages, variable bindings can be written as expressions, not just
607 statements. Like Ruby:
613 In Rust, however, using `let` to introduce a binding is _not_ an expression. The
614 following will produce a compile-time error:
617 let x = (let y = 5i); // expected identifier, found keyword `let`
620 The compiler is telling us here that it was expecting to see the beginning of
621 an expression, and a `let` can only begin a statement, not an expression.
623 Note that assigning to an already-bound variable (e.g. `y = 5i`) is still an
624 expression, although its value is not particularly useful. Unlike C, where an
625 assignment evaluates to the assigned value (e.g. `5i` in the previous example),
626 in Rust the value of an assignment is the unit type `()` (which we'll cover later).
628 The second kind of statement in Rust is the **expression statement**. Its
629 purpose is to turn any expression into a statement. In practical terms, Rust's
630 grammar expects statements to follow other statements. This means that you use
631 semicolons to separate expressions from each other. This means that Rust
632 looks a lot like most other languages that require you to use semicolons
633 at the end of every line, and you will see semicolons at the end of almost
634 every line of Rust code you see.
636 What is this exception that makes us say 'almost?' You saw it already, in this
642 let y: int = if x == 5i { 10i } else { 15i };
645 Note that I've added the type annotation to `y`, to specify explicitly that I
646 want `y` to be an integer.
648 This is not the same as this, which won't compile:
653 let y: int = if x == 5i { 10i; } else { 15i; };
656 Note the semicolons after the 10 and 15. Rust will give us the following error:
659 error: mismatched types: expected `int` but found `()` (expected int but found ())
662 We expected an integer, but we got `()`. `()` is pronounced 'unit', and is a
663 special type in Rust's type system. `()` is different than `null` in other
664 languages, because `()` is distinct from other types. For example, in C, `null`
665 is a valid value for a variable of type `int`. In Rust, `()` is _not_ a valid
666 value for a variable of type `int`. It's only a valid value for variables of
667 the type `()`, which aren't very useful. Remember how we said statements don't
668 return a value? Well, that's the purpose of unit in this case. The semicolon
669 turns any expression into a statement by throwing away its value and returning
672 There's one more time in which you won't see a semicolon at the end of a line
673 of Rust code. For that, we'll need our next concept: functions.
677 You've already seen one function so far, the `main` function:
684 This is the simplest possible function declaration. As we mentioned before,
685 `fn` says 'this is a function,' followed by the name, some parenthesis because
686 this function takes no arguments, and then some curly braces to indicate the
687 body. Here's a function named `foo`:
694 So, what about taking arguments? Here's a function that prints a number:
697 fn print_number(x: int) {
698 println!("x is: {}", x);
702 Here's a complete program that uses `print_number`:
709 fn print_number(x: int) {
710 println!("x is: {}", x);
714 As you can see, function arguments work very similar to `let` declarations:
715 you add a type to the argument name, after a colon.
717 Here's a complete program that adds two numbers together and prints them:
724 fn print_sum(x: int, y: int) {
725 println!("sum is: {}", x + y);
729 You separate arguments with a comma, both when you call the function, as well
730 as when you declare it.
732 Unlike `let`, you _must_ declare the types of function arguments. This does
736 fn print_number(x, y) {
737 println!("x is: {}", x + y);
744 hello.rs:5:18: 5:19 error: expected `:` but found `,`
745 hello.rs:5 fn print_number(x, y) {
748 This is a deliberate design decision. While full-program inference is possible,
749 languages which have it, like Haskell, often suggest that documenting your
750 types explicitly is a best-practice. We agree that forcing functions to declare
751 types while allowing for inference inside of function bodies is a wonderful
752 sweet spot between full inference and no inference.
754 What about returning a value? Here's a function that adds one to an integer:
757 fn add_one(x: int) -> int {
762 Rust functions return exactly one value, and you declare the type after an
763 'arrow', which is a dash (`-`) followed by a greater-than sign (`>`).
765 You'll note the lack of a semicolon here. If we added it in:
768 fn add_one(x: int) -> int {
773 We would get an error:
776 error: not all control paths return a value
777 fn add_one(x: int) -> int {
781 note: consider removing this semicolon:
786 Remember our earlier discussions about semicolons and `()`? Our function claims
787 to return an `int`, but with a semicolon, it would return `()` instead. Rust
788 realizes this probably isn't what we want, and suggests removing the semicolon.
790 This is very much like our `if` statement before: the result of the block
791 (`{}`) is the value of the expression. Other expression-oriented languages,
792 such as Ruby, work like this, but it's a bit unusual in the systems programming
793 world. When people first learn about this, they usually assume that it
794 introduces bugs. But because Rust's type system is so strong, and because unit
795 is its own unique type, we have never seen an issue where adding or removing a
796 semicolon in a return position would cause a bug.
798 But what about early returns? Rust does have a keyword for that, `return`:
801 fn foo(x: int) -> int {
802 if x < 5 { return x; }
808 Using a `return` as the last line of a function works, but is considered poor
812 fn foo(x: int) -> int {
813 if x < 5 { return x; }
819 There are some additional ways to define functions, but they involve features
820 that we haven't learned about yet, so let's just leave it at that for now.
825 Now that we have some functions, it's a good idea to learn about comments.
826 Comments are notes that you leave to other programmers to help explain things
827 about your code. The compiler mostly ignores them.
829 Rust has two kinds of comments that you should care about: **line comment**s
830 and **doc comment**s.
833 // Line comments are anything after '//' and extend to the end of the line.
835 let x = 5i; // this is also a line comment.
837 // If you have a long explanation for something, you can put line comments next
838 // to each other. Put a space between the // and your comment so that it's
842 The other kind of comment is a doc comment. Doc comments use `///` instead of
843 `//`, and support Markdown notation inside:
846 /// `hello` is a function that prints a greeting that is personalized based on
851 /// * `name` - The name of the person you'd like to greet.
856 /// let name = "Steve";
857 /// hello(name); // prints "Hello, Steve!"
859 fn hello(name: &str) {
860 println!("Hello, {}!", name);
864 When writing doc comments, adding sections for any arguments, return values,
865 and providing some examples of usage is very, very helpful.
867 You can use the `rustdoc` tool to generate HTML documentation from these doc
868 comments. We will talk more about `rustdoc` when we get to modules, as
869 generally, you want to export documentation for a full module.
871 # Compound Data Types
873 Rust, like many programming languages, has a number of different data types
874 that are built-in. You've already done some simple work with integers and
875 strings, but next, let's talk about some more complicated ways of storing data.
879 The first compound data type we're going to talk about are called **tuple**s.
880 Tuples are an ordered list of a fixed size. Like this:
883 let x = (1i, "hello");
886 The parenthesis and commas form this two-length tuple. Here's the same code, but
887 with the type annotated:
890 let x: (int, &str) = (1, "hello");
893 As you can see, the type of a tuple looks just like the tuple, but with each
894 position having a type name rather than the value. Careful readers will also
895 note that tuples are heterogeneous: we have an `int` and a `&str` in this tuple.
896 You haven't seen `&str` as a type before, and we'll discuss the details of
897 strings later. In systems programming languages, strings are a bit more complex
898 than in other languages. For now, just read `&str` as "a string slice," and
899 we'll learn more soon.
901 You can access the fields in a tuple through a **destructuring let**. Here's
905 let (x, y, z) = (1i, 2i, 3i);
907 println!("x is {}", x);
910 Remember before when I said the left hand side of a `let` statement was more
911 powerful than just assigning a binding? Here we are. We can put a pattern on
912 the left hand side of the `let`, and if it matches up to the right hand side,
913 we can assign multiple bindings at once. In this case, `let` 'destructures,'
914 or 'breaks up,' the tuple, and assigns the bits to three bindings.
916 This pattern is very powerful, and we'll see it repeated more later.
918 The last thing to say about tuples is that they are only equivalent if
919 the arity, types, and values are all identical.
922 let x = (1i, 2i, 3i);
923 let y = (2i, 3i, 4i);
932 This will print `no`, as the values aren't equal.
934 One other use of tuples is to return multiple values from a function:
937 fn next_two(x: int) -> (int, int) { (x + 1i, x + 2i) }
940 let (x, y) = next_two(5i);
941 println!("x, y = {}, {}", x, y);
945 Even though Rust functions can only return one value, a tuple _is_ one value,
946 that happens to be made up of two. You can also see in this example how you
947 can destructure a pattern returned by a function, as well.
949 Tuples are a very simple data structure, and so are not often what you want.
950 Let's move on to their bigger sibling, structs.
954 A struct is another form of a 'record type,' just like a tuple. There's a
955 difference: structs give each element that they contain a name, called a
956 'field' or a 'member.' Check it out:
965 let origin = Point { x: 0i, y: 0i };
967 println!("The origin is at ({}, {})", origin.x, origin.y);
971 There's a lot going on here, so let's break it down. We declare a struct with
972 the `struct` keyword, and then with a name. By convention, structs begin with a
973 capital letter and are also camel cased: `PointInSpace`, not `Point_In_Space`.
975 We can create an instance of our struct via `let`, as usual, but we use a `key:
976 value` style syntax to set each field. The order doesn't need to be the same as
977 in the original declaration.
979 Finally, because fields have names, we can access the field through dot
980 notation: `origin.x`.
982 The values in structs are immutable, like other bindings in Rust. However, you
983 can use `mut` to make them mutable:
992 let mut point = Point { x: 0i, y: 0i };
996 println!("The point is at ({}, {})", point.x, point.y);
1000 This will print `The point is at (5, 0)`.
1002 ## Tuple Structs and Newtypes
1004 Rust has another data type that's like a hybrid between a tuple and a struct,
1005 called a **tuple struct**. Tuple structs do have a name, but their fields
1010 struct Color(int, int, int);
1011 struct Point(int, int, int);
1014 These two will not be equal, even if they have the same values:
1017 let black = Color(0, 0, 0);
1018 let origin = Point(0, 0, 0);
1021 It is almost always better to use a struct than a tuple struct. We would write
1022 `Color` and `Point` like this instead:
1038 Now, we have actual names, rather than positions. Good names are important,
1039 and with a struct, we have actual names.
1041 There _is_ one case when a tuple struct is very useful, though, and that's a
1042 tuple struct with only one element. We call this a 'newtype,' because it lets
1043 you create a new type that's a synonym for another one:
1048 let length = Inches(10);
1050 let Inches(integer_length) = length;
1051 println!("length is {} inches", integer_length);
1054 As you can see here, you can extract the inner integer type through a
1055 destructuring `let`.
1059 Finally, Rust has a "sum type", an **enum**. Enums are an incredibly useful
1060 feature of Rust, and are used throughout the standard library. This is an enum
1061 that is provided by the Rust standard library:
1071 An `Ordering` can only be _one_ of `Less`, `Equal`, or `Greater` at any given
1072 time. Here's an example:
1075 fn cmp(a: int, b: int) -> Ordering {
1077 else if a > b { Greater }
1085 let ordering = cmp(x, y);
1087 if ordering == Less {
1089 } else if ordering == Greater {
1090 println!("greater");
1091 } else if ordering == Equal {
1097 `cmp` is a function that compares two things, and returns an `Ordering`. We
1098 return either `Less`, `Greater`, or `Equal`, depending on if the two values
1099 are greater, less, or equal.
1101 The `ordering` variable has the type `Ordering`, and so contains one of the
1102 three values. We can then do a bunch of `if`/`else` comparisons to check
1105 However, repeated `if`/`else` comparisons get quite tedious. Rust has a feature
1106 that not only makes them nicer to read, but also makes sure that you never
1107 miss a case. Before we get to that, though, let's talk about another kind of
1108 enum: one with values.
1110 This enum has two variants, one of which has a value:
1123 Value(n) => println!("x is {:d}", n),
1124 Missing => println!("x is missing!"),
1128 Value(n) => println!("y is {:d}", n),
1129 Missing => println!("y is missing!"),
1134 This enum represents an `int` that we may or may not have. In the `Missing`
1135 case, we have no value, but in the `Value` case, we do. This enum is specific
1136 to `int`s, though. We can make it usable by any type, but we haven't quite
1139 You can have any number of values in an enum:
1142 enum OptionalColor {
1143 Color(int, int, int),
1148 Enums with values are quite useful, but as I mentioned, they're even more
1149 useful when they're generic across types. But before we get to generics, let's
1150 talk about how to fix this big `if`/`else` statements we've been writing. We'll
1151 do that with `match`.
1155 Often, a simple `if`/`else` isn't enough, because you have more than two
1156 possible options. And `else` conditions can get incredibly complicated. So
1157 what's the solution?
1159 Rust has a keyword, `match`, that allows you to replace complicated `if`/`else`
1160 groupings with something more powerful. Check it out:
1166 1 => println!("one"),
1167 2 => println!("two"),
1168 3 => println!("three"),
1169 4 => println!("four"),
1170 5 => println!("five"),
1171 _ => println!("something else"),
1175 `match` takes an expression, and then branches based on its value. Each 'arm' of
1176 the branch is of the form `val => expression`. When the value matches, that arm's
1177 expression will be evaluated. It's called `match` because of the term 'pattern
1178 matching,' which `match` is an implementation of.
1180 So what's the big advantage here? Well, there are a few. First of all, `match`
1181 does 'exhaustiveness checking.' Do you see that last arm, the one with the
1182 underscore (`_`)? If we remove that arm, Rust will give us an error:
1185 error: non-exhaustive patterns: `_` not covered
1188 In other words, Rust is trying to tell us we forgot a value. Because `x` is an
1189 integer, Rust knows that it can have a number of different values. For example,
1190 `6i`. But without the `_`, there is no arm that could match, and so Rust refuses
1191 to compile. `_` is sort of like a catch-all arm. If none of the other arms match,
1192 the arm with `_` will. And since we have this catch-all arm, we now have an arm
1193 for every possible value of `x`, and so our program will now compile.
1195 `match` statements also destructure enums, as well. Remember this code from the
1199 fn cmp(a: int, b: int) -> Ordering {
1201 else if a > b { Greater }
1209 let ordering = cmp(x, y);
1211 if ordering == Less {
1213 } else if ordering == Greater {
1214 println!("greater");
1215 } else if ordering == Equal {
1221 We can re-write this as a `match`:
1224 fn cmp(a: int, b: int) -> Ordering {
1226 else if a > b { Greater }
1235 Less => println!("less"),
1236 Greater => println!("greater"),
1237 Equal => println!("equal"),
1242 This version has way less noise, and it also checks exhaustively to make sure
1243 that we have covered all possible variants of `Ordering`. With our `if`/`else`
1244 version, if we had forgotten the `Greater` case, for example, our program would
1245 have happily compiled. If we forget in the `match`, it will not. Rust helps us
1246 make sure to cover all of our bases.
1248 `match` is also an expression, which means we can use it on the right hand side
1249 of a `let` binding. We could also implement the previous line like this:
1252 fn cmp(a: int, b: int) -> Ordering {
1254 else if a > b { Greater }
1262 let result = match cmp(x, y) {
1264 Greater => "greater",
1268 println!("{}", result);
1272 In this case, it doesn't make a lot of sense, as we are just making a temporary
1273 string where we don't need to, but sometimes, it's a nice pattern.
1277 Looping is the last basic construct that we haven't learned yet in Rust. Rust has
1278 two main looping constructs: `for` and `while`.
1282 The `for` loop is used to loop a particular number of times. Rust's `for` loops
1283 work a bit differently than in other systems languages, however. Rust's `for`
1284 loop doesn't look like this C `for` loop:
1287 for (x = 0; x < 10; x++) {
1288 printf( "%d\n", x );
1295 for x in range(0i, 10i) {
1296 println!("{:d}", x);
1300 In slightly more abstract terms,
1303 for var in expression {
1308 The expression is an iterator, which we will discuss in more depth later in the
1309 guide. The iterator gives back a series of elements. Each element is one
1310 iteration of the loop. That value is then bound to the name `var`, which is
1311 valid for the loop body. Once the body is over, the next value is fetched from
1312 the iterator, and we loop another time. When there are no more values, the
1315 In our example, the `range` function is a function, provided by Rust, that
1316 takes a start and an end position, and gives an iterator over those values. The
1317 upper bound is exclusive, though, so our loop will print `0` through `9`, not
1320 Rust does not have the "C style" `for` loop on purpose. Manually controlling
1321 each element of the loop is complicated and error prone, even for experienced C
1324 We'll talk more about `for` when we cover **iterator**s, later in the Guide.
1328 The other kind of looping construct in Rust is the `while` loop. It looks like
1333 let mut done = false;
1338 if x % 5 == 0 { done = true; }
1342 `while` loops are the correct choice when you're not sure how many times
1345 If you need an infinite loop, you may be tempted to write this:
1351 Rust has a dedicated keyword, `loop`, to handle this case:
1357 Rust's control-flow analysis treats this construct differently than a
1358 `while true`, since we know that it will always loop. The details of what
1359 that _means_ aren't super important to understand at this stage, but in
1360 general, the more information we can give to the compiler, the better it
1361 can do with safety and code generation. So you should always prefer
1362 `loop` when you plan to loop infinitely.
1364 ## Ending iteration early
1366 Let's take a look at that `while` loop we had earlier:
1370 let mut done = false;
1375 if x % 5 == 0 { done = true; }
1379 We had to keep a dedicated `mut` boolean variable binding, `done`, to know
1380 when we should skip out of the loop. Rust has two keywords to help us with
1381 modifying iteration: `break` and `continue`.
1383 In this case, we can write the loop in a better way with `break`:
1391 if x % 5 == 0 { break; }
1395 We now loop forever with `loop`, and use `break` to break out early.
1397 `continue` is similar, but instead of ending the loop, goes to the next
1398 iteration: This will only print the odd numbers:
1401 for x in range(0i, 10i) {
1402 if x % 2 == 0 { continue; }
1404 println!("{:d}", x);
1408 Both `continue` and `break` are valid in both kinds of loops.
1412 Strings are an important concept for any programmer to master. Rust's string
1413 handling system is a bit different than in other languages, due to its systems
1414 focus. Any time you have a data structure of variable size, things can get
1415 tricky, and strings are a re-sizable data structure. That said, Rust's strings
1416 also work differently than in some other systems languages, such as C.
1418 Let's dig into the details. A **string** is a sequence of unicode scalar values
1419 encoded as a stream of UTF-8 bytes. All strings are guaranteed to be
1420 validly-encoded UTF-8 sequences. Additionally, strings are not null-terminated
1421 and can contain null bytes.
1423 Rust has two main types of strings: `&str` and `String`.
1425 The first kind is a `&str`. This is pronounced a 'string slice.' String literals
1426 are of the type `&str`:
1429 let string = "Hello there.";
1432 This string is statically allocated, meaning that it's saved inside our
1433 compiled program, and exists for the entire duration it runs. The `string`
1434 binding is a reference to this statically allocated string. String slices
1435 have a fixed size, and cannot be mutated.
1437 A `String`, on the other hand, is an in-memory string. This string is
1438 growable, and is also guaranteed to be UTF-8.
1441 let mut s = "Hello".to_string();
1444 s.push_str(", world.");
1448 You can coerce a `String` into a `&str` with the `as_slice()` method:
1451 fn takes_slice(slice: &str) {
1452 println!("Got: {}", slice);
1456 let s = "Hello".to_string();
1457 takes_slice(s.as_slice());
1461 To compare a String to a constant string, prefer `as_slice()`...
1464 fn compare(string: String) {
1465 if string.as_slice() == "Hello" {
1471 ... over `to_string()`:
1474 fn compare(string: String) {
1475 if string == "Hello".to_string() {
1481 Converting a `String` to a `&str` is cheap, but converting the `&str` to a
1482 `String` involves allocating memory. No reason to do that unless you have to!
1484 That's the basics of strings in Rust! They're probably a bit more complicated
1485 than you are used to, if you come from a scripting language, but when the
1486 low-level details matter, they really matter. Just remember that `String`s
1487 allocate memory and control their data, while `&str`s are a reference to
1488 another string, and you'll be all set.
1492 Like many programming languages, Rust has a list type for when you want a list
1493 of things. But similar to strings, Rust has different types to represent this
1494 idea: `Vec<T>` (a 'vector'), `[T, .. N]` (an 'array'), and `&[T]` (a 'slice').
1497 Vectors are similar to `String`s: they have a dynamic length, and they
1498 allocate enough memory to fit. You can create a vector with the `vec!` macro:
1501 let nums = vec![1i, 2i, 3i];
1504 Notice that unlike the `println!` macro we've used in the past, we use square
1505 brackets (`[]`) with `vec!`. Rust allows you to use either in either situation,
1506 this is just convention.
1508 You can create an array with just square brackets:
1511 let nums = [1i, 2i, 3i];
1514 So what's the difference? An array has a fixed size, so you can't add or
1518 let mut nums = vec![1i, 2i, 3i];
1519 nums.push(4i); // works
1521 let mut nums = [1i, 2i, 3i];
1522 nums.push(4i); // error: type `[int, .. 3]` does not implement any method
1523 // in scope named `push`
1526 The `push()` method lets you append a value to the end of the vector. But
1527 since arrays have fixed sizes, adding an element doesn't make any sense.
1528 You can see how it has the exact type in the error message: `[int, .. 3]`.
1529 An array of `int`s, with length 3.
1531 Similar to `&str`, a slice is a reference to another array. We can get a
1532 slice from a vector by using the `as_slice()` method:
1535 let vec = vec![1i, 2i, 3i];
1536 let slice = vec.as_slice();
1539 All three types implement an `iter()` method, which returns an iterator. We'll
1540 talk more about the details of iterators later, but for now, the `iter()` method
1541 allows you to write a `for` loop that prints out the contents of a vector, array,
1545 let vec = vec![1i, 2i, 3i];
1547 for i in vec.iter() {
1552 This code will print each number in order, on its own line.
1554 You can access a particular element of a vector, array, or slice by using
1555 **subscript notation**:
1558 let names = ["Graydon", "Brian", "Niko"];
1560 println!("The second name is: {}", names[1]);
1563 These subscripts start at zero, like in most programming languages, so the
1564 first name is `names[0]` and the second name is `names[1]`. The above example
1565 prints `The second name is Brian`.
1567 There's a whole lot more to vectors, but that's enough to get started. We have
1568 now learned all of the most basic Rust concepts. We're ready to start building
1569 our guessing game, but we need to know how to do one last thing first: get
1570 input from the keyboard. You can't have a guessing game without the ability to
1575 Getting input from the keyboard is pretty easy, but uses some things
1576 we haven't seen before. Here's a simple program that reads some input,
1577 and then prints it back out:
1581 println!("Type something!");
1583 let input = std::io::stdin().read_line().ok().expect("Failed to read line");
1585 println!("{}", input);
1589 Let's go over these chunks, one by one:
1595 This calls a function, `stdin()`, that lives inside the `std::io` module. As
1596 you can imagine, everything in `std` is provided by Rust, the 'standard
1597 library.' We'll talk more about the module system later.
1599 Since writing the fully qualified name all the time is annoying, we can use
1600 the `use` statement to import it in:
1608 However, it's considered better practice to not import individual functions, but
1609 to import the module, and only use one level of qualification:
1617 Let's update our example to use this style:
1623 println!("Type something!");
1625 let input = io::stdin().read_line().ok().expect("Failed to read line");
1627 println!("{}", input);
1637 The `read_line()` method can be called on the result of `stdin()` to return
1638 a full line of input. Nice and easy.
1641 .ok().expect("Failed to read line");
1644 Do you remember this code?
1657 Value(n) => println!("x is {:d}", n),
1658 Missing => println!("x is missing!"),
1662 Value(n) => println!("y is {:d}", n),
1663 Missing => println!("y is missing!"),
1668 We had to match each time, to see if we had a value or not. In this case,
1669 though, we _know_ that `x` has a `Value`. But `match` forces us to handle
1670 the `missing` case. This is what we want 99% of the time, but sometimes, we
1671 know better than the compiler.
1673 Likewise, `read_line()` does not return a line of input. It _might_ return a
1674 line of input. It might also fail to do so. This could happen if our program
1675 isn't running in a terminal, but as part of a cron job, or some other context
1676 where there's no standard input. Because of this, `read_line` returns a type
1677 very similar to our `OptionalInt`: an `IoResult<T>`. We haven't talked about
1678 `IoResult<T>` yet because it is the **generic** form of our `OptionalInt`.
1679 Until then, you can think of it as being the same thing, just for any type, not
1682 Rust provides a method on these `IoResult<T>`s called `ok()`, which does the
1683 same thing as our `match` statement, but assuming that we have a valid value.
1684 If we don't, it will terminate our program. In this case, if we can't get
1685 input, our program doesn't work, so we're okay with that. In most cases, we
1686 would want to handle the error case explicitly. The result of `ok()` has a
1687 method, `expect()`, which allows us to give an error message if this crash
1690 We will cover the exact details of how all of this works later in the Guide.
1691 For now, this gives you enough of a basic understanding to work with.
1693 Back to the code we were working on! Here's a refresher:
1699 println!("Type something!");
1701 let input = io::stdin().read_line().ok().expect("Failed to read line");
1703 println!("{}", input);
1707 With long lines like this, Rust gives you some flexibility with the whitespace.
1708 We _could_ write the example like this:
1714 println!("Type something!");
1716 let input = io::stdin()
1719 .expect("Failed to read line");
1721 println!("{}", input);
1725 Sometimes, this makes things more readable. Sometimes, less. Use your judgment
1728 That's all you need to get basic input from the standard input! It's not too
1729 complicated, but there are a number of small parts.
1733 Okay! We've got the basics of Rust down. Let's write a bigger program.
1735 For our first project, we'll implement a classic beginner programming problem:
1736 the guessing game. Here's how it works: Our program will generate a random
1737 integer between one and a hundred. It will then prompt us to enter a guess.
1738 Upon entering our guess, it will tell us if we're too low or too high. Once we
1739 guess correctly, it will congratulate us, and print the number of guesses we've
1740 taken to the screen. Sound good?
1744 Let's set up a new project. Go to your projects directory. Remember how we
1745 had to create our directory structure and a `Cargo.toml` for `hello_world`? Cargo
1746 has a command that does that for us. Let's give it a shot:
1750 $ cargo new guessing_game --bin
1754 We pass the name of our project to `cargo new`, and then the `--bin` flag,
1755 since we're making a binary, rather than a library.
1757 Check out the generated `Cargo.toml`:
1762 name = "guessing_game"
1764 authors = ["Your Name <you@example.com>"]
1767 Cargo gets this information from your environment. If it's not correct, go ahead
1770 Finally, Cargo generated a hello, world for us. Check out `src/main.rs`:
1774 println!("Hello, world!");
1778 Let's try compiling what Cargo gave us:
1782 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1785 Excellent! Open up your `src/main.rs` again. We'll be writing all of
1786 our code in this file. We'll talk about multiple-file projects later on in the
1789 Before we move on, let me show you one more Cargo command: `run`. `cargo run`
1790 is kind of like `cargo build`, but it also then runs the produced executable.
1795 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1796 Running `target/guessing_game`
1800 Great! The `run` command comes in handy when you need to rapidly iterate on a project.
1801 Our game is just such a project, we need to quickly test each iteration before moving on to the next one.
1803 ## Processing a Guess
1805 Let's get to it! The first thing we need to do for our guessing game is
1806 allow our player to input a guess. Put this in your `src/main.rs`:
1812 println!("Guess the number!");
1814 println!("Please input your guess.");
1816 let input = io::stdin().read_line()
1818 .expect("Failed to read line");
1820 println!("You guessed: {}", input);
1824 You've seen this code before, when we talked about standard input. We
1825 import the `std::io` module with `use`, and then our `main` function contains
1826 our program's logic. We print a little message announcing the game, ask the
1827 user to input a guess, get their input, and then print it out.
1829 Because we talked about this in the section on standard I/O, I won't go into
1830 more details here. If you need a refresher, go re-read that section.
1832 ## Generating a secret number
1834 Next, we need to generate a secret number. To do that, we need to use Rust's
1835 random number generation, which we haven't talked about yet. Rust includes a
1836 bunch of interesting functions in its standard library. If you need a bit of
1837 code, it's possible that it's already been written for you! In this case,
1838 we do know that Rust has random number generation, but we don't know how to
1841 Enter the docs. Rust has a page specifically to document the standard library.
1842 You can find that page [here](std/index.html). There's a lot of information on
1843 that page, but the best part is the search bar. Right up at the top, there's
1844 a box that you can enter in a search term. The search is pretty primitive
1845 right now, but is getting better all the time. If you type 'random' in that
1846 box, the page will update to [this
1847 one](http://doc.rust-lang.org/std/index.html?search=random). The very first
1849 [std::rand::random](http://doc.rust-lang.org/std/rand/fn.random.html). If we
1850 click on that result, we'll be taken to its documentation page.
1852 This page shows us a few things: the type signature of the function, some
1853 explanatory text, and then an example. Let's modify our code to add in the
1861 println!("Guess the number!");
1863 let secret_number = (rand::random() % 100i) + 1i;
1865 println!("The secret number is: {}", secret_number);
1867 println!("Please input your guess.");
1869 let input = io::stdin().read_line()
1871 .expect("Failed to read line");
1874 println!("You guessed: {}", input);
1878 The first thing we changed was to `use std::rand`, as the docs
1879 explained. We then added in a `let` expression to create a variable binding
1880 named `secret_number`, and we printed out its result.
1882 Also, you may wonder why we are using `%` on the result of `rand::random()`.
1883 This operator is called 'modulo', and it returns the remainder of a division.
1884 By taking the modulo of the result of `rand::random()`, we're limiting the
1885 values to be between 0 and 99. Then, we add one to the result, making it from 1
1886 to 100. Using modulo can give you a very, very small bias in the result, but
1887 for this example, it is not important.
1889 Let's try to compile this using `cargo build`:
1893 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1894 src/main.rs:7:26: 7:34 error: the type of this value must be known in this context
1895 src/main.rs:7 let secret_number = (rand::random() % 100i) + 1i;
1897 error: aborting due to previous error
1900 It didn't work! Rust says "the type of this value must be known in this
1901 context." What's up with that? Well, as it turns out, `rand::random()` can
1902 generate many kinds of random values, not just integers. And in this case, Rust
1903 isn't sure what kind of value `random()` should generate. So we have to help
1904 it. With number literals, we just add an `i` onto the end to tell Rust they're
1905 integers, but that does not work with functions. There's a different syntax,
1906 and it looks like this:
1909 rand::random::<int>();
1912 This says "please give me a random `int` value." We can change our code to use
1920 println!("Guess the number!");
1922 let secret_number = (rand::random::<int>() % 100i) + 1i;
1924 println!("The secret number is: {}", secret_number);
1926 println!("Please input your guess.");
1928 let input = io::stdin().read_line()
1930 .expect("Failed to read line");
1933 println!("You guessed: {}", input);
1937 Try running our new program a few times:
1941 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1942 Running `target/guessing_game`
1944 The secret number is: 7
1945 Please input your guess.
1948 $ ./target/guessing_game
1950 The secret number is: 83
1951 Please input your guess.
1954 $ ./target/guessing_game
1956 The secret number is: -29
1957 Please input your guess.
1962 Wait. Negative 29? We wanted a number between one and a hundred! We have two
1963 options here: we can either ask `random()` to generate an unsigned integer, which
1964 can only be positive, or we can use the `abs()` function. Let's go with the
1965 unsigned integer approach. If we want a random positive number, we should ask for
1966 a random positive number. Our code looks like this now:
1973 println!("Guess the number!");
1975 let secret_number = (rand::random::<uint>() % 100u) + 1u;
1977 println!("The secret number is: {}", secret_number);
1979 println!("Please input your guess.");
1981 let input = io::stdin().read_line()
1983 .expect("Failed to read line");
1986 println!("You guessed: {}", input);
1994 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1995 Running `target/guessing_game`
1997 The secret number is: 57
1998 Please input your guess.
2003 Great! Next up: let's compare our guess to the secret guess.
2005 ## Comparing guesses
2007 If you remember, earlier in the guide, we made a `cmp` function that compared
2008 two numbers. Let's add that in, along with a `match` statement to compare the
2009 guess to the secret guess:
2016 println!("Guess the number!");
2018 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2020 println!("The secret number is: {}", secret_number);
2022 println!("Please input your guess.");
2024 let input = io::stdin().read_line()
2026 .expect("Failed to read line");
2029 println!("You guessed: {}", input);
2031 match cmp(input, secret_number) {
2032 Less => println!("Too small!"),
2033 Greater => println!("Too big!"),
2034 Equal => { println!("You win!"); },
2038 fn cmp(a: int, b: int) -> Ordering {
2040 else if a > b { Greater }
2045 If we try to compile, we'll get some errors:
2049 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2050 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)
2051 src/main.rs:20 match cmp(input, secret_number) {
2053 src/main.rs:20:22: 20:35 error: mismatched types: expected `int` but found `uint` (expected int but found uint)
2054 src/main.rs:20 match cmp(input, secret_number) {
2056 error: aborting due to 2 previous errors
2059 This often happens when writing Rust programs, and is one of Rust's greatest
2060 strengths. You try out some code, see if it compiles, and Rust tells you that
2061 you've done something wrong. In this case, our `cmp` function works on integers,
2062 but we've given it unsigned integers. In this case, the fix is easy, because
2063 we wrote the `cmp` function! Let's change it to take `uint`s:
2070 println!("Guess the number!");
2072 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2074 println!("The secret number is: {}", secret_number);
2076 println!("Please input your guess.");
2078 let input = io::stdin().read_line()
2080 .expect("Failed to read line");
2083 println!("You guessed: {}", input);
2085 match cmp(input, secret_number) {
2086 Less => println!("Too small!"),
2087 Greater => println!("Too big!"),
2088 Equal => { println!("You win!"); },
2092 fn cmp(a: uint, b: uint) -> Ordering {
2094 else if a > b { Greater }
2099 And try compiling again:
2103 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2104 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)
2105 src/main.rs:20 match cmp(input, secret_number) {
2107 error: aborting due to previous error
2110 This error is similar to the last one: we expected to get a `uint`, but we got
2111 a `String` instead! That's because our `input` variable is coming from the
2112 standard input, and you can guess anything. Try it:
2115 $ ./target/guessing_game
2117 The secret number is: 73
2118 Please input your guess.
2123 Oops! Also, you'll note that we just ran our program even though it didn't compile.
2124 This works because the older version we did successfully compile was still lying
2125 around. Gotta be careful!
2127 Anyway, we have a `String`, but we need a `uint`. What to do? Well, there's
2128 a function for that:
2131 let input = io::stdin().read_line()
2133 .expect("Failed to read line");
2134 let input_num: Option<uint> = from_str(input.as_slice());
2137 The `from_str` function takes in a `&str` value and converts it into something.
2138 We tell it what kind of something with a type hint. Remember our type hint with
2139 `random()`? It looked like this:
2142 rand::random::<uint>();
2145 There's an alternate way of providing a hint too, and that's declaring the type
2149 let x: uint = rand::random();
2152 In this case, we say `x` is a `uint` explicitly, so Rust is able to properly
2153 tell `random()` what to generate. In a similar fashion, both of these work:
2156 let input_num = from_str::<Option<uint>>("5");
2157 let input_num: Option<uint> = from_str("5");
2160 In this case, I happen to prefer the latter, and in the `random()` case, I prefer
2161 the former. I think the nested `<>`s make the first option especially ugly and
2162 a bit harder to read.
2164 Anyway, with us now converting our input to a number, our code looks like this:
2171 println!("Guess the number!");
2173 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2175 println!("The secret number is: {}", secret_number);
2177 println!("Please input your guess.");
2179 let input = io::stdin().read_line()
2181 .expect("Failed to read line");
2182 let input_num: Option<uint> = from_str(input.as_slice());
2186 println!("You guessed: {}", input_num);
2188 match cmp(input_num, secret_number) {
2189 Less => println!("Too small!"),
2190 Greater => println!("Too big!"),
2191 Equal => { println!("You win!"); },
2195 fn cmp(a: uint, b: uint) -> Ordering {
2197 else if a > b { Greater }
2206 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2207 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)
2208 src/main.rs:22 match cmp(input_num, secret_number) {
2210 error: aborting due to previous error
2213 Oh yeah! Our `input_num` has the type `Option<uint>`, rather than `uint`. We
2214 need to unwrap the Option. If you remember from before, `match` is a great way
2215 to do that. Try this code:
2222 println!("Guess the number!");
2224 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2226 println!("The secret number is: {}", secret_number);
2228 println!("Please input your guess.");
2230 let input = io::stdin().read_line()
2232 .expect("Failed to read line");
2233 let input_num: Option<uint> = from_str(input.as_slice());
2235 let num = match input_num {
2238 println!("Please input a number!");
2244 println!("You guessed: {}", num);
2246 match cmp(num, secret_number) {
2247 Less => println!("Too small!"),
2248 Greater => println!("Too big!"),
2249 Equal => { println!("You win!"); },
2253 fn cmp(a: uint, b: uint) -> Ordering {
2255 else if a > b { Greater }
2260 We use a `match` to either give us the `uint` inside of the `Option`, or we
2261 print an error message and return. Let's give this a shot:
2265 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2266 Running `target/guessing_game`
2268 The secret number is: 17
2269 Please input your guess.
2271 Please input a number!
2274 Uh, what? But we did!
2276 ... actually, we didn't. See, when you get a line of input from `stdin()`,
2277 you get all the input. Including the `\n` character from you pressing Enter.
2278 So, `from_str()` sees the string `"5\n"` and says "nope, that's not a number,
2279 there's non-number stuff in there!" Luckily for us, `&str`s have an easy
2280 method we can use defined on them: `trim()`. One small modification, and our
2281 code looks like this:
2288 println!("Guess the number!");
2290 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2292 println!("The secret number is: {}", secret_number);
2294 println!("Please input your guess.");
2296 let input = io::stdin().read_line()
2298 .expect("Failed to read line");
2299 let input_num: Option<uint> = from_str(input.as_slice().trim());
2301 let num = match input_num {
2304 println!("Please input a number!");
2310 println!("You guessed: {}", num);
2312 match cmp(num, secret_number) {
2313 Less => println!("Too small!"),
2314 Greater => println!("Too big!"),
2315 Equal => { println!("You win!"); },
2319 fn cmp(a: uint, b: uint) -> Ordering {
2321 else if a > b { Greater }
2330 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2331 Running `target/guessing_game`
2333 The secret number is: 58
2334 Please input your guess.
2340 Nice! You can see I even added spaces before my guess, and it still figured
2341 out that I guessed 76. Run the program a few times, and verify that guessing
2342 the number works, as well as guessing a number too small.
2344 The Rust compiler helped us out quite a bit there! This technique is called
2345 "lean on the compiler," and it's often useful when working on some code. Let
2346 the error messages help guide you towards the correct types.
2348 Now we've got most of the game working, but we can only make one guess. Let's
2349 change that by adding loops!
2353 As we already discussed, the `loop` keyword gives us an infinite loop. So
2361 println!("Guess the number!");
2363 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2365 println!("The secret number is: {}", secret_number);
2369 println!("Please input your guess.");
2371 let input = io::stdin().read_line()
2373 .expect("Failed to read line");
2374 let input_num: Option<uint> = from_str(input.as_slice().trim());
2376 let num = match input_num {
2379 println!("Please input a number!");
2385 println!("You guessed: {}", num);
2387 match cmp(num, secret_number) {
2388 Less => println!("Too small!"),
2389 Greater => println!("Too big!"),
2390 Equal => { println!("You win!"); },
2395 fn cmp(a: uint, b: uint) -> Ordering {
2397 else if a > b { Greater }
2402 And try it out. But wait, didn't we just add an infinite loop? Yup. Remember
2403 that `return`? If we give a non-number answer, we'll `return` and quit. Observe:
2407 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2408 Running `target/guessing_game`
2410 The secret number is: 59
2411 Please input your guess.
2415 Please input your guess.
2419 Please input your guess.
2423 Please input your guess.
2425 Please input a number!
2428 Ha! `quit` actually quits. As does any other non-number input. Well, this is
2429 suboptimal to say the least. First, let's actually quit when you win the game:
2436 println!("Guess the number!");
2438 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2440 println!("The secret number is: {}", secret_number);
2444 println!("Please input your guess.");
2446 let input = io::stdin().read_line()
2448 .expect("Failed to read line");
2449 let input_num: Option<uint> = from_str(input.as_slice().trim());
2451 let num = match input_num {
2454 println!("Please input a number!");
2460 println!("You guessed: {}", num);
2462 match cmp(num, secret_number) {
2463 Less => println!("Too small!"),
2464 Greater => println!("Too big!"),
2466 println!("You win!");
2473 fn cmp(a: uint, b: uint) -> Ordering {
2475 else if a > b { Greater }
2480 By adding the `return` line after the `You win!`, we'll exit the program when
2481 we win. We have just one more tweak to make: when someone inputs a non-number,
2482 we don't want to quit, we just want to ignore it. Change that `return` to
2491 println!("Guess the number!");
2493 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2495 println!("The secret number is: {}", secret_number);
2499 println!("Please input your guess.");
2501 let input = io::stdin().read_line()
2503 .expect("Failed to read line");
2504 let input_num: Option<uint> = from_str(input.as_slice().trim());
2506 let num = match input_num {
2509 println!("Please input a number!");
2515 println!("You guessed: {}", num);
2517 match cmp(num, secret_number) {
2518 Less => println!("Too small!"),
2519 Greater => println!("Too big!"),
2521 println!("You win!");
2528 fn cmp(a: uint, b: uint) -> Ordering {
2530 else if a > b { Greater }
2535 Now we should be good! Let's try:
2539 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2540 Running `target/guessing_game`
2542 The secret number is: 61
2543 Please input your guess.
2547 Please input your guess.
2551 Please input your guess.
2553 Please input a number!
2554 Please input your guess.
2560 Awesome! With one tiny last tweak, we have finished the guessing game. Can you
2561 think of what it is? That's right, we don't want to print out the secret number.
2562 It was good for testing, but it kind of ruins the game. Here's our final source:
2569 println!("Guess the number!");
2571 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2575 println!("Please input your guess.");
2577 let input = io::stdin().read_line()
2579 .expect("Failed to read line");
2580 let input_num: Option<uint> = from_str(input.as_slice().trim());
2582 let num = match input_num {
2585 println!("Please input a number!");
2591 println!("You guessed: {}", num);
2593 match cmp(num, secret_number) {
2594 Less => println!("Too small!"),
2595 Greater => println!("Too big!"),
2597 println!("You win!");
2604 fn cmp(a: uint, b: uint) -> Ordering {
2606 else if a > b { Greater }
2613 At this point, you have successfully built the Guessing Game! Congratulations!
2615 You've now learned the basic syntax of Rust. All of this is relatively close to
2616 various other programming languages you have used in the past. These
2617 fundamental syntactical and semantic elements will form the foundation for the
2618 rest of your Rust education.
2620 Now that you're an expert at the basics, it's time to learn about some of
2621 Rust's more unique features.
2623 # Crates and Modules
2625 Rust features a strong module system, but it works a bit differently than in
2626 other programming languages. Rust's module system has two main components:
2627 **crate**s, and **module**s.
2629 A crate is Rust's unit of independent compilation. Rust always compiles one
2630 crate at a time, producing either a library or an executable. However, executables
2631 usually depend on libraries, and many libraries depend on other libraries as well.
2632 To support this, crates can depend on other crates.
2634 Each crate contains a hierarchy of modules. This tree starts off with a single
2635 module, called the **crate root**. Within the crate root, we can declare other
2636 modules, which can contain other modules, as deeply as you'd like.
2638 Note that we haven't mentioned anything about files yet. Rust does not impose a
2639 particular relationship between your filesystem structure and your module
2640 structure. That said, there is a conventional approach to how Rust looks for
2641 modules on the file system, but it's also overridable.
2643 Enough talk, let's build something! Let's make a new project called `modules`.
2647 $ cargo new modules --bin
2650 Let's double check our work by compiling:
2654 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2655 Running `target/modules`
2659 Excellent! So, we already have a single crate here: our `src/main.rs` is a crate.
2660 Everything in that file is in the crate root. A crate that generates an executable
2661 defines a `main` function inside its root, as we've done here.
2663 Let's define a new module inside our crate. Edit `src/main.rs` to look
2668 println!("Hello, world!");
2673 println!("Hello, world!");
2678 We now have a module named `hello` inside of our crate root. Modules use
2679 `snake_case` naming, like functions and variable bindings.
2681 Inside the `hello` module, we've defined a `print_hello` function. This will
2682 also print out our hello world message. Modules allow you to split up your
2683 program into nice neat boxes of functionality, grouping common things together,
2684 and keeping different things apart. It's kinda like having a set of shelves:
2685 a place for everything and everything in its place.
2687 To call our `print_hello` function, we use the double colon (`::`):
2690 hello::print_hello();
2693 You've seen this before, with `io::stdin()` and `rand::random()`. Now you know
2694 how to make your own. However, crates and modules have rules about
2695 **visibility**, which controls who exactly may use the functions defined in a
2696 given module. By default, everything in a module is private, which means that
2697 it can only be used by other functions in the same module. This will not
2702 hello::print_hello();
2707 println!("Hello, world!");
2715 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2716 src/main.rs:2:5: 2:23 error: function `print_hello` is private
2717 src/main.rs:2 hello::print_hello();
2721 To make it public, we use the `pub` keyword:
2725 hello::print_hello();
2729 pub fn print_hello() {
2730 println!("Hello, world!");
2739 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2740 Running `target/modules`
2744 Nice! There are more things we can do with modules, including moving them into
2745 their own files. This is enough detail for now.
2749 Traditionally, testing has not been a strong suit of most systems programming
2750 languages. Rust, however, has very basic testing built into the language
2751 itself. While automated testing cannot prove that your code is bug-free, it is
2752 useful for verifying that certain behaviors work as intended.
2754 Here's a very basic test:
2758 fn is_one_equal_to_one() {
2763 You may notice something new: that `#[test]`. Before we get into the mechanics
2764 of testing, let's talk about attributes.
2768 Rust's testing system uses **attribute**s to mark which functions are tests.
2769 Attributes can be placed on any Rust **item**. Remember how most things in
2770 Rust are an expression, but `let` is not? Item declarations are also not
2771 expressions. Here's a list of things that qualify as an item:
2782 You haven't learned about all of these things yet, but that's the list. As
2783 you can see, functions are at the top of it.
2785 Attributes can appear in three ways:
2787 1. A single identifier, the attribute name. `#[test]` is an example of this.
2788 2. An identifier followed by an equals sign (`=`) and a literal. `#[cfg=test]`
2789 is an example of this.
2790 3. An identifier followed by a parenthesized list of sub-attribute arguments.
2791 `#[cfg(unix, target_word_size = "32")]` is an example of this, where one of
2792 the sub-arguments is of the second kind.
2794 There are a number of different kinds of attributes, enough that we won't go
2795 over them all here. Before we talk about the testing-specific attributes, I
2796 want to call out one of the most important kinds of attributes: stability
2799 ## Stability attributes
2801 Rust provides six attributes to indicate the stability level of various
2802 parts of your library. The six levels are:
2804 * deprecated: This item should no longer be used. No guarantee of backwards
2806 * experimental: This item was only recently introduced or is otherwise in a
2807 state of flux. It may change significantly, or even be removed. No guarantee
2808 of backwards-compatibility.
2809 * unstable: This item is still under development, but requires more testing to
2810 be considered stable. No guarantee of backwards-compatibility.
2811 * stable: This item is considered stable, and will not change significantly.
2812 Guarantee of backwards-compatibility.
2813 * frozen: This item is very stable, and is unlikely to change. Guarantee of
2814 backwards-compatibility.
2815 * locked: This item will never change unless a serious bug is found. Guarantee
2816 of backwards-compatibility.
2818 All of Rust's standard library uses these attribute markers to communicate
2819 their relative stability, and you should use them in your code, as well.
2820 There's an associated attribute, `warn`, that allows you to warn when you
2821 import an item marked with certain levels: deprecated, experimental and
2822 unstable. For now, only deprecated warns by default, but this will change once
2823 the standard library has been stabilized.
2825 You can use the `warn` attribute like this:
2831 And later, when you import a crate:
2834 extern crate some_crate;
2837 You'll get a warning if you use something marked unstable.
2839 You may have noticed an exclamation point in the `warn` attribute declaration.
2840 The `!` in this attribute means that this attribute applies to the enclosing
2841 item, rather than to the item that follows the attribute. So this `warn`
2842 attribute declaration applies to the enclosing crate itself, rather than
2843 to whatever item statement follows it:
2846 // applies to the crate we're in
2849 extern crate some_crate;
2851 // applies to the following `fn`.
2860 Let's write a very simple crate in a test-driven manner. You know the drill by
2861 now: make a new project:
2865 $ cargo new testing --bin
2873 Compiling testing v0.0.1 (file:///home/you/projects/testing)
2874 Running `target/testing`
2878 Great. Rust's infrastructure supports tests in two sorts of places, and they're
2879 for two kinds of tests: you include **unit test**s inside of the crate itself,
2880 and you place **integration test**s inside a `tests` directory. "Unit tests"
2881 are small tests that test one focused unit, "integration tests" tests multiple
2882 units in integration. That said, this is a social convention, they're no different
2883 in syntax. Let's make a `tests` directory:
2889 Next, let's create an integration test in `tests/lib.rs`:
2898 It doesn't matter what you name your test functions, though it's nice if
2899 you give them descriptive names. You'll see why in a moment. We then use a
2900 macro, `assert!`, to assert that something is true. In this case, we're giving
2901 it `false`, so this test should fail. Let's try it!
2905 Compiling testing v0.0.1 (file:///home/you/projects/testing)
2906 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
2907 /home/you/projects/testing/src/main.rs:1 fn main() {
2908 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
2909 /home/you/projects/testing/src/main.rs:3 }
2913 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
2921 ---- foo stdout ----
2922 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
2929 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
2931 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
2934 Lots of output! Let's break this down:
2938 Compiling testing v0.0.1 (file:///home/you/projects/testing)
2941 You can run all of your tests with `cargo test`. This runs both your tests in
2942 `tests`, as well as the tests you put inside of your crate.
2945 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
2946 /home/you/projects/testing/src/main.rs:1 fn main() {
2947 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
2948 /home/you/projects/testing/src/main.rs:3 }
2951 Rust has a **lint** called 'warn on dead code' used by default. A lint is a
2952 bit of code that checks your code, and can tell you things about it. In this
2953 case, Rust is warning us that we've written some code that's never used: our
2954 `main` function. Of course, since we're running tests, we don't use `main`.
2955 We'll turn this lint off for just this function soon. For now, just ignore this
2961 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
2964 Wait a minute, zero tests? Didn't we define one? Yup. This output is from
2965 attempting to run the tests in our crate, of which we don't have any.
2966 You'll note that Rust reports on several kinds of tests: passed, failed,
2967 ignored, and measured. The 'measured' tests refer to benchmark tests, which
2968 we'll cover soon enough!
2975 Now we're getting somewhere. Remember when we talked about naming our tests
2976 with good names? This is why. Here, it says 'test foo' because we called our
2977 test 'foo.' If we had given it a good name, it'd be more clear which test
2978 failed, especially as we accumulate more tests.
2983 ---- foo stdout ----
2984 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
2991 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
2993 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
2996 After all the tests run, Rust will show us any output from our failed tests.
2997 In this instance, Rust tells us that our assertion failed, with false. This was
3000 Whew! Let's fix our test:
3009 And then try to run our tests again:
3013 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3014 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3015 /home/you/projects/testing/src/main.rs:1 fn main() {
3016 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3017 /home/you/projects/testing/src/main.rs:3 }
3021 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3027 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3030 Nice! Our test passes, as we expected. Let's get rid of that warning for our `main`
3031 function. Change your `src/main.rs` to look like this:
3036 println!("Hello, world");
3040 This attribute combines two things: `cfg` and `not`. The `cfg` attribute allows
3041 you to conditionally compile code based on something. The following item will
3042 only be compiled if the configuration says it's true. And when Cargo compiles
3043 our tests, it sets things up so that `cfg(test)` is true. But we want to only
3044 include `main` when it's _not_ true. So we use `not` to negate things:
3045 `cfg(not(test))` will only compile our code when the `cfg(test)` is false.
3047 With this attribute, we won't get the warning:
3051 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3055 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3061 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3064 Nice. Okay, let's write a real test now. Change your `tests/lib.rs`
3069 fn math_checks_out() {
3070 let result = add_three_times_four(5i);
3072 assert_eq!(32i, result);
3076 And try to run the test:
3080 Compiling testing v0.0.1 (file:///home/youg/projects/testing)
3081 /home/youg/projects/testing/tests/lib.rs:3:18: 3:38 error: unresolved name `add_three_times_four`.
3082 /home/youg/projects/testing/tests/lib.rs:3 let result = add_three_times_four(5i);
3083 ^~~~~~~~~~~~~~~~~~~~
3084 error: aborting due to previous error
3085 Build failed, waiting for other jobs to finish...
3086 Could not compile `testing`.
3088 To learn more, run the command again with --verbose.
3091 Rust can't find this function. That makes sense, as we didn't write it yet!
3093 In order to share this code with our tests, we'll need to make a library crate.
3094 This is also just good software design: as we mentioned before, it's a good idea
3095 to put most of your functionality into a library crate, and have your executable
3096 crate use that library. This allows for code re-use.
3098 To do that, we'll need to make a new module. Make a new file, `src/lib.rs`,
3103 pub fn add_three_times_four(x: int) -> int {
3108 We're calling this file `lib.rs` because it has the same name as our project,
3109 and so it's named this, by convention.
3111 We'll then need to use this crate in our `src/main.rs`:
3114 extern crate testing;
3118 println!("Hello, world");
3122 Finally, let's import this function in our `tests/lib.rs`:
3125 extern crate testing;
3126 use testing::add_three_times_four;
3129 fn math_checks_out() {
3130 let result = add_three_times_four(5i);
3132 assert_eq!(32i, result);
3136 Let's give it a run:
3140 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3144 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3149 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3153 test math_checks_out ... ok
3155 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3158 Great! One test passed. We've got an integration test showing that our public
3159 method works, but maybe we want to test some of the internal logic as well.
3160 While this function is simple, if it were more complicated, you can imagine
3161 we'd need more tests. So let's break it up into two helper functions, and
3162 write some unit tests to test those.
3164 Change your `src/lib.rs` to look like this:
3167 pub fn add_three_times_four(x: int) -> int {
3168 times_four(add_three(x))
3171 fn add_three(x: int) -> int { x + 3 }
3173 fn times_four(x: int) -> int { x * 4 }
3176 If you run `cargo test`, you should get the same output:
3180 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3184 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3189 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3193 test math_checks_out ... ok
3195 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3198 If we tried to write a test for these two new functions, it wouldn't
3202 extern crate testing;
3203 use testing::add_three_times_four;
3204 use testing::add_three;
3207 fn math_checks_out() {
3208 let result = add_three_times_four(5i);
3210 assert_eq!(32i, result);
3214 fn test_add_three() {
3215 let result = add_three(5i);
3217 assert_eq!(8i, result);
3221 We'd get this error:
3224 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3225 /home/you/projects/testing/tests/lib.rs:3:5: 3:24 error: function `add_three` is private
3226 /home/you/projects/testing/tests/lib.rs:3 use testing::add_three;
3230 Right. It's private. So external, integration tests won't work. We need a
3231 unit test. Open up your `src/lib.rs` and add this:
3234 pub fn add_three_times_four(x: int) -> int {
3235 times_four(add_three(x))
3238 fn add_three(x: int) -> int { x + 3 }
3240 fn times_four(x: int) -> int { x * 4 }
3244 use super::add_three;
3245 use super::times_four;
3248 fn test_add_three() {
3249 let result = add_three(5i);
3251 assert_eq!(8i, result);
3255 fn test_times_four() {
3256 let result = times_four(5i);
3258 assert_eq!(20i, result);
3263 Let's give it a shot:
3267 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3270 test test::test_times_four ... ok
3271 test test::test_add_three ... ok
3273 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3278 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3282 test math_checks_out ... ok
3284 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3287 Cool! We now have two tests of our internal functions. You'll note that there
3288 are three sets of output now: one for `src/main.rs`, one for `src/lib.rs`, and
3289 one for `tests/lib.rs`. There's one interesting thing that we haven't talked
3290 about yet, and that's these lines:
3293 use super::add_three;
3294 use super::times_four;
3297 Because we've made a nested module, we can import functions from the parent
3298 module by using `super`. Sub-modules are allowed to 'see' private functions in
3299 the parent. We sometimes call this usage of `use` a 're-export,' because we're
3300 exporting the name again, somewhere else.
3302 We've now covered the basics of testing. Rust's tools are primitive, but they
3303 work well in the simple cases. There are some Rustaceans working on building
3304 more complicated frameworks on top of all of this, but they're just starting
3309 In systems programming, pointers are an incredibly important topic. Rust has a
3310 very rich set of pointers, and they operate differently than in many other
3311 languages. They are important enough that we have a specific [Pointer
3312 Guide](guide-pointers.html) that goes into pointers in much detail. In fact,
3313 while you're currently reading this guide, which covers the language in broad
3314 overview, there are a number of other guides that put a specific topic under a
3315 microscope. You can find the list of guides on the [documentation index
3316 page](index.html#guides).
3318 In this section, we'll assume that you're familiar with pointers as a general
3319 concept. If you aren't, please read the [introduction to
3320 pointers](guide-pointers.html#an-introduction) section of the Pointer Guide,
3321 and then come back here. We'll wait.
3323 Got the gist? Great. Let's talk about pointers in Rust.
3327 The most primitive form of pointer in Rust is called a **reference**.
3328 References are created using the ampersand (`&`). Here's a simple
3336 `y` is a reference to `x`. To dereference (get the value being referred to
3337 rather than the reference itself) `y`, we use the asterisk (`*`):
3346 Like any `let` binding, references are immutable by default.
3348 You can declare that functions take a reference:
3351 fn add_one(x: &int) -> int { *x + 1 }
3354 assert_eq!(6, add_one(&5));
3358 As you can see, we can make a reference from a literal by applying `&` as well.
3359 Of course, in this simple function, there's not a lot of reason to take `x` by
3360 reference. It's just an example of the syntax.
3362 Because references are immutable, you can have multiple references that
3363 **alias** (point to the same place):
3371 We can make a mutable reference by using `&mut` instead of `&`:
3378 Note that `x` must also be mutable. If it isn't, like this:
3388 6:19 error: cannot borrow immutable local variable `x` as mutable
3393 We don't want a mutable reference to immutable data! This error message uses a
3394 term we haven't talked about yet, 'borrow.' We'll get to that in just a moment.
3396 This simple example actually illustrates a lot of Rust's power: Rust has
3397 prevented us, at compile time, from breaking our own rules. Because Rust's
3398 references check these kinds of rules entirely at compile time, there's no
3399 runtime overhead for this safety. At runtime, these are the same as a raw
3400 machine pointer, like in C or C++. We've just double-checked ahead of time
3401 that we haven't done anything dangerous.
3403 Rust will also prevent us from creating two mutable references that alias.
3412 It gives us this error:
3415 error: cannot borrow `x` as mutable more than once at a time
3418 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3421 note: previous borrow ends here
3430 This is a big error message. Let's dig into it for a moment. There are three
3431 parts: the error and two notes. The error says what we expected, we cannot have
3432 two pointers that point to the same memory.
3434 The two notes give some extra context. Rust's error messages often contain this
3435 kind of extra information when the error is complex. Rust is telling us two
3436 things: first, that the reason we cannot **borrow** `x` as `z` is that we
3437 previously borrowed `x` as `y`. The second note shows where `y`'s borrowing
3442 In order to truly understand this error, we have to learn a few new concepts:
3443 **ownership**, **borrowing**, and **lifetimes**.
3445 ## Ownership, borrowing, and lifetimes
3447 Whenever a resource of some kind is created, something must be responsible
3448 for destroying that resource as well. Given that we're discussing pointers
3449 right now, let's discuss this in the context of memory allocation, though
3450 it applies to other resources as well.
3452 When you allocate heap memory, you need a mechanism to free that memory. Many
3453 languages let the programmer control the allocation, and then use a garbage
3454 collector to handle the deallocation. This is a valid, time-tested strategy,
3455 but it's not without its drawbacks. Because the programmer does not have to
3456 think as much about deallocation, allocation becomes something commonplace,
3457 because it's easy. And if you need precise control over when something is
3458 deallocated, leaving it up to your runtime can make this difficult.
3460 Rust chooses a different path, and that path is called **ownership**. Any
3461 binding that creates a resource is the **owner** of that resource.
3463 Being an owner affords you some privileges:
3465 1. You control when that resource is deallocated.
3466 2. You may lend that resource, immutably, to as many borrowers as you'd like.
3467 3. You may lend that resource, mutably, to a single borrower.
3469 But it also comes with some restrictions:
3471 1. If someone is borrowing your resource (either mutably or immutably), you may
3472 not mutate the resource or mutably lend it to someone.
3473 2. If someone is mutably borrowing your resource, you may not lend it out at
3474 all (mutably or immutably) or access it in any way.
3476 What's up with all this 'lending' and 'borrowing'? When you allocate memory,
3477 you get a pointer to that memory. This pointer allows you to manipulate said
3478 memory. If you are the owner of a pointer, then you may allow another
3479 binding to temporarily borrow that pointer, and then they can manipulate the
3480 memory. The length of time that the borrower is borrowing the pointer
3481 from you is called a **lifetime**.
3483 If two distinct bindings share a pointer, and the memory that pointer points to
3484 is immutable, then there are no problems. But if it's mutable, both pointers
3485 can attempt to write to the memory at the same time, causing a **race
3486 condition**. Therefore, if someone wants to mutate something that they've
3487 borrowed from you, you must not have lent out that pointer to anyone else.
3489 Rust has a sophisticated system called the **borrow checker** to make sure that
3490 everyone plays by these rules. At compile time, it verifies that none of these
3491 rules are broken. If there's no problem, our program compiles successfully, and
3492 there is no runtime overhead for any of this. The borrow checker works only at
3493 compile time. If the borrow checker did find a problem, it will report a
3494 **lifetime error**, and your program will refuse to compile.
3496 That's a lot to take in. It's also one of the _most_ important concepts in
3497 all of Rust. Let's see this syntax in action:
3501 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3503 // other code here...
3505 } // privilege 1: when x goes out of scope, this memory is deallocated
3507 /// this function borrows an integer. It's given back automatically when the
3508 /// function returns.
3509 fn foo(x: &int) -> &int { x }
3512 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3514 // privilege 2: you may lend that resource, to as many borrowers as you'd like
3518 foo(&x); // functions can borrow too!
3520 let a = &x; // we can do this alllllll day!
3524 let mut x = 5i; // x is the owner of this integer, which is memory on the stack.
3526 let y = &mut x; // privilege 3: you may lend that resource to a single borrower,
3531 If you are a borrower, you get a few privileges as well, but must also obey a
3534 1. If the borrow is immutable, you may read the data the pointer points to.
3535 2. If the borrow is mutable, you may read and write the data the pointer points to.
3536 3. You may lend the pointer to someone else in an immutable fashion, **BUT**
3537 4. When you do so, they must return it to you before you must give your own
3540 This last requirement can seem odd, but it also makes sense. If you have to
3541 return something, and you've lent it to someone, they need to give it back to
3542 you for you to give it back! If we didn't, then the owner could deallocate
3543 the memory, and the person we've loaned it out to would have a pointer to
3544 invalid memory. This is called a 'dangling pointer.'
3546 Let's re-examine the error that led us to talk about all of this, which was a
3547 violation of the restrictions placed on owners who lend something out mutably.
3559 error: cannot borrow `x` as mutable more than once at a time
3562 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3565 note: previous borrow ends here
3574 This error comes in three parts. Let's go over each in turn.
3577 error: cannot borrow `x` as mutable more than once at a time
3582 This error states the restriction: you cannot lend out something mutable more
3583 than once at the same time. The borrow checker knows the rules!
3586 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3591 Some compiler errors come with notes to help you fix the error. This error comes
3592 with two notes, and this is the first. This note informs us of exactly where
3593 the first mutable borrow occurred. The error showed us the second. So now we
3594 see both parts of the problem. It also alludes to rule #3, by reminding us that
3595 we can't change `x` until the borrow is over.
3598 note: previous borrow ends here
3607 Here's the second note, which lets us know where the first borrow would be over.
3608 This is useful, because if we wait to try to borrow `x` after this borrow is
3609 over, then everything will work.
3611 For more advanced patterns, please consult the [Lifetime
3612 Guide](guide-lifetimes.html). You'll also learn what this type signature with
3616 pub fn as_maybe_owned(&self) -> MaybeOwned<'a> { ... }
3621 All of our references so far have been to variables we've created on the stack.
3622 In Rust, the simplest way to allocate heap variables is using a *box*. To
3623 create a box, use the `box` keyword:
3629 This allocates an integer `5` on the heap, and creates a binding `x` that
3630 refers to it.. The great thing about boxed pointers is that we don't have to
3631 manually free this allocation! If we write
3640 then Rust will automatically free `x` at the end of the block. This isn't
3641 because Rust has a garbage collector -- it doesn't. Instead, when `x` goes out
3642 of scope, Rust `free`s `x`. This Rust code will do the same thing as the
3647 int *x = (int *)malloc(sizeof(int));
3653 This means we get the benefits of manual memory management, but the compiler
3654 ensures that we don't do something wrong. We can't forget to `free` our memory.
3656 Boxes are the sole owner of their contents, so you cannot take a mutable
3657 reference to them and then use the original box:
3663 *x; // you might expect 5, but this is actually an error
3666 This gives us this error:
3669 8:7 error: cannot use `*x` because it was mutably borrowed
3672 6:19 note: borrow of `x` occurs here
3677 As long as `y` is borrowing the contents, we cannot use `x`. After `y` is
3678 done borrowing the value, we can use it again. This works fine:
3685 } // y goes out of scope at the end of the block
3692 Sometimes, you need to allocate something on the heap, but give out multiple
3693 references to the memory. Rust's `Rc<T>` (pronounced 'arr cee tee') and
3694 `Arc<T>` types (again, the `T` is for generics, we'll learn more later) provide
3695 you with this ability. **Rc** stands for 'reference counted,' and **Arc** for
3696 'atomically reference counted.' This is how Rust keeps track of the multiple
3697 owners: every time we make a new reference to the `Rc<T>`, we add one to its
3698 internal 'reference count.' Every time a reference goes out of scope, we
3699 subtract one from the count. When the count is zero, the `Rc<T>` can be safely
3700 deallocated. `Arc<T>` is almost identical to `Rc<T>`, except for one thing: The
3701 'atomically' in 'Arc' means that increasing and decreasing the count uses a
3702 thread-safe mechanism to do so. Why two types? `Rc<T>` is faster, so if you're
3703 not in a multi-threaded scenario, you can have that advantage. Since we haven't
3704 talked about threading yet in Rust, we'll show you `Rc<T>` for the rest of this
3707 To create an `Rc<T>`, use `Rc::new()`:
3712 let x = Rc::new(5i);
3715 To create a second reference, use the `.clone()` method:
3720 let x = Rc::new(5i);
3724 The `Rc<T>` will live as long as any of its references are alive. After they
3725 all go out of scope, the memory will be `free`d.
3727 If you use `Rc<T>` or `Arc<T>`, you have to be careful about introducing
3728 cycles. If you have two `Rc<T>`s that point to each other, the reference counts
3729 will never drop to zero, and you'll have a memory leak. To learn more, check
3730 out [the section on `Rc<T>` and `Arc<T>` in the pointers
3731 guide](http://doc.rust-lang.org/guide-pointers.html#rc-and-arc).
3735 We've made use of patterns a few times in the guide: first with `let` bindings,
3736 then with `match` statements. Let's go on a whirlwind tour of all of the things
3739 A quick refresher: you can match against literals directly, and `_` acts as an
3746 1 => println!("one"),
3747 2 => println!("two"),
3748 3 => println!("three"),
3749 _ => println!("anything"),
3753 You can match multiple patterns with `|`:
3759 1 | 2 => println!("one or two"),
3760 3 => println!("three"),
3761 _ => println!("anything"),
3765 You can match a range of values with `..`:
3771 1 .. 5 => println!("one through five"),
3772 _ => println!("anything"),
3776 Ranges are mostly used with integers and single characters.
3778 If you're matching multiple things, via a `|` or a `..`, you can bind
3779 the value to a name with `@`:
3785 x @ 1 .. 5 => println!("got {}", x),
3786 _ => println!("anything"),
3790 If you're matching on an enum which has variants, you can use `..` to
3791 ignore the value in the variant:
3802 Value(..) => println!("Got an int!"),
3803 Missing => println!("No such luck."),
3807 You can introduce **match guards** with `if`:
3818 Value(x) if x > 5 => println!("Got an int bigger than five!"),
3819 Value(..) => println!("Got an int!"),
3820 Missing => println!("No such luck."),
3824 If you're matching on a pointer, you can use the same syntax as you declared it
3831 &x => println!("Got a value: {}", x),
3835 Here, the `x` inside the `match` has type `int`. In other words, the left hand
3836 side of the pattern destructures the value. If we have `&5i`, then in `&x`, `x`
3839 If you want to get a reference, use the `ref` keyword:
3845 ref x => println!("Got a reference to {}", x),
3849 Here, the `x` inside the `match` has the type `&int`. In other words, the `ref`
3850 keyword _creates_ a reference, for use in the pattern. If you need a mutable
3851 reference, `ref mut` will work in the same way:
3857 ref mut x => println!("Got a mutable reference to {}", x),
3861 If you have a struct, you can destructure it inside of a pattern:
3869 let origin = Point { x: 0i, y: 0i };
3872 Point { x: x, y: y } => println!("({},{})", x, y),
3876 If we only care about some of the values, we don't have to give them all names:
3884 let origin = Point { x: 0i, y: 0i };
3887 Point { x: x, .. } => println!("x is {}", x),
3891 Whew! That's a lot of different ways to match things, and they can all be
3892 mixed and matched, depending on what you're doing:
3896 Foo { x: Some(ref name), y: None } => ...
3900 Patterns are very powerful. Make good use of them.
3904 Functions are great, but if you want to call a bunch of them on some data, it
3905 can be awkward. Consider this code:
3911 We would read this left-to right, and so we see 'baz bar foo.' But this isn't the
3912 order that the functions would get called in, that's inside-out: 'foo bar baz.'
3913 Wouldn't it be nice if we could do this instead?
3916 x.foo().bar().baz();
3919 Luckily, as you may have guessed with the leading question, you can! Rust provides
3920 the ability to use this **method call syntax** via the `impl` keyword.
3922 Here's how it works:
3932 fn area(&self) -> f64 {
3933 std::f64::consts::PI * (self.radius * self.radius)
3938 let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
3939 println!("{}", c.area());
3943 This will print `12.566371`.
3945 We've made a struct that represents a circle. We then write an `impl` block,
3946 and inside it, define a method, `area`. Methods take a special first
3947 parameter, `&self`. There are three variants: `self`, `&self`, and `&mut self`.
3948 You can think of this first parameter as being the `x` in `x.foo()`. The three
3949 variants correspond to the three kinds of thing `x` could be: `self` if it's
3950 just a value on the stack, `&self` if it's a reference, and `&mut self` if it's
3951 a mutable reference. We should default to using `&self`, as it's the most
3954 Finally, as you may remember, the value of the area of a circle is `π*r²`.
3955 Because we took the `&self` parameter to `area`, we can use it just like any
3956 other parameter. Because we know it's a `Circle`, we can access the `radius`
3957 just like we would with any other struct. An import of π and some
3958 multiplications later, and we have our area.
3960 You can also define methods that do not take a `self` parameter. Here's a
3961 pattern that's very common in Rust code:
3971 fn new(x: f64, y: f64, radius: f64) -> Circle {
3981 let c = Circle::new(0.0, 0.0, 2.0);
3985 This **static method** builds a new `Circle` for us. Note that static methods
3986 are called with the `Struct::method()` syntax, rather than the `ref.method()`
3991 So far, we've made lots of functions in Rust. But we've given them all names.
3992 Rust also allows us to create anonymous functions too. Rust's anonymous
3993 functions are called **closure**s. By themselves, closures aren't all that
3994 interesting, but when you combine them with functions that take closures as
3995 arguments, really powerful things are possible.
3997 Let's make a closure:
4000 let add_one = |x| { 1i + x };
4002 println!("The 5 plus 1 is {}.", add_one(5i));
4005 We create a closure using the `|...| { ... }` syntax, and then we create a
4006 binding so we can use it later. Note that we call the function using the
4007 binding name and two parentheses, just like we would for a named function.
4009 Let's compare syntax. The two are pretty close:
4012 let add_one = |x: int| -> int { 1i + x };
4013 fn add_one (x: int) -> int { 1i + x }
4016 As you may have noticed, closures infer their argument and return types, so you
4017 don't need to declare one. This is different from named functions, which
4018 default to returning unit (`()`).
4020 There's one big difference between a closure and named functions, and it's in
4021 the name: a closure "closes over its environment." What's that mean? It means
4028 let printer = || { println!("x is: {}", x); };
4030 printer(); // prints "x is: 5"
4034 The `||` syntax means this is an anonymous closure that takes no arguments.
4035 Without it, we'd just have a block of code in `{}`s.
4037 In other words, a closure has access to variables in the scope that it's
4038 defined. The closure borrows any variables that it uses. This will error:
4044 let printer = || { println!("x is: {}", x); };
4046 x = 6i; // error: cannot assign to `x` because it is borrowed
4052 Rust has a second type of closure, called a **proc**. Procs are created
4053 with the `proc` keyword:
4058 let p = proc() { x * x };
4059 println!("{}", p()); // prints 25
4062 Procs have a big difference from closures: they may only be called once. This
4063 will error when we try to compile:
4068 let p = proc() { x * x };
4069 println!("{}", p());
4070 println!("{}", p()); // error: use of moved value `p`
4073 This restriction is important. Procs are allowed to consume values that they
4074 capture, and thus have to be restricted to being called once for soundness
4075 reasons: any value consumed would be invalid on a second call.
4077 Procs are most useful with Rust's concurrency features, and so we'll just leave
4078 it at this for now. We'll talk about them more in the "Tasks" section of the
4081 ## Accepting closures as arguments
4083 Closures are most useful as an argument to another function. Here's an example:
4086 fn twice(x: int, f: |int| -> int) -> int {
4091 let square = |x: int| { x * x };
4093 twice(5i, square); // evaluates to 50
4097 Let's break example down, starting with `main`:
4100 let square = |x: int| { x * x };
4103 We've seen this before. We make a closure that takes an integer, and returns
4107 twice(5i, square); // evaluates to 50
4110 This line is more interesting. Here, we call our function, `twice`, and we pass
4111 it two arguments: an integer, `5`, and our closure, `square`. This is just like
4112 passing any other two variable bindings to a function, but if you've never
4113 worked with closures before, it can seem a little complex. Just think: "I'm
4114 passing two variables, one is an int, and one is a function."
4116 Next, let's look at how `twice` is defined:
4119 fn twice(x: int, f: |int| -> int) -> int {
4122 `twice` takes two arguments, `x` and `f`. That's why we called it with two
4123 arguments. `x` is an `int`, we've done that a ton of times. `f` is a function,
4124 though, and that function takes an `int` and returns an `int`. Notice
4125 how the `|int| -> int` syntax looks a lot like our definition of `square`
4126 above, if we added the return type in:
4129 let square = |x: int| -> int { x * x };
4133 This function takes an `int` and returns an `int`.
4135 This is the most complicated function signature we've seen yet! Give it a read
4136 a few times until you can see how it works. It takes a teeny bit of practice, and
4139 Finally, `twice` returns an `int` as well.
4141 Okay, let's look at the body of `twice`:
4144 fn twice(x: int, f: |int| -> int) -> int {
4149 Since our closure is named `f`, we can call it just like we called our closures
4150 before. And we pass in our `x` argument to each one. Hence 'twice.'
4152 If you do the math, `(5 * 5) + (5 * 5) == 50`, so that's the output we get.
4154 Play around with this concept until you're comfortable with it. Rust's standard
4155 library uses lots of closures, where appropriate, so you'll be using
4156 this technique a lot.
4158 If we didn't want to give `square` a name, we could also just define it inline.
4159 This example is the same as the previous one:
4162 fn twice(x: int, f: |int| -> int) -> int {
4167 twice(5i, |x: int| { x * x }); // evaluates to 50
4171 A named function's name can be used wherever you'd use a closure. Another
4172 way of writing the previous example:
4175 fn twice(x: int, f: |int| -> int) -> int {
4179 fn square(x: int) -> int { x * x }
4182 twice(5i, square); // evaluates to 50
4186 Doing this is not particularly common, but every once in a while, it's useful.
4188 That's all you need to get the hang of closures! Closures are a little bit
4189 strange at first, but once you're used to using them, you'll miss them in any
4190 language that doesn't have them. Passing functions to other functions is
4191 incredibly powerful. Next, let's look at one of those things: iterators.
4195 Let's talk about loops.
4197 Remember Rust's `for` loop? Here's an example:
4200 for x in range(0i, 10i) {
4201 println!("{:d}", x);
4205 Now that you know more Rust, we can talk in detail about how this works. The
4206 `range` function returns an **iterator**. An iterator is something that we can
4207 call the `.next()` method on repeatedly, and it gives us a sequence of things.
4212 let mut range = range(0i, 10i);
4215 match range.next() {
4224 We make a mutable binding to the return value of `range`, which is our iterator.
4225 We then `loop`, with an inner `match`. This `match` is used on the result of
4226 `range.next()`, which gives us a reference to the next value of the iterator.
4227 `next` returns an `Option<int>`, in this case, which will be `Some(int)` when
4228 we have a value and `None` once we run out. If we get `Some(int)`, we print it
4229 out, and if we get `None`, we `break` out of the loop.
4231 This code sample is basically the same as our `for` loop version. The `for`
4232 loop is just a handy way to write this `loop`/`match`/`break` construct.
4234 `for` loops aren't the only thing that uses iterators, however. Writing your
4235 own iterator involves implementing the `Iterator` trait. While doing that is
4236 outside of the scope of this guide, Rust provides a number of useful iterators
4237 to accomplish various tasks. Before we talk about those, we should talk about a
4238 Rust anti-pattern. And that's `range`.
4240 Yes, we just talked about how `range` is cool. But `range` is also very
4241 primitive. For example, if you needed to iterate over the contents of
4242 a vector, you may be tempted to write this:
4245 let nums = vec![1i, 2i, 3i];
4247 for i in range(0u, nums.len()) {
4248 println!("{}", nums[i]);
4252 This is strictly worse than using an actual iterator. The `.iter()` method on
4253 vectors returns an iterator which iterates through a reference to each element
4254 of the vector in turn. So write this:
4257 let nums = vec![1i, 2i, 3i];
4259 for num in nums.iter() {
4260 println!("{}", num);
4264 There are two reasons for this. First, this more directly expresses what we
4265 mean. We iterate through the entire vector, rather than iterating through
4266 indexes, and then indexing the vector. Second, this version is more efficient:
4267 the first version will have extra bounds checking because it used indexing,
4268 `nums[i]`. But since we yield a reference to each element of the vector in turn
4269 with the iterator, there's no bounds checking in the second example. This is
4270 very common with iterators: we can ignore unnecessary bounds checks, but still
4271 know that we're safe.
4273 There's another detail here that's not 100% clear because of how `println!`
4274 works. `num` is actually of type `&int`, that is, it's a reference to an `int`,
4275 not an `int` itself. `println!` handles the dereferencing for us, so we don't
4276 see it. This code works fine too:
4279 let nums = vec![1i, 2i, 3i];
4281 for num in nums.iter() {
4282 println!("{}", *num);
4286 Now we're explicitly dereferencing `num`. Why does `iter()` give us references?
4287 Well, if it gave us the data itself, we would have to be its owner, which would
4288 involve making a copy of the data and giving us the copy. With references,
4289 we're just borrowing a reference to the data, and so it's just passing
4290 a reference, without needing to do the copy.
4292 So, now that we've established that `range` is often not what you want, let's
4293 talk about what you do want instead.
4295 There are three broad classes of things that are relevant here: iterators,
4296 **iterator adapters**, and **consumers**. Here's some definitions:
4298 * 'iterators' give you a sequence of values.
4299 * 'iterator adapters' operate on an iterator, producing a new iterator with a
4300 different output sequence.
4301 * 'consumers' operate on an iterator, producing some final set of values.
4303 Let's talk about consumers first, since you've already seen an iterator,
4308 A 'consumer' operates on an iterator, returning some kind of value or values.
4309 The most common consumer is `collect()`. This code doesn't quite compile,
4310 but it shows the intention:
4313 let one_to_one_hundred = range(0i, 100i).collect();
4316 As you can see, we call `collect()` on our iterator. `collect()` takes
4317 as many values as the iterator will give it, and returns a collection
4318 of the results. So why won't this compile? Rust can't determine what
4319 type of things you want to collect, and so you need to let it know.
4320 Here's the version that does compile:
4323 let one_to_one_hundred = range(0i, 100i).collect::<Vec<int>>();
4326 If you remember, the `::<>` syntax allows us to give a type hint,
4327 and so we tell it that we want a vector of integers.
4329 `collect()` is the most common consumer, but there are others too. `find()`
4333 let one_to_one_hundred = range(0i, 100i);
4335 let greater_than_forty_two = range(0i, 100i)
4336 .find(|x| *x >= 42);
4338 match greater_than_forty_two {
4339 Some(_) => println!("We got some numbers!"),
4340 None => println!("No numbers found :("),
4344 `find` takes a closure, and works on a reference to each element of an
4345 iterator. This closure returns `true` if the element is the element we're
4346 looking for, and `false` otherwise. Because we might not find a matching
4347 element, `find` returns an `Option` rather than the element itself.
4349 Another important consumer is `fold`. Here's what it looks like:
4352 let sum = range(1i, 100i)
4353 .fold(0i, |sum, x| sum + x);
4356 `fold()` is a consumer that looks like this:
4357 `fold(base, |accumulator, element| ...)`. It takes two arguments: the first
4358 is an element called the "base". The second is a closure that itself takes two
4359 arguments: the first is called the "accumulator," and the second is an
4360 "element." Upon each iteration, the closure is called, and the result is the
4361 value of the accumulator on the next iteration. On the first iteration, the
4362 base is the value of the accumulator.
4364 Okay, that's a bit confusing. Let's examine the values of all of these things
4367 | base | accumulator | element | closure result |
4368 |------|-------------|---------|----------------|
4369 | 0i | 0i | 1i | 1i |
4370 | 0i | 1i | 2i | 3i |
4371 | 0i | 3i | 3i | 6i |
4373 We called `fold()` with these arguments:
4377 .fold(0i, |sum, x| sum + x);
4380 So, `0i` is our base, `sum` is our accumulator, and `x` is our element. On the
4381 first iteration, we set `sum` to `0i`, and `x` is the first element of `nums`,
4382 `1i`. We then add `sum` and `x`, which gives us `0i + 1i = 1i`. On the second
4383 iteration, that value becomes our accumulator, `sum`, and the element is
4384 the second element of the array, `2i`. `1i + 2i = 3i`, and so that becomes
4385 the value of the accumulator for the last iteration. On that iteration,
4386 `x` is the last element, `3i`, and `3i + 3i = 6i`, which is our final
4387 result for our sum. `1 + 2 + 3 = 6`, and that's the result we got.
4389 Whew. `fold` can be a bit strange the first few times you see it, but once it
4390 clicks, you can use it all over the place. Any time you have a list of things,
4391 and you want a single result, `fold` is appropriate.
4393 Consumers are important due to one additional property of iterators we haven't
4394 talked about yet: laziness. Let's talk some more about iterators, and you'll
4395 see why consumers matter.
4399 As we've said before, an iterator is something that we can call the `.next()`
4400 method on repeatedly, and it gives us a sequence of things. Because you need
4401 to call the method, this means that iterators are **lazy**. This code, for
4402 example, does not actually generate the numbers `1-100`, and just creates a
4403 value that represents the sequence:
4406 let nums = range(1i, 100i);
4409 Since we didn't do anything with the range, it didn't generate the sequence.
4410 Once we add the consumer:
4413 let nums = range(1i, 100i).collect::<Vec<int>>();
4416 Now, `collect()` will require that `range()` give it some numbers, and so
4417 it will do the work of generating the sequence.
4419 `range` is one of two basic iterators that you'll see. The other is `iter()`,
4420 which you've used before. `iter()` can turn a vector into a simple iterator
4421 that gives you each element in turn:
4424 let nums = [1i, 2i, 3i];
4426 for num in nums.iter() {
4427 println!("{}", num);
4431 These two basic iterators should serve you well. There are some more
4432 advanced iterators, including ones that are infinite. Like `count`:
4435 std::iter::count(1i, 5i);
4438 This iterator counts up from one, adding five each time. It will give
4439 you a new integer every time, forever. Well, technically, until the
4440 maximum number that an `int` can represent. But since iterators are lazy,
4441 that's okay! You probably don't want to use `collect()` on it, though...
4443 That's enough about iterators. Iterator adapters are the last concept
4444 we need to talk about with regards to iterators. Let's get to it!
4446 ## Iterator adapters
4448 "Iterator adapters" take an iterator and modify it somehow, producing
4449 a new iterator. The simplest one is called `map`:
4452 range(1i, 100i).map(|x| x + 1i);
4455 `map` is called upon another iterator, and produces a new iterator where each
4456 element reference has the closure it's been given as an argument called on it.
4457 So this would give us the numbers from `2-101`. Well, almost! If you
4458 compile the example, you'll get a warning:
4461 2:37 warning: unused result which must be used: iterator adaptors are lazy and
4462 do nothing unless consumed, #[warn(unused_must_use)] on by default
4463 range(1i, 100i).map(|x| x + 1i);
4464 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4467 Laziness strikes again! That closure will never execute. This example
4468 doesn't print any numbers:
4471 range(1i, 100i).map(|x| println!("{}", x));
4474 If you are trying to execute a closure on an iterator for its side effects,
4475 just use `for` instead.
4477 There are tons of interesting iterator adapters. `take(n)` will get the
4478 first `n` items out of an iterator, and return them as a list. Let's
4479 try it out with our infinite iterator from before, `count()`:
4482 for i in std::iter::count(1i, 5i).take(5) {
4497 `filter()` is an adapter that takes a closure as an argument. This closure
4498 returns `true` or `false`. The new iterator `filter()` produces returns
4499 only the elements that that closure returned `true` for:
4502 for i in range(1i, 100i).filter(|x| x % 2 == 0) {
4507 This will print all of the even numbers between one and a hundred.
4509 You can chain all three things together: start with an iterator, adapt it
4510 a few times, and then consume the result. Check it out:
4514 .filter(|x| x % 2 == 0)
4515 .filter(|x| x % 3 == 0)
4517 .collect::<Vec<int>>();
4520 This will give you a vector containing `6`, `12`, `18`, `24`, and `30`.
4522 This is just a small taste of what iterators, iterator adapters, and consumers
4523 can help you with. There are a number of really useful iterators, and you can
4524 write your own as well. Iterators provide a safe, efficient way to manipulate
4525 all kinds of lists. They're a little unusual at first, but if you play with
4526 them, you'll get hooked. For a full list of the different iterators and
4527 consumers, check out the [iterator module documentation](std/iter/index.html).
4531 Sometimes, when writing a function or data type, we may want it to work for
4532 multiple types of arguments. For example, remember our `OptionalInt` type?
4541 If we wanted to also have an `OptionalFloat64`, we would need a new enum:
4544 enum OptionalFloat64 {
4550 This is really unfortunate. Luckily, Rust has a feature that gives us a better
4551 way: generics. Generics are called **parametric polymorphism** in type theory,
4552 which means that they are types or functions that have multiple forms ("poly"
4553 is multiple, "morph" is form) over a given parameter ("parametric").
4555 Anyway, enough with type theory declarations, let's check out the generic form
4556 of `OptionalInt`. It is actually provided by Rust itself, and looks like this:
4565 The `<T>` part, which you've seen a few times before, indicates that this is
4566 a generic data type. Inside the declaration of our enum, wherever we see a `T`,
4567 we substitute that type for the same type used in the generic. Here's an
4568 example of using `Option<T>`, with some extra type annotations:
4571 let x: Option<int> = Some(5i);
4574 In the type declaration, we say `Option<int>`. Note how similar this looks to
4575 `Option<T>`. So, in this particular `Option`, `T` has the value of `int`. On
4576 the right hand side of the binding, we do make a `Some(T)`, where `T` is `5i`.
4577 Since that's an `int`, the two sides match, and Rust is happy. If they didn't
4578 match, we'd get an error:
4581 let x: Option<f64> = Some(5i);
4582 // error: mismatched types: expected `core::option::Option<f64>`
4583 // but found `core::option::Option<int>` (expected f64 but found int)
4586 That doesn't mean we can't make `Option<T>`s that hold an `f64`! They just have to
4590 let x: Option<int> = Some(5i);
4591 let y: Option<f64> = Some(5.0f64);
4594 This is just fine. One definition, multiple uses.
4596 Generics don't have to only be generic over one type. Consider Rust's built-in
4597 `Result<T, E>` type:
4606 This type is generic over _two_ types: `T` and `E`. By the way, the capital letters
4607 can be any letter you'd like. We could define `Result<T, E>` as:
4616 if we wanted to. Convention says that the first generic parameter should be
4617 `T`, for 'type,' and that we use `E` for 'error.' Rust doesn't care, however.
4619 The `Result<T, E>` type is intended to
4620 be used to return the result of a computation, and to have the ability to
4621 return an error if it didn't work out. Here's an example:
4624 let x: Result<f64, String> = Ok(2.3f64);
4625 let y: Result<f64, String> = Err("There was an error.".to_string());
4628 This particular Result will return an `f64` if there's a success, and a
4629 `String` if there's a failure. Let's write a function that uses `Result<T, E>`:
4632 fn inverse(x: f64) -> Result<f64, String> {
4633 if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4639 We don't want to take the inverse of zero, so we check to make sure that we
4640 weren't passed zero. If we were, then we return an `Err`, with a message. If
4641 it's okay, we return an `Ok`, with the answer.
4643 Why does this matter? Well, remember how `match` does exhaustive matches?
4644 Here's how this function gets used:
4647 # fn inverse(x: f64) -> Result<f64, String> {
4648 # if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4651 let x = inverse(25.0f64);
4654 Ok(x) => println!("The inverse of 25 is {}", x),
4655 Err(msg) => println!("Error: {}", msg),
4659 The `match` enforces that we handle the `Err` case. In addition, because the
4660 answer is wrapped up in an `Ok`, we can't just use the result without doing
4664 let x = inverse(25.0f64);
4665 println!("{}", x + 2.0f64); // error: binary operation `+` cannot be applied
4666 // to type `core::result::Result<f64,collections::string::String>`
4669 This function is great, but there's one other problem: it only works for 64 bit
4670 floating point values. What if we wanted to handle 32 bit floating point as
4671 well? We'd have to write this:
4674 fn inverse32(x: f32) -> Result<f32, String> {
4675 if x == 0.0f32 { return Err("x cannot be zero!".to_string()); }
4681 Bummer. What we need is a **generic function**. Luckily, we can write one!
4682 However, it won't _quite_ work yet. Before we get into that, let's talk syntax.
4683 A generic version of `inverse` would look something like this:
4686 fn inverse<T>(x: T) -> Result<T, String> {
4687 if x == 0.0 { return Err("x cannot be zero!".to_string()); }
4693 Just like how we had `Option<T>`, we use a similar syntax for `inverse<T>`.
4694 We can then use `T` inside the rest of the signature: `x` has type `T`, and half
4695 of the `Result` has type `T`. However, if we try to compile that example, we'll get
4699 error: binary operation `==` cannot be applied to type `T`
4702 Because `T` can be _any_ type, it may be a type that doesn't implement `==`,
4703 and therefore, the first line would be wrong. What do we do?
4705 To fix this example, we need to learn about another Rust feature: traits.
4709 Do you remember the `impl` keyword, used to call a function with method
4720 fn area(&self) -> f64 {
4721 std::f64::consts::PI * (self.radius * self.radius)
4726 Traits are similar, except that we define a trait with just the method
4727 signature, then implement the trait for that struct. Like this:
4737 fn area(&self) -> f64;
4740 impl HasArea for Circle {
4741 fn area(&self) -> f64 {
4742 std::f64::consts::PI * (self.radius * self.radius)
4747 As you can see, the `trait` block looks very similar to the `impl` block,
4748 but we don't define a body, just a type signature. When we `impl` a trait,
4749 we use `impl Trait for Item`, rather than just `impl Item`.
4751 So what's the big deal? Remember the error we were getting with our generic
4755 error: binary operation `==` cannot be applied to type `T`
4758 We can use traits to constrain our generics. Consider this function, which
4759 does not compile, and gives us a similar error:
4762 fn print_area<T>(shape: T) {
4763 println!("This shape has an area of {}", shape.area());
4770 error: type `T` does not implement any method in scope named `area`
4773 Because `T` can be any type, we can't be sure that it implements the `area`
4774 method. But we can add a **trait constraint** to our generic `T`, ensuring
4779 # fn area(&self) -> f64;
4781 fn print_area<T: HasArea>(shape: T) {
4782 println!("This shape has an area of {}", shape.area());
4786 The syntax `<T: HasArea>` means `any type that implements the HasArea trait`.
4787 Because traits define function type signatures, we can be sure that any type
4788 which implements `HasArea` will have an `.area()` method.
4790 Here's an extended example of how this works:
4794 fn area(&self) -> f64;
4803 impl HasArea for Circle {
4804 fn area(&self) -> f64 {
4805 std::f64::consts::PI * (self.radius * self.radius)
4815 impl HasArea for Square {
4816 fn area(&self) -> f64 {
4817 self.side * self.side
4821 fn print_area<T: HasArea>(shape: T) {
4822 println!("This shape has an area of {}", shape.area());
4843 This program outputs:
4846 This shape has an area of 3.141593
4847 This shape has an area of 1
4850 As you can see, `print_area` is now generic, but also ensures that we
4851 have passed in the correct types. If we pass in an incorrect type:
4857 We get a compile-time error:
4860 error: failed to find an implementation of trait main::HasArea for int
4863 So far, we've only added trait implementations to structs, but you can
4864 implement a trait for any type. So technically, we _could_ implement
4865 `HasArea` for `int`:
4869 fn area(&self) -> f64;
4872 impl HasArea for int {
4873 fn area(&self) -> f64 {
4874 println!("this is silly");
4883 It is considered poor style to implement methods on such primitive types, even
4884 though it is possible.
4886 This may seem like the Wild West, but there are two other restrictions around
4887 implementing traits that prevent this from getting out of hand. First, traits
4888 must be `use`d in any scope where you wish to use the trait's method. So for
4889 example, this does not work:
4893 use std::f64::consts;
4896 fn area(&self) -> f64;
4905 impl HasArea for Circle {
4906 fn area(&self) -> f64 {
4907 consts::PI * (self.radius * self.radius)
4913 let c = shapes::Circle {
4919 println!("{}", c.area());
4923 Now that we've moved the structs and traits into their own module, we get an
4927 error: type `shapes::Circle` does not implement any method in scope named `area`
4930 If we add a `use` line right above `main` and make the right things public,
4934 use shapes::HasArea;
4937 use std::f64::consts;
4940 fn area(&self) -> f64;
4949 impl HasArea for Circle {
4950 fn area(&self) -> f64 {
4951 consts::PI * (self.radius * self.radius)
4958 let c = shapes::Circle {
4964 println!("{}", c.area());
4968 This means that even if someone does something bad like add methods to `int`,
4969 it won't affect you, unless you `use` that trait.
4971 There's one more restriction on implementing traits. Either the trait or the
4972 type you're writing the `impl` for must be inside your crate. So, we could
4973 implement the `HasArea` type for `int`, because `HasArea` is in our crate. But
4974 if we tried to implement `Float`, a trait provided by Rust, for `int`, we could
4975 not, because both the trait and the type aren't in our crate.
4977 One last thing about traits: generic functions with a trait bound use
4978 **monomorphization** ("mono": one, "morph": form), so they are statically
4979 dispatched. What's that mean? Well, let's take a look at `print_area` again:
4982 fn print_area<T: HasArea>(shape: T) {
4983 println!("This shape has an area of {}", shape.area());
4987 let c = Circle { ... };
4989 let s = Square { ... };
4996 When we use this trait with `Circle` and `Square`, Rust ends up generating
4997 two different functions with the concrete type, and replacing the call sites with
4998 calls to the concrete implementations. In other words, you get something like
5002 fn __print_area_circle(shape: Circle) {
5003 println!("This shape has an area of {}", shape.area());
5006 fn __print_area_square(shape: Square) {
5007 println!("This shape has an area of {}", shape.area());
5011 let c = Circle { ... };
5013 let s = Square { ... };
5015 __print_area_circle(c);
5016 __print_area_square(s);
5020 The names don't actually change to this, it's just for illustration. But
5021 as you can see, there's no overhead of deciding which version to call here,
5022 hence 'statically dispatched.' The downside is that we have two copies of
5023 the same function, so our binary is a little bit larger.
5027 Concurrency and parallelism are topics that are of increasing interest to a
5028 broad subsection of software developers. Modern computers are often multi-core,
5029 to the point that even embedded devices like cell phones have more than one
5030 processor. Rust's semantics lend themselves very nicely to solving a number of
5031 issues that programmers have with concurrency. Many concurrency errors that are
5032 runtime errors in other languages are compile-time errors in Rust.
5034 Rust's concurrency primitive is called a **task**. Tasks are lightweight, and
5035 do not share memory in an unsafe manner, preferring message passing to
5036 communicate. It's worth noting that tasks are implemented as a library, and
5037 not part of the language. This means that in the future, other concurrency
5038 libraries can be written for Rust to help in specific scenarios. Here's an
5039 example of creating a task:
5043 println!("Hello from a task!");
5047 The `spawn` function takes a proc as an argument, and runs that proc in a new
5048 task. A proc takes ownership of its entire environment, and so any variables
5049 that you use inside the proc will not be usable afterward:
5052 let mut x = vec![1i, 2i, 3i];
5055 println!("The value of x[0] is: {}", x[0]);
5058 println!("The value of x[0] is: {}", x[0]); // error: use of moved value: `x`
5061 `x` is now owned by the proc, and so we can't use it anymore. Many other
5062 languages would let us do this, but it's not safe to do so. Rust's type system
5065 If tasks were only able to capture these values, they wouldn't be very useful.
5066 Luckily, tasks can communicate with each other through **channel**s. Channels
5070 let (tx, rx) = channel();
5073 tx.send("Hello from a task!".to_string());
5076 let message = rx.recv();
5077 println!("{}", message);
5080 The `channel()` function returns two endpoints: a `Receiver<T>` and a
5081 `Sender<T>`. You can use the `.send()` method on the `Sender<T>` end, and
5082 receive the message on the `Receiver<T>` side with the `recv()` method. This
5083 method blocks until it gets a message. There's a similar method, `.try_recv()`,
5084 which returns an `Option<T>` and does not block.
5086 If you want to send messages to the task as well, create two channels!
5089 let (tx1, rx1) = channel();
5090 let (tx2, rx2) = channel();
5093 tx1.send("Hello from a task!".to_string());
5094 let message = rx2.recv();
5095 println!("{}", message);
5098 let message = rx1.recv();
5099 println!("{}", message);
5101 tx2.send("Goodbye from main!".to_string());
5104 The proc has one sending end and one receiving end, and the main task has one
5105 of each as well. Now they can talk back and forth in whatever way they wish.
5107 Notice as well that because `Sender` and `Receiver` are generic, while you can
5108 pass any kind of information through the channel, the ends are strongly typed.
5109 If you try to pass a string, and then an integer, Rust will complain.
5113 With these basic primitives, many different concurrency patterns can be
5114 developed. Rust includes some of these types in its standard library. For
5115 example, if you wish to compute some value in the background, `Future` is
5116 a useful thing to use:
5119 use std::sync::Future;
5121 let mut delayed_value = Future::spawn(proc() {
5122 // just return anything for examples' sake
5126 println!("value = {}", delayed_value.get());
5129 Calling `Future::spawn` works just like `spawn()`: it takes a proc. In this
5130 case, though, you don't need to mess with the channel: just have the proc
5133 `Future::spawn` will return a value which we can bind with `let`. It needs
5134 to be mutable, because once the value is computed, it saves a copy of the
5135 value, and if it were immutable, it couldn't update itself.
5137 The proc will go on processing in the background, and when we need the final
5138 value, we can call `get()` on it. This will block until the result is done,
5139 but if it's finished computing in the background, we'll just get the value
5142 ## Success and failure
5144 Tasks don't always succeed, they can also fail. A task that wishes to fail
5145 can call the `fail!` macro, passing a message:
5153 If a task fails, it is not possible for it to recover. However, it can
5154 notify other tasks that it has failed. We can do this with `task::try`:
5160 let result = task::try(proc() {
5169 This task will randomly fail or succeed. `task::try` returns a `Result`
5170 type, so we can handle the response like any other computation that may
5175 One of Rust's most advanced features is its system of **macro**s. While
5176 functions allow you to provide abstractions over values and operations, macros
5177 allow you to provide abstractions over syntax. Do you wish Rust had the ability
5178 to do something that it can't currently do? You may be able to write a macro
5179 to extend Rust's capabilities.
5181 You've already used one macro extensively: `println!`. When we invoke
5182 a Rust macro, we need to use the exclamation mark (`!`). There's two reasons
5183 that this is true: the first is that it makes it clear when you're using a
5184 macro. The second is that macros allow for flexible syntax, and so Rust must
5185 be able to tell where a macro starts and ends. The `!(...)` helps with this.
5187 Let's talk some more about `println!`. We could have implemented `println!` as
5188 a function, but it would be worse. Why? Well, what macros allow you to do
5189 is write code that generates more code. So when we call `println!` like this:
5193 println!("x is: {}", x);
5196 The `println!` macro does a few things:
5198 1. It parses the string to find any `{}`s
5199 2. It checks that the number of `{}`s matches the number of other arguments.
5200 3. It generates a bunch of Rust code, taking this in mind.
5202 What this means is that you get type checking at compile time, because
5203 Rust will generate code that takes all of the types into account. If
5204 `println!` was a function, it could still do this type checking, but it
5205 would happen at run time rather than compile time.
5207 We can check this out using a special flag to `rustc`. This code, in a file
5213 println!("x is: {:s}", x);
5217 Can have its macros expanded like this: `rustc print.rs --pretty=expanded`, will
5218 give us this huge result:
5224 #[phase(plugin, link)]
5225 extern crate std = "std";
5226 extern crate rt = "native";
5227 use std::prelude::*;
5234 static __STATIC_FMTSTR: [::std::fmt::rt::Piece<'static>, ..2u] =
5235 [::std::fmt::rt::String("x is: "),
5236 ::std::fmt::rt::Argument(::std::fmt::rt::Argument{position:
5237 ::std::fmt::rt::ArgumentNext,
5239 ::std::fmt::rt::FormatSpec{fill:
5242 ::std::fmt::rt::AlignUnknown,
5246 ::std::fmt::rt::CountImplied,
5248 ::std::fmt::rt::CountImplied,},})];
5250 &[::std::fmt::argument(::std::fmt::secret_string, __arg0)];
5253 ::std::fmt::Arguments::new(__STATIC_FMTSTR, __args_vec)
5255 ::std::io::stdio::println_args(&__args)
5261 Intense. Here's a trimmed down version that's a bit easier to read:
5268 static __STATIC_FMTSTR: =
5271 position: ArgumentNext,
5272 format: FormatSpec {
5274 align: AlignUnknown,
5276 precision: CountImplied,
5277 width: CountImplied,
5281 let __args_vec = &[argument(secret_string, __arg0)];
5282 let __args = unsafe { Arguments::new(__STATIC_FMTSTR, __args_vec) };
5284 println_args(&__args)
5290 Whew! This isn't too terrible. You can see that we still `let x = 5i`,
5291 but then things get a little bit hairy. Three more bindings get set: a
5292 static format string, an argument vector, and the arguments. We then
5293 invoke the `println_args` function with the generated arguments.
5295 This is the code (well, the full version) that Rust actually compiles. You can
5296 see all of the extra information that's here. We get all of the type safety and
5297 options that it provides, but at compile time, and without needing to type all
5298 of this out. This is how macros are powerful. Without them, you would need to
5299 type all of this by hand to get a type checked `println`.
5301 For more on macros, please consult [the Macros Guide](guide-macros.html).
5302 Macros are a very advanced and still slightly experimental feature, but don't
5303 require a deep understanding to call, since they look just like functions. The
5304 Guide can help you if you want to write your own.
5308 Finally, there's one more Rust concept that you should be aware of: `unsafe`.
5309 There are two circumstances where Rust's safety provisions don't work well.
5310 The first is when interfacing with C code, and the second is when building
5311 certain kinds of abstractions.
5313 Rust has support for FFI (which you can read about in the [FFI
5314 Guide](guide-ffi.html)), but can't guarantee that the C code will be safe.
5315 Therefore, Rust marks such functions with the `unsafe`
5316 keyword, which indicates that the function may not behave properly.
5318 Second, if you'd like to create some sort of shared-memory data structure, Rust
5319 won't allow it, because memory must be owned by a single owner. However, if
5320 you're planning on making access to that shared memory safe, such as with a
5321 mutex, _you_ know that it's safe, but Rust can't know. Writing an `unsafe`
5322 block allows you to ask the compiler to trust you. In this case, the _internal_
5323 implementation of the mutex is considered unsafe, but the _external_ interface
5324 we present is safe. This allows it to be effectively used in normal Rust, while
5325 being able to implement functionality that the compiler can't double check for
5328 Doesn't an escape hatch undermine the safety of the entire system? Well, if
5329 Rust code segfaults, it _must_ be because of unsafe code somewhere. By
5330 annotating exactly where that is, you have a significantly smaller area to
5333 We haven't even talked about any examples here, and that's because I want to
5334 emphasize that you should not be writing unsafe code unless you know exactly
5335 what you're doing. The vast majority of Rust developers will only interact with
5336 it when doing FFI, and advanced library authors may use it to build certain
5337 kinds of abstraction.
5341 We covered a lot of ground here. When you've mastered everything in this Guide,
5342 you will have a firm grasp of basic Rust development. There's a whole lot more
5343 out there, we've just covered the surface. There's tons of topics that you can
5344 dig deeper into, and we've built specialized guides for many of them. To learn
5345 more, dig into the [full documentation
5346 index](http://doc.rust-lang.org/index.html).