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 either the [32-bit
33 installer](https://static.rust-lang.org/dist/rust-nightly-i686-w64-mingw32.exe)
35 installer](https://static.rust-lang.org/dist/rust-nightly-x86_64-w64-mingw32.exe)
38 If you decide you don't want Rust anymore, we'll be a bit sad, but that's okay.
39 Not every programming language is great for everyone. Just pass an argument to
43 $ curl -s https://static.rust-lang.org/rustup.sh | sudo sh -s -- --uninstall
46 If you used the Windows installer, just re-run the `.exe` and it will give you
49 You can re-run this script any time you want to update Rust. Which, at this
50 point, is often. Rust is still pre-1.0, and so people assume that you're using
53 This brings me to one other point: some people, and somewhat rightfully so, get
54 very upset when we tell you to `curl | sudo sh`. And they should be! Basically,
55 when you do this, you are trusting that the good people who maintain Rust
56 aren't going to hack your computer and do bad things. That's a good instinct!
57 If you're one of those people, please check out the documentation on [building
58 Rust from Source](https://github.com/rust-lang/rust#building-from-source), or
59 [the official binary downloads](http://www.rust-lang.org/install.html). And we
60 promise that this method will not be the way to install Rust forever: it's just
61 the easiest way to keep people updated while Rust is in its alpha state.
63 Oh, we should also mention the officially supported platforms:
65 * Windows (7, 8, Server 2008 R2), x86 only
66 * Linux (2.6.18 or later, various distributions), x86 and x86-64
67 * OSX 10.7 (Lion) or greater, x86 and x86-64
69 We extensively test Rust on these platforms, and a few others, too, like
70 Android. But these are the ones most likely to work, as they have the most
73 Finally, a comment about Windows. Rust considers Windows to be a first-class
74 platform upon release, but if we're honest, the Windows experience isn't as
75 integrated as the Linux/OS X experience is. We're working on it! If anything
76 does not work, it is a bug. Please let us know if that happens. Each and every
77 commit is tested against Windows just like any other platform.
79 If you've got Rust installed, you can open up a shell, and type this:
85 You should see some output that looks something like this:
88 rustc 0.12.0-nightly (b7aa03a3c 2014-09-28 11:38:01 +0000)
91 If you did, Rust has been installed successfully! Congrats!
93 If not, there are a number of places where you can get help. The easiest is
94 [the #rust IRC channel on irc.mozilla.org](irc://irc.mozilla.org/#rust), which
95 you can access through
96 [Mibbit](http://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust). Click
97 that link, and you'll be chatting with other Rustaceans (a silly nickname we
98 call ourselves), and we can help you out. Other great resources include [our
99 mailing list](https://mail.mozilla.org/listinfo/rust-dev), [the /r/rust
100 subreddit](http://www.reddit.com/r/rust), and [Stack
101 Overflow](http://stackoverflow.com/questions/tagged/rust).
105 Now that you have Rust installed, let's write your first Rust program. It's
106 traditional to make your first program in any new language one that prints the
107 text "Hello, world!" to the screen. The nice thing about starting with such a
108 simple program is that you can verify that your compiler isn't just installed,
109 but also working properly. And printing information to the screen is a pretty
112 The first thing that we need to do is make a file to put our code in. I like
113 to make a `projects` directory in my home directory, and keep all my projects
114 there. Rust does not care where your code lives.
116 This actually leads to one other concern we should address: this guide will
117 assume that you have basic familiarity with the command line. Rust does not
118 require that you know a whole ton about the command line, but until the
119 language is in a more finished state, IDE support is spotty. Rust makes no
120 specific demands on your editing tooling, or where your code lives.
122 With that said, let's make a directory in our projects directory.
131 If you're on Windows and not using PowerShell, the `~` may not work. Consult
132 the documentation for your shell for more details.
134 Let's make a new source file next. I'm going to use the syntax `editor
135 filename` to represent editing a file in these examples, but you should use
136 whatever method you want. We'll call our file `main.rs`:
142 Rust files always end in a `.rs` extension. If you're using more than one word
143 in your file name, use an underscore. `hello_world.rs` rather than
146 Now that you've got your file open, type this in:
150 println!("Hello, world!");
154 Save the file, and then type this into your terminal window:
158 $ ./main # or main.exe on Windows
162 Success! Let's go over what just happened in detail.
170 These lines define a **function** in Rust. The `main` function is special:
171 it's the beginning of every Rust program. The first line says "I'm declaring a
172 function named `main`, which takes no arguments and returns nothing." If there
173 were arguments, they would go inside the parentheses (`(` and `)`), and because
174 we aren't returning anything from this function, we've dropped that notation
175 entirely. We'll get to it later.
177 You'll also note that the function is wrapped in curly braces (`{` and `}`).
178 Rust requires these around all function bodies. It is also considered good
179 style to put the opening curly brace on the same line as the function
180 declaration, with one space in between.
182 Next up is this line:
185 println!("Hello, world!");
188 This line does all of the work in our little program. There are a number of
189 details that are important here. The first is that it's indented with four
190 spaces, not tabs. Please configure your editor of choice to insert four spaces
191 with the tab key. We provide some [sample configurations for various
192 editors](https://github.com/rust-lang/rust/tree/master/src/etc).
194 The second point is the `println!()` part. This is calling a Rust **macro**,
195 which is how metaprogramming is done in Rust. If it were a function instead, it
196 would look like this: `println()`. For our purposes, we don't need to worry
197 about this difference. Just know that sometimes, you'll see a `!`, and that
198 means that you're calling a macro instead of a normal function. Rust implements
199 `println!` as a macro rather than a function for good reasons, but that's a
200 very advanced topic. You'll learn more when we talk about macros later. One
201 last thing to mention: Rust's macros are significantly different than C macros,
202 if you've used those. Don't be scared of using macros. We'll get to the details
203 eventually, you'll just have to trust us for now.
205 Next, `"Hello, world!"` is a **string**. Strings are a surprisingly complicated
206 topic in a systems programming language, and this is a **statically allocated**
207 string. We will talk more about different kinds of allocation later. We pass
208 this string as an argument to `println!`, which prints the string to the
211 Finally, the line ends with a semicolon (`;`). Rust is an **expression
212 oriented** language, which means that most things are expressions. The `;` is
213 used to indicate that this expression is over, and the next one is ready to
214 begin. Most lines of Rust code end with a `;`. We will cover this in-depth
217 Finally, actually **compiling** and **running** our program. We can compile
218 with our compiler, `rustc`, by passing it the name of our source file:
224 This is similar to `gcc` or `clang`, if you come from a C or C++ background. Rust
225 will output a binary executable. You can see it with `ls`:
239 There are now two files: our source code, with the `.rs` extension, and the
240 executable (`main.exe` on Windows, `main` everywhere else)
243 $ ./main # or main.exe on Windows
246 This prints out our `Hello, world!` text to our terminal.
248 If you come from a dynamically typed language like Ruby, Python, or JavaScript,
249 you may not be used to these two steps being separate. Rust is an
250 **ahead-of-time compiled language**, which means that you can compile a
251 program, give it to someone else, and they don't need to have Rust installed.
252 If you give someone a `.rb` or `.py` or `.js` file, they need to have
253 Ruby/Python/JavaScript installed, but you just need one command to both compile
254 and run your program. Everything is a tradeoff in language design, and Rust has
257 Congratulations! You have officially written a Rust program. That makes you a
258 Rust programmer! Welcome.
260 Next, I'd like to introduce you to another tool, Cargo, which is used to write
261 real-world Rust programs. Just using `rustc` is nice for simple things, but as
262 your project grows, you'll want something to help you manage all of the options
263 that it has, and to make it easy to share your code with other people and
268 [Cargo](http://crates.io) is a tool that Rustaceans use to help manage their
269 Rust projects. Cargo is currently in an alpha state, just like Rust, and so it
270 is still a work in progress. However, it is already good enough to use for many
271 Rust projects, and so it is assumed that Rust projects will use Cargo from the
274 Cargo manages three things: building your code, downloading the dependencies
275 your code needs, and building the dependencies your code needs. At first, your
276 program doesn't have any dependencies, so we'll only be using the first part of
277 its functionality. Eventually, we'll add more. Since we started off by using
278 Cargo, it'll be easy to add later.
280 Let's convert Hello World to Cargo. The first thing we need to do to begin
281 using Cargo is to install Cargo. Luckily for us, the script we ran to install
282 Rust includes Cargo by default. If you installed Rust some other way, you may
283 want to [check the Cargo
284 README](https://github.com/rust-lang/cargo#installing-cargo-from-nightlies)
285 for specific instructions about installing it.
287 To Cargo-ify our project, we need to do two things: Make a `Cargo.toml`
288 configuration file, and put our source file in the right place. Let's
293 $ mv main.rs src/main.rs
296 Cargo expects your source files to live inside a `src` directory. That leaves
297 the top level for other things, like READMEs, license information, and anything
298 not related to your code. Cargo helps us keep our projects nice and tidy. A
299 place for everything, and everything in its place.
301 Next, our configuration file:
307 Make sure to get this name right: you need the capital `C`!
316 authors = [ "Your name <you@example.com>" ]
323 This file is in the [TOML](https://github.com/toml-lang/toml) format. Let's let
324 it explain itself to you:
326 > TOML aims to be a minimal configuration file format that's easy to read due
327 > to obvious semantics. TOML is designed to map unambiguously to a hash table.
328 > TOML should be easy to parse into data structures in a wide variety of
331 TOML is very similar to INI, but with some extra goodies.
333 Anyway, there are two **table**s in this file: `package` and `bin`. The first
334 tells Cargo metadata about your package. The second tells Cargo that we're
335 interested in building a binary, not a library (though we could do both!), as
336 well as what it is named.
338 Once you have this file in place, we should be ready to build! Try this:
342 Compiling hello_world v0.0.1 (file:///home/yourname/projects/hello_world)
343 $ ./target/hello_world
347 Bam! We build our project with `cargo build`, and run it with
348 `./target/hello_world`. This hasn't bought us a whole lot over our simple use
349 of `rustc`, but think about the future: when our project has more than one
350 file, we would need to call `rustc` twice, and pass it a bunch of options to
351 tell it to build everything together. With Cargo, as our project grows, we can
352 just `cargo build` and it'll work the right way.
354 You'll also notice that Cargo has created a new file: `Cargo.lock`.
362 This file is used by Cargo to keep track of dependencies in your application.
363 Right now, we don't have any, so it's a bit sparse. You won't ever need
364 to touch this file yourself, just let Cargo handle it.
366 That's it! We've successfully built `hello_world` with Cargo. Even though our
367 program is simple, it's using much of the real tooling that you'll use for the
368 rest of your Rust career.
370 Now that you've got the tools down, let's actually learn more about the Rust
371 language itself. These are the basics that will serve you well through the rest
372 of your time with Rust.
376 The first thing we'll learn about are 'variable bindings.' They look like this:
382 In many languages, this is called a 'variable.' But Rust's variable bindings
383 have a few tricks up their sleeves. Rust has a very powerful feature called
384 'pattern matching' that we'll get into detail with later, but the left
385 hand side of a `let` expression is a full pattern, not just a variable name.
386 This means we can do things like:
389 let (x, y) = (1i, 2i);
392 After this expression is evaluated, `x` will be one, and `y` will be two.
393 Patterns are really powerful, but this is about all we can do with them so far.
394 So let's just keep this in the back of our minds as we go forward.
396 By the way, in these examples, `i` indicates that the number is an integer.
398 Rust is a statically typed language, which means that we specify our types up
399 front. So why does our first example compile? Well, Rust has this thing called
400 "type inference." If it can figure out what the type of something is, Rust
401 doesn't require you to actually type it out.
403 We can add the type if we want to, though. Types come after a colon (`:`):
409 If I asked you to read this out loud to the rest of the class, you'd say "`x`
410 is a binding with the type `int` and the value `five`."
412 By default, bindings are **immutable**. This code will not compile:
419 It will give you this error:
422 error: re-assignment of immutable variable `x`
427 If you want a binding to be mutable, you can use `mut`:
434 There is no single reason that bindings are immutable by default, but we can
435 think about it through one of Rust's primary focuses: safety. If you forget to
436 say `mut`, the compiler will catch it, and let you know that you have mutated
437 something you may not have cared to mutate. If bindings were mutable by
438 default, the compiler would not be able to tell you this. If you _did_ intend
439 mutation, then the solution is quite easy: add `mut`.
441 There are other good reasons to avoid mutable state when possible, but they're
442 out of the scope of this guide. In general, you can often avoid explicit
443 mutation, and so it is preferable in Rust. That said, sometimes, mutation is
444 what you need, so it's not verboten.
446 Let's get back to bindings. Rust variable bindings have one more aspect that
447 differs from other languages: bindings are required to be initialized with a
448 value before you're allowed to use them. If we try...
454 ...we'll get an error:
457 src/main.rs:2:9: 2:10 error: cannot determine a type for this local variable: unconstrained type
462 Giving it a type will compile, though:
468 Let's try it out. Change your `src/main.rs` file to look like this:
474 println!("Hello world!");
478 You can use `cargo build` on the command line to build it. You'll get a warning,
479 but it will still print "Hello, world!":
482 Compiling hello_world v0.0.1 (file:///home/you/projects/hello_world)
483 src/main.rs:2:9: 2:10 warning: unused variable: `x`, #[warn(unused_variable)] on by default
484 src/main.rs:2 let x: int;
488 Rust warns us that we never use the variable binding, but since we never use it,
489 no harm, no foul. Things change if we try to actually use this `x`, however. Let's
490 do that. Change your program to look like this:
496 println!("The value of x is: {}", x);
500 And try to build it. You'll get an error:
504 Compiling hello_world v0.0.1 (file:///home/you/projects/hello_world)
505 src/main.rs:4:39: 4:40 error: use of possibly uninitialized variable: `x`
506 src/main.rs:4 println!("The value of x is: {}", x);
508 note: in expansion of format_args!
509 <std macros>:2:23: 2:77 note: expansion site
510 <std macros>:1:1: 3:2 note: in expansion of println!
511 src/main.rs:4:5: 4:42 note: expansion site
512 error: aborting due to previous error
513 Could not compile `hello_world`.
516 Rust will not let us use a value that has not been initialized. Next, let's
517 talk about this stuff we've added to `println!`.
519 If you include two curly braces (`{}`, some call them moustaches...) in your
520 string to print, Rust will interpret this as a request to interpolate some sort
521 of value. **String interpolation** is a computer science term that means "stick
522 in the middle of a string." We add a comma, and then `x`, to indicate that we
523 want `x` to be the value we're interpolating. The comma is used to separate
524 arguments we pass to functions and macros, if you're passing more than one.
526 When you just use the curly braces, Rust will attempt to display the
527 value in a meaningful way by checking out its type. If you want to specify the
528 format in a more detailed manner, there are a [wide number of options
529 available](std/fmt/index.html). For now, we'll just stick to the default:
530 integers aren't very complicated to print.
534 Rust's take on `if` is not particularly complex, but it's much more like the
535 `if` you'll find in a dynamically typed language than in a more traditional
536 systems language. So let's talk about it, to make sure you grasp the nuances.
538 `if` is a specific form of a more general concept, the 'branch.' The name comes
539 from a branch in a tree: a decision point, where depending on a choice,
540 multiple paths can be taken.
542 In the case of `if`, there is one choice that leads down two paths:
548 println!("x is five!");
552 If we changed the value of `x` to something else, this line would not print.
553 More specifically, if the expression after the `if` evaluates to `true`, then
554 the block is executed. If it's `false`, then it is not.
556 If you want something to happen in the `false` case, use an `else`:
562 println!("x is five!");
564 println!("x is not five :(");
568 This is all pretty standard. However, you can also do this:
581 Which we can (and probably should) write like this:
586 let y = if x == 5i { 10i } else { 15i };
589 This reveals two interesting things about Rust: it is an expression-based
590 language, and semicolons are different than in other 'curly brace and
591 semicolon'-based languages. These two things are related.
593 ## Expressions vs. Statements
595 Rust is primarily an expression based language. There are only two kinds of
596 statements, and everything else is an expression.
598 So what's the difference? Expressions return a value, and statements do not.
599 In many languages, `if` is a statement, and therefore, `let x = if ...` would
600 make no sense. But in Rust, `if` is an expression, which means that it returns
601 a value. We can then use this value to initialize the binding.
603 Speaking of which, bindings are a kind of the first of Rust's two statements.
604 The proper name is a **declaration statement**. So far, `let` is the only kind
605 of declaration statement we've seen. Let's talk about that some more.
607 In some languages, variable bindings can be written as expressions, not just
608 statements. Like Ruby:
614 In Rust, however, using `let` to introduce a binding is _not_ an expression. The
615 following will produce a compile-time error:
618 let x = (let y = 5i); // expected identifier, found keyword `let`
621 The compiler is telling us here that it was expecting to see the beginning of
622 an expression, and a `let` can only begin a statement, not an expression.
624 Note that assigning to an already-bound variable (e.g. `y = 5i`) is still an
625 expression, although its value is not particularly useful. Unlike C, where an
626 assignment evaluates to the assigned value (e.g. `5i` in the previous example),
627 in Rust the value of an assignment is the unit type `()` (which we'll cover later).
629 The second kind of statement in Rust is the **expression statement**. Its
630 purpose is to turn any expression into a statement. In practical terms, Rust's
631 grammar expects statements to follow other statements. This means that you use
632 semicolons to separate expressions from each other. This means that Rust
633 looks a lot like most other languages that require you to use semicolons
634 at the end of every line, and you will see semicolons at the end of almost
635 every line of Rust code you see.
637 What is this exception that makes us say 'almost?' You saw it already, in this
643 let y: int = if x == 5i { 10i } else { 15i };
646 Note that I've added the type annotation to `y`, to specify explicitly that I
647 want `y` to be an integer.
649 This is not the same as this, which won't compile:
654 let y: int = if x == 5i { 10i; } else { 15i; };
657 Note the semicolons after the 10 and 15. Rust will give us the following error:
660 error: mismatched types: expected `int` but found `()` (expected int but found ())
663 We expected an integer, but we got `()`. `()` is pronounced 'unit', and is a
664 special type in Rust's type system. In Rust, `()` is _not_ a valid value for a
665 variable of type `int`. It's only a valid value for variables of the type `()`,
666 which aren't very useful. Remember how we said statements don't return a value?
667 Well, that's the purpose of unit in this case. The semicolon turns any
668 expression into a statement by throwing away its value and returning unit
671 There's one more time in which you won't see a semicolon at the end of a line
672 of Rust code. For that, we'll need our next concept: functions.
676 You've already seen one function so far, the `main` function:
683 This is the simplest possible function declaration. As we mentioned before,
684 `fn` says 'this is a function,' followed by the name, some parenthesis because
685 this function takes no arguments, and then some curly braces to indicate the
686 body. Here's a function named `foo`:
693 So, what about taking arguments? Here's a function that prints a number:
696 fn print_number(x: int) {
697 println!("x is: {}", x);
701 Here's a complete program that uses `print_number`:
708 fn print_number(x: int) {
709 println!("x is: {}", x);
713 As you can see, function arguments work very similar to `let` declarations:
714 you add a type to the argument name, after a colon.
716 Here's a complete program that adds two numbers together and prints them:
723 fn print_sum(x: int, y: int) {
724 println!("sum is: {}", x + y);
728 You separate arguments with a comma, both when you call the function, as well
729 as when you declare it.
731 Unlike `let`, you _must_ declare the types of function arguments. This does
735 fn print_number(x, y) {
736 println!("x is: {}", x + y);
743 hello.rs:5:18: 5:19 error: expected `:` but found `,`
744 hello.rs:5 fn print_number(x, y) {
747 This is a deliberate design decision. While full-program inference is possible,
748 languages which have it, like Haskell, often suggest that documenting your
749 types explicitly is a best-practice. We agree that forcing functions to declare
750 types while allowing for inference inside of function bodies is a wonderful
751 sweet spot between full inference and no inference.
753 What about returning a value? Here's a function that adds one to an integer:
756 fn add_one(x: int) -> int {
761 Rust functions return exactly one value, and you declare the type after an
762 'arrow', which is a dash (`-`) followed by a greater-than sign (`>`).
764 You'll note the lack of a semicolon here. If we added it in:
767 fn add_one(x: int) -> int {
772 We would get an error:
775 error: not all control paths return a value
776 fn add_one(x: int) -> int {
780 note: consider removing this semicolon:
785 Remember our earlier discussions about semicolons and `()`? Our function claims
786 to return an `int`, but with a semicolon, it would return `()` instead. Rust
787 realizes this probably isn't what we want, and suggests removing the semicolon.
789 This is very much like our `if` statement before: the result of the block
790 (`{}`) is the value of the expression. Other expression-oriented languages,
791 such as Ruby, work like this, but it's a bit unusual in the systems programming
792 world. When people first learn about this, they usually assume that it
793 introduces bugs. But because Rust's type system is so strong, and because unit
794 is its own unique type, we have never seen an issue where adding or removing a
795 semicolon in a return position would cause a bug.
797 But what about early returns? Rust does have a keyword for that, `return`:
800 fn foo(x: int) -> int {
801 if x < 5 { return x; }
807 Using a `return` as the last line of a function works, but is considered poor
811 fn foo(x: int) -> int {
812 if x < 5 { return x; }
818 There are some additional ways to define functions, but they involve features
819 that we haven't learned about yet, so let's just leave it at that for now.
824 Now that we have some functions, it's a good idea to learn about comments.
825 Comments are notes that you leave to other programmers to help explain things
826 about your code. The compiler mostly ignores them.
828 Rust has two kinds of comments that you should care about: **line comment**s
829 and **doc comment**s.
832 // Line comments are anything after '//' and extend to the end of the line.
834 let x = 5i; // this is also a line comment.
836 // If you have a long explanation for something, you can put line comments next
837 // to each other. Put a space between the // and your comment so that it's
841 The other kind of comment is a doc comment. Doc comments use `///` instead of
842 `//`, and support Markdown notation inside:
845 /// `hello` is a function that prints a greeting that is personalized based on
850 /// * `name` - The name of the person you'd like to greet.
855 /// let name = "Steve";
856 /// hello(name); // prints "Hello, Steve!"
858 fn hello(name: &str) {
859 println!("Hello, {}!", name);
863 When writing doc comments, adding sections for any arguments, return values,
864 and providing some examples of usage is very, very helpful.
866 You can use the `rustdoc` tool to generate HTML documentation from these doc
867 comments. We will talk more about `rustdoc` when we get to modules, as
868 generally, you want to export documentation for a full module.
870 # Compound Data Types
872 Rust, like many programming languages, has a number of different data types
873 that are built-in. You've already done some simple work with integers and
874 strings, but next, let's talk about some more complicated ways of storing data.
878 The first compound data type we're going to talk about are called **tuple**s.
879 Tuples are an ordered list of a fixed size. Like this:
882 let x = (1i, "hello");
885 The parenthesis and commas form this two-length tuple. Here's the same code, but
886 with the type annotated:
889 let x: (int, &str) = (1, "hello");
892 As you can see, the type of a tuple looks just like the tuple, but with each
893 position having a type name rather than the value. Careful readers will also
894 note that tuples are heterogeneous: we have an `int` and a `&str` in this tuple.
895 You haven't seen `&str` as a type before, and we'll discuss the details of
896 strings later. In systems programming languages, strings are a bit more complex
897 than in other languages. For now, just read `&str` as "a string slice," and
898 we'll learn more soon.
900 You can access the fields in a tuple through a **destructuring let**. Here's
904 let (x, y, z) = (1i, 2i, 3i);
906 println!("x is {}", x);
909 Remember before when I said the left hand side of a `let` statement was more
910 powerful than just assigning a binding? Here we are. We can put a pattern on
911 the left hand side of the `let`, and if it matches up to the right hand side,
912 we can assign multiple bindings at once. In this case, `let` 'destructures,'
913 or 'breaks up,' the tuple, and assigns the bits to three bindings.
915 This pattern is very powerful, and we'll see it repeated more later.
917 There also a few things you can do with a tuple as a whole, without
918 destructuring. You can assign one tuple into another, if they have the same
919 arity and contained types.
922 let mut x = (1i, 2i);
928 You can also check for equality with `==`. Again, this will only compile if the
929 tuples have the same type.
932 let x = (1i, 2i, 3i);
933 let y = (2i, 2i, 4i);
942 This will print `no`, because some of the values aren't equal.
944 One other use of tuples is to return multiple values from a function:
947 fn next_two(x: int) -> (int, int) { (x + 1i, x + 2i) }
950 let (x, y) = next_two(5i);
951 println!("x, y = {}, {}", x, y);
955 Even though Rust functions can only return one value, a tuple _is_ one value,
956 that happens to be made up of two. You can also see in this example how you
957 can destructure a pattern returned by a function, as well.
959 Tuples are a very simple data structure, and so are not often what you want.
960 Let's move on to their bigger sibling, structs.
964 A struct is another form of a 'record type,' just like a tuple. There's a
965 difference: structs give each element that they contain a name, called a
966 'field' or a 'member.' Check it out:
975 let origin = Point { x: 0i, y: 0i };
977 println!("The origin is at ({}, {})", origin.x, origin.y);
981 There's a lot going on here, so let's break it down. We declare a struct with
982 the `struct` keyword, and then with a name. By convention, structs begin with a
983 capital letter and are also camel cased: `PointInSpace`, not `Point_In_Space`.
985 We can create an instance of our struct via `let`, as usual, but we use a `key:
986 value` style syntax to set each field. The order doesn't need to be the same as
987 in the original declaration.
989 Finally, because fields have names, we can access the field through dot
990 notation: `origin.x`.
992 The values in structs are immutable, like other bindings in Rust. However, you
993 can use `mut` to make them mutable:
1002 let mut point = Point { x: 0i, y: 0i };
1006 println!("The point is at ({}, {})", point.x, point.y);
1010 This will print `The point is at (5, 0)`.
1012 ## Tuple Structs and Newtypes
1014 Rust has another data type that's like a hybrid between a tuple and a struct,
1015 called a **tuple struct**. Tuple structs do have a name, but their fields
1020 struct Color(int, int, int);
1021 struct Point(int, int, int);
1024 These two will not be equal, even if they have the same values:
1027 let black = Color(0, 0, 0);
1028 let origin = Point(0, 0, 0);
1031 It is almost always better to use a struct than a tuple struct. We would write
1032 `Color` and `Point` like this instead:
1048 Now, we have actual names, rather than positions. Good names are important,
1049 and with a struct, we have actual names.
1051 There _is_ one case when a tuple struct is very useful, though, and that's a
1052 tuple struct with only one element. We call this a 'newtype,' because it lets
1053 you create a new type that's a synonym for another one:
1058 let length = Inches(10);
1060 let Inches(integer_length) = length;
1061 println!("length is {} inches", integer_length);
1064 As you can see here, you can extract the inner integer type through a
1065 destructuring `let`.
1069 Finally, Rust has a "sum type", an **enum**. Enums are an incredibly useful
1070 feature of Rust, and are used throughout the standard library. This is an enum
1071 that is provided by the Rust standard library:
1081 An `Ordering` can only be _one_ of `Less`, `Equal`, or `Greater` at any given
1082 time. Here's an example:
1085 fn cmp(a: int, b: int) -> Ordering {
1087 else if a > b { Greater }
1095 let ordering = cmp(x, y);
1097 if ordering == Less {
1099 } else if ordering == Greater {
1100 println!("greater");
1101 } else if ordering == Equal {
1107 `cmp` is a function that compares two things, and returns an `Ordering`. We
1108 return either `Less`, `Greater`, or `Equal`, depending on if the two values
1109 are greater, less, or equal.
1111 The `ordering` variable has the type `Ordering`, and so contains one of the
1112 three values. We can then do a bunch of `if`/`else` comparisons to check
1115 However, repeated `if`/`else` comparisons get quite tedious. Rust has a feature
1116 that not only makes them nicer to read, but also makes sure that you never
1117 miss a case. Before we get to that, though, let's talk about another kind of
1118 enum: one with values.
1120 This enum has two variants, one of which has a value:
1133 Value(n) => println!("x is {:d}", n),
1134 Missing => println!("x is missing!"),
1138 Value(n) => println!("y is {:d}", n),
1139 Missing => println!("y is missing!"),
1144 This enum represents an `int` that we may or may not have. In the `Missing`
1145 case, we have no value, but in the `Value` case, we do. This enum is specific
1146 to `int`s, though. We can make it usable by any type, but we haven't quite
1149 You can have any number of values in an enum:
1152 enum OptionalColor {
1153 Color(int, int, int),
1158 Enums with values are quite useful, but as I mentioned, they're even more
1159 useful when they're generic across types. But before we get to generics, let's
1160 talk about how to fix these big `if`/`else` statements we've been writing. We'll
1161 do that with `match`.
1165 Often, a simple `if`/`else` isn't enough, because you have more than two
1166 possible options. And `else` conditions can get incredibly complicated. So
1167 what's the solution?
1169 Rust has a keyword, `match`, that allows you to replace complicated `if`/`else`
1170 groupings with something more powerful. Check it out:
1176 1 => println!("one"),
1177 2 => println!("two"),
1178 3 => println!("three"),
1179 4 => println!("four"),
1180 5 => println!("five"),
1181 _ => println!("something else"),
1185 `match` takes an expression, and then branches based on its value. Each 'arm' of
1186 the branch is of the form `val => expression`. When the value matches, that arm's
1187 expression will be evaluated. It's called `match` because of the term 'pattern
1188 matching,' which `match` is an implementation of.
1190 So what's the big advantage here? Well, there are a few. First of all, `match`
1191 does 'exhaustiveness checking.' Do you see that last arm, the one with the
1192 underscore (`_`)? If we remove that arm, Rust will give us an error:
1195 error: non-exhaustive patterns: `_` not covered
1198 In other words, Rust is trying to tell us we forgot a value. Because `x` is an
1199 integer, Rust knows that it can have a number of different values. For example,
1200 `6i`. But without the `_`, there is no arm that could match, and so Rust refuses
1201 to compile. `_` is sort of like a catch-all arm. If none of the other arms match,
1202 the arm with `_` will. And since we have this catch-all arm, we now have an arm
1203 for every possible value of `x`, and so our program will now compile.
1205 `match` statements also destructure enums, as well. Remember this code from the
1209 fn cmp(a: int, b: int) -> Ordering {
1211 else if a > b { Greater }
1219 let ordering = cmp(x, y);
1221 if ordering == Less {
1223 } else if ordering == Greater {
1224 println!("greater");
1225 } else if ordering == Equal {
1231 We can re-write this as a `match`:
1234 fn cmp(a: int, b: int) -> Ordering {
1236 else if a > b { Greater }
1245 Less => println!("less"),
1246 Greater => println!("greater"),
1247 Equal => println!("equal"),
1252 This version has way less noise, and it also checks exhaustively to make sure
1253 that we have covered all possible variants of `Ordering`. With our `if`/`else`
1254 version, if we had forgotten the `Greater` case, for example, our program would
1255 have happily compiled. If we forget in the `match`, it will not. Rust helps us
1256 make sure to cover all of our bases.
1258 `match` is also an expression, which means we can use it on the right
1259 hand side of a `let` binding or directly where an expression is
1260 used. We could also implement the previous line like this:
1263 fn cmp(a: int, b: int) -> Ordering {
1265 else if a > b { Greater }
1273 println!("{}", match cmp(x, y) {
1275 Greater => "greater",
1281 Sometimes, it's a nice pattern.
1285 Looping is the last basic construct that we haven't learned yet in Rust. Rust has
1286 two main looping constructs: `for` and `while`.
1290 The `for` loop is used to loop a particular number of times. Rust's `for` loops
1291 work a bit differently than in other systems languages, however. Rust's `for`
1292 loop doesn't look like this "C style" `for` loop:
1295 for (x = 0; x < 10; x++) {
1296 printf( "%d\n", x );
1300 Instead, it looks like this:
1303 for x in range(0i, 10i) {
1304 println!("{:d}", x);
1308 In slightly more abstract terms,
1311 for var in expression {
1316 The expression is an iterator, which we will discuss in more depth later in the
1317 guide. The iterator gives back a series of elements. Each element is one
1318 iteration of the loop. That value is then bound to the name `var`, which is
1319 valid for the loop body. Once the body is over, the next value is fetched from
1320 the iterator, and we loop another time. When there are no more values, the
1323 In our example, `range` is a function that takes a start and an end position,
1324 and gives an iterator over those values. The upper bound is exclusive, though,
1325 so our loop will print `0` through `9`, not `10`.
1327 Rust does not have the "C style" `for` loop on purpose. Manually controlling
1328 each element of the loop is complicated and error prone, even for experienced C
1331 We'll talk more about `for` when we cover **iterator**s, later in the Guide.
1335 The other kind of looping construct in Rust is the `while` loop. It looks like
1340 let mut done = false;
1345 if x % 5 == 0 { done = true; }
1349 `while` loops are the correct choice when you're not sure how many times
1352 If you need an infinite loop, you may be tempted to write this:
1358 Rust has a dedicated keyword, `loop`, to handle this case:
1364 Rust's control-flow analysis treats this construct differently than a
1365 `while true`, since we know that it will always loop. The details of what
1366 that _means_ aren't super important to understand at this stage, but in
1367 general, the more information we can give to the compiler, the better it
1368 can do with safety and code generation. So you should always prefer
1369 `loop` when you plan to loop infinitely.
1371 ## Ending iteration early
1373 Let's take a look at that `while` loop we had earlier:
1377 let mut done = false;
1382 if x % 5 == 0 { done = true; }
1386 We had to keep a dedicated `mut` boolean variable binding, `done`, to know
1387 when we should skip out of the loop. Rust has two keywords to help us with
1388 modifying iteration: `break` and `continue`.
1390 In this case, we can write the loop in a better way with `break`:
1398 if x % 5 == 0 { break; }
1402 We now loop forever with `loop`, and use `break` to break out early.
1404 `continue` is similar, but instead of ending the loop, goes to the next
1405 iteration: This will only print the odd numbers:
1408 for x in range(0i, 10i) {
1409 if x % 2 == 0 { continue; }
1411 println!("{:d}", x);
1415 Both `continue` and `break` are valid in both kinds of loops.
1419 Strings are an important concept for any programmer to master. Rust's string
1420 handling system is a bit different than in other languages, due to its systems
1421 focus. Any time you have a data structure of variable size, things can get
1422 tricky, and strings are a re-sizable data structure. That said, Rust's strings
1423 also work differently than in some other systems languages, such as C.
1425 Let's dig into the details. A **string** is a sequence of unicode scalar values
1426 encoded as a stream of UTF-8 bytes. All strings are guaranteed to be
1427 validly-encoded UTF-8 sequences. Additionally, strings are not null-terminated
1428 and can contain null bytes.
1430 Rust has two main types of strings: `&str` and `String`.
1432 The first kind is a `&str`. This is pronounced a 'string slice.' String literals
1433 are of the type `&str`:
1436 let string = "Hello there.";
1439 This string is statically allocated, meaning that it's saved inside our
1440 compiled program, and exists for the entire duration it runs. The `string`
1441 binding is a reference to this statically allocated string. String slices
1442 have a fixed size, and cannot be mutated.
1444 A `String`, on the other hand, is an in-memory string. This string is
1445 growable, and is also guaranteed to be UTF-8.
1448 let mut s = "Hello".to_string();
1451 s.push_str(", world.");
1455 You can coerce a `String` into a `&str` with the `as_slice()` method:
1458 fn takes_slice(slice: &str) {
1459 println!("Got: {}", slice);
1463 let s = "Hello".to_string();
1464 takes_slice(s.as_slice());
1468 To compare a String to a constant string, prefer `as_slice()`...
1471 fn compare(string: String) {
1472 if string.as_slice() == "Hello" {
1478 ... over `to_string()`:
1481 fn compare(string: String) {
1482 if string == "Hello".to_string() {
1488 Converting a `String` to a `&str` is cheap, but converting the `&str` to a
1489 `String` involves allocating memory. No reason to do that unless you have to!
1491 That's the basics of strings in Rust! They're probably a bit more complicated
1492 than you are used to, if you come from a scripting language, but when the
1493 low-level details matter, they really matter. Just remember that `String`s
1494 allocate memory and control their data, while `&str`s are a reference to
1495 another string, and you'll be all set.
1497 # Arrays, Vectors, and Slices
1499 Like many programming languages, Rust has list types to represent a sequence of
1500 things. The most basic is the **array**, a fixed-size list of elements of the
1501 same type. By default, arrays are immutable.
1504 let a = [1i, 2i, 3i];
1505 let mut m = [1i, 2i, 3i];
1508 You can create an array with a given number of elements, all initialized to the
1509 same value, with `[val, ..N]` syntax. The compiler ensures that arrays are
1513 let a = [0i, ..20]; // Shorthand for array of 20 elements all initialized to 0
1516 Arrays have type `[T,..N]`. We'll talk about this `T` notation later, when we
1519 You can get the number of elements in an array `a` with `a.len()`, and use
1520 `a.iter()` to iterate over them with a for loop. This code will print each
1524 let a = [1i, 2, 3]; // Only the first item needs a type suffix
1526 println!("a has {} elements", a.len());
1532 You can access a particular element of an array with **subscript notation**:
1535 let names = ["Graydon", "Brian", "Niko"];
1537 println!("The second name is: {}", names[1]);
1540 Subscripts start at zero, like in most programming languages, so the first name
1541 is `names[0]` and the second name is `names[1]`. The above example prints
1542 `The second name is: Brian`. If you try to use a subscript that is not in the
1543 array, you will get an error: array access is bounds-checked at run-time. Such
1544 errant access is the source of many bugs in other systems programming
1547 A **vector** is a dynamic or "growable" array, implemented as the standard
1548 library type [`Vec<T>`](std/vec/) (we'll talk about what the `<T>` means
1549 later). Vectors are to arrays what `String` is to `&str`. You can create them
1550 with the `vec!` macro:
1553 let v = vec![1i, 2, 3];
1556 (Notice that unlike the `println!` macro we've used in the past, we use square
1557 brackets `[]` with `vec!`. Rust allows you to use either in either situation,
1558 this is just convention.)
1560 You can get the length of, iterate over, and subscript vectors just like
1561 arrays. In addition, (mutable) vectors can grow automatically:
1564 let mut nums = vec![1i, 2, 3];
1566 println!("The length of nums is now {}", nums.len()); // Prints 4
1569 Vectors have many more useful methods.
1571 A **slice** is a reference to (or "view" into) an array. They are useful for
1572 allowing safe, efficient access to a portion of an array without copying. For
1573 example, you might want to reference just one line of a file read into memory.
1574 By nature, a slice is not created directly, but from an existing variable.
1575 Slices have a length, can be mutable or not, and in many ways behave like
1579 let a = [0i, 1, 2, 3, 4];
1580 let middle = a.slice(1, 4); // A slice of a: just the elements [1,2,3]
1582 for e in middle.iter() {
1583 println!("{}", e); // Prints 1, 2, 3
1587 You can also take a slice of a vector, `String`, or `&str`, because they are
1588 backed by arrays. Slices have type `&[T]`, which we'll talk about when we cover
1591 We have now learned all of the most basic Rust concepts. We're ready to start
1592 building our guessing game, we just need to know one last thing: how to get
1593 input from the keyboard. You can't have a guessing game without the ability to
1598 Getting input from the keyboard is pretty easy, but uses some things
1599 we haven't seen before. Here's a simple program that reads some input,
1600 and then prints it back out:
1604 println!("Type something!");
1606 let input = std::io::stdin().read_line().ok().expect("Failed to read line");
1608 println!("{}", input);
1612 Let's go over these chunks, one by one:
1618 This calls a function, `stdin()`, that lives inside the `std::io` module. As
1619 you can imagine, everything in `std` is provided by Rust, the 'standard
1620 library.' We'll talk more about the module system later.
1622 Since writing the fully qualified name all the time is annoying, we can use
1623 the `use` statement to import it in:
1631 However, it's considered better practice to not import individual functions, but
1632 to import the module, and only use one level of qualification:
1640 Let's update our example to use this style:
1646 println!("Type something!");
1648 let input = io::stdin().read_line().ok().expect("Failed to read line");
1650 println!("{}", input);
1660 The `read_line()` method can be called on the result of `stdin()` to return
1661 a full line of input. Nice and easy.
1664 .ok().expect("Failed to read line");
1667 Do you remember this code?
1680 Value(n) => println!("x is {:d}", n),
1681 Missing => println!("x is missing!"),
1685 Value(n) => println!("y is {:d}", n),
1686 Missing => println!("y is missing!"),
1691 We had to match each time, to see if we had a value or not. In this case,
1692 though, we _know_ that `x` has a `Value`. But `match` forces us to handle
1693 the `missing` case. This is what we want 99% of the time, but sometimes, we
1694 know better than the compiler.
1696 Likewise, `read_line()` does not return a line of input. It _might_ return a
1697 line of input. It might also fail to do so. This could happen if our program
1698 isn't running in a terminal, but as part of a cron job, or some other context
1699 where there's no standard input. Because of this, `read_line` returns a type
1700 very similar to our `OptionalInt`: an `IoResult<T>`. We haven't talked about
1701 `IoResult<T>` yet because it is the **generic** form of our `OptionalInt`.
1702 Until then, you can think of it as being the same thing, just for any type, not
1705 Rust provides a method on these `IoResult<T>`s called `ok()`, which does the
1706 same thing as our `match` statement, but assuming that we have a valid value.
1707 We then call `expect()` on the result, which will terminate our program if we
1708 don't have a valid value. In this case, if we can't get input, our program
1709 doesn't work, so we're okay with that. In most cases, we would want to handle
1710 the error case explicitly. `expect()` allows us to give an error message if
1713 We will cover the exact details of how all of this works later in the Guide.
1714 For now, this gives you enough of a basic understanding to work with.
1716 Back to the code we were working on! Here's a refresher:
1722 println!("Type something!");
1724 let input = io::stdin().read_line().ok().expect("Failed to read line");
1726 println!("{}", input);
1730 With long lines like this, Rust gives you some flexibility with the whitespace.
1731 We _could_ write the example like this:
1737 println!("Type something!");
1739 let input = io::stdin()
1742 .expect("Failed to read line");
1744 println!("{}", input);
1748 Sometimes, this makes things more readable. Sometimes, less. Use your judgment
1751 That's all you need to get basic input from the standard input! It's not too
1752 complicated, but there are a number of small parts.
1756 Okay! We've got the basics of Rust down. Let's write a bigger program.
1758 For our first project, we'll implement a classic beginner programming problem:
1759 the guessing game. Here's how it works: Our program will generate a random
1760 integer between one and a hundred. It will then prompt us to enter a guess.
1761 Upon entering our guess, it will tell us if we're too low or too high. Once we
1762 guess correctly, it will congratulate us. Sound good?
1766 Let's set up a new project. Go to your projects directory. Remember how we
1767 had to create our directory structure and a `Cargo.toml` for `hello_world`? Cargo
1768 has a command that does that for us. Let's give it a shot:
1772 $ cargo new guessing_game --bin
1776 We pass the name of our project to `cargo new`, and then the `--bin` flag,
1777 since we're making a binary, rather than a library.
1779 Check out the generated `Cargo.toml`:
1784 name = "guessing_game"
1786 authors = ["Your Name <you@example.com>"]
1789 Cargo gets this information from your environment. If it's not correct, go ahead
1792 Finally, Cargo generated a hello, world for us. Check out `src/main.rs`:
1796 println!("Hello, world!");
1800 Let's try compiling what Cargo gave us:
1804 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1807 Excellent! Open up your `src/main.rs` again. We'll be writing all of
1808 our code in this file. We'll talk about multiple-file projects later on in the
1811 Before we move on, let me show you one more Cargo command: `run`. `cargo run`
1812 is kind of like `cargo build`, but it also then runs the produced executable.
1817 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1818 Running `target/guessing_game`
1822 Great! The `run` command comes in handy when you need to rapidly iterate on a project.
1823 Our game is just such a project, we need to quickly test each iteration before moving on to the next one.
1825 ## Processing a Guess
1827 Let's get to it! The first thing we need to do for our guessing game is
1828 allow our player to input a guess. Put this in your `src/main.rs`:
1834 println!("Guess the number!");
1836 println!("Please input your guess.");
1838 let input = io::stdin().read_line()
1840 .expect("Failed to read line");
1842 println!("You guessed: {}", input);
1846 You've seen this code before, when we talked about standard input. We
1847 import the `std::io` module with `use`, and then our `main` function contains
1848 our program's logic. We print a little message announcing the game, ask the
1849 user to input a guess, get their input, and then print it out.
1851 Because we talked about this in the section on standard I/O, I won't go into
1852 more details here. If you need a refresher, go re-read that section.
1854 ## Generating a secret number
1856 Next, we need to generate a secret number. To do that, we need to use Rust's
1857 random number generation, which we haven't talked about yet. Rust includes a
1858 bunch of interesting functions in its standard library. If you need a bit of
1859 code, it's possible that it's already been written for you! In this case,
1860 we do know that Rust has random number generation, but we don't know how to
1863 Enter the docs. Rust has a page specifically to document the standard library.
1864 You can find that page [here](std/index.html). There's a lot of information on
1865 that page, but the best part is the search bar. Right up at the top, there's
1866 a box that you can enter in a search term. The search is pretty primitive
1867 right now, but is getting better all the time. If you type 'random' in that
1868 box, the page will update to [this
1869 one](std/index.html?search=random). The very first
1871 [std::rand::random](std/rand/fn.random.html). If we
1872 click on that result, we'll be taken to its documentation page.
1874 This page shows us a few things: the type signature of the function, some
1875 explanatory text, and then an example. Let's modify our code to add in the
1883 println!("Guess the number!");
1885 let secret_number = (rand::random() % 100i) + 1i;
1887 println!("The secret number is: {}", secret_number);
1889 println!("Please input your guess.");
1891 let input = io::stdin().read_line()
1893 .expect("Failed to read line");
1896 println!("You guessed: {}", input);
1900 The first thing we changed was to `use std::rand`, as the docs
1901 explained. We then added in a `let` expression to create a variable binding
1902 named `secret_number`, and we printed out its result.
1904 Also, you may wonder why we are using `%` on the result of `rand::random()`.
1905 This operator is called 'modulo', and it returns the remainder of a division.
1906 By taking the modulo of the result of `rand::random()`, we're limiting the
1907 values to be between 0 and 99. Then, we add one to the result, making it from 1
1908 to 100. Using modulo can give you a very, very small bias in the result, but
1909 for this example, it is not important.
1911 Let's try to compile this using `cargo build`:
1915 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1916 src/main.rs:7:26: 7:34 error: the type of this value must be known in this context
1917 src/main.rs:7 let secret_number = (rand::random() % 100i) + 1i;
1919 error: aborting due to previous error
1922 It didn't work! Rust says "the type of this value must be known in this
1923 context." What's up with that? Well, as it turns out, `rand::random()` can
1924 generate many kinds of random values, not just integers. And in this case, Rust
1925 isn't sure what kind of value `random()` should generate. So we have to help
1926 it. With number literals, we just add an `i` onto the end to tell Rust they're
1927 integers, but that does not work with functions. There's a different syntax,
1928 and it looks like this:
1931 rand::random::<int>();
1934 This says "please give me a random `int` value." We can change our code to use
1942 println!("Guess the number!");
1944 let secret_number = (rand::random::<int>() % 100i) + 1i;
1946 println!("The secret number is: {}", secret_number);
1948 println!("Please input your guess.");
1950 let input = io::stdin().read_line()
1952 .expect("Failed to read line");
1955 println!("You guessed: {}", input);
1959 Try running our new program a few times:
1963 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
1964 Running `target/guessing_game`
1966 The secret number is: 7
1967 Please input your guess.
1970 $ ./target/guessing_game
1972 The secret number is: 83
1973 Please input your guess.
1976 $ ./target/guessing_game
1978 The secret number is: -29
1979 Please input your guess.
1984 Wait. Negative 29? We wanted a number between one and a hundred! We have two
1985 options here: we can either ask `random()` to generate an unsigned integer, which
1986 can only be positive, or we can use the `abs()` function. Let's go with the
1987 unsigned integer approach. If we want a random positive number, we should ask for
1988 a random positive number. Our code looks like this now:
1995 println!("Guess the number!");
1997 let secret_number = (rand::random::<uint>() % 100u) + 1u;
1999 println!("The secret number is: {}", secret_number);
2001 println!("Please input your guess.");
2003 let input = io::stdin().read_line()
2005 .expect("Failed to read line");
2008 println!("You guessed: {}", input);
2016 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2017 Running `target/guessing_game`
2019 The secret number is: 57
2020 Please input your guess.
2025 Great! Next up: let's compare our guess to the secret guess.
2027 ## Comparing guesses
2029 If you remember, earlier in the guide, we made a `cmp` function that compared
2030 two numbers. Let's add that in, along with a `match` statement to compare the
2031 guess to the secret guess:
2038 println!("Guess the number!");
2040 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2042 println!("The secret number is: {}", secret_number);
2044 println!("Please input your guess.");
2046 let input = io::stdin().read_line()
2048 .expect("Failed to read line");
2051 println!("You guessed: {}", input);
2053 match cmp(input, secret_number) {
2054 Less => println!("Too small!"),
2055 Greater => println!("Too big!"),
2056 Equal => println!("You win!"),
2060 fn cmp(a: int, b: int) -> Ordering {
2062 else if a > b { Greater }
2067 If we try to compile, we'll get some errors:
2071 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2072 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)
2073 src/main.rs:20 match cmp(input, secret_number) {
2075 src/main.rs:20:22: 20:35 error: mismatched types: expected `int` but found `uint` (expected int but found uint)
2076 src/main.rs:20 match cmp(input, secret_number) {
2078 error: aborting due to 2 previous errors
2081 This often happens when writing Rust programs, and is one of Rust's greatest
2082 strengths. You try out some code, see if it compiles, and Rust tells you that
2083 you've done something wrong. In this case, our `cmp` function works on integers,
2084 but we've given it unsigned integers. In this case, the fix is easy, because
2085 we wrote the `cmp` function! Let's change it to take `uint`s:
2092 println!("Guess the number!");
2094 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2096 println!("The secret number is: {}", secret_number);
2098 println!("Please input your guess.");
2100 let input = io::stdin().read_line()
2102 .expect("Failed to read line");
2105 println!("You guessed: {}", input);
2107 match cmp(input, secret_number) {
2108 Less => println!("Too small!"),
2109 Greater => println!("Too big!"),
2110 Equal => println!("You win!"),
2114 fn cmp(a: uint, b: uint) -> Ordering {
2116 else if a > b { Greater }
2121 And try compiling again:
2125 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2126 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)
2127 src/main.rs:20 match cmp(input, secret_number) {
2129 error: aborting due to previous error
2132 This error is similar to the last one: we expected to get a `uint`, but we got
2133 a `String` instead! That's because our `input` variable is coming from the
2134 standard input, and you can guess anything. Try it:
2137 $ ./target/guessing_game
2139 The secret number is: 73
2140 Please input your guess.
2145 Oops! Also, you'll note that we just ran our program even though it didn't compile.
2146 This works because the older version we did successfully compile was still lying
2147 around. Gotta be careful!
2149 Anyway, we have a `String`, but we need a `uint`. What to do? Well, there's
2150 a function for that:
2153 let input = io::stdin().read_line()
2155 .expect("Failed to read line");
2156 let input_num: Option<uint> = from_str(input.as_slice());
2159 The `from_str` function takes in a `&str` value and converts it into something.
2160 We tell it what kind of something with a type hint. Remember our type hint with
2161 `random()`? It looked like this:
2164 rand::random::<uint>();
2167 There's an alternate way of providing a hint too, and that's declaring the type
2171 let x: uint = rand::random();
2174 In this case, we say `x` is a `uint` explicitly, so Rust is able to properly
2175 tell `random()` what to generate. In a similar fashion, both of these work:
2178 let input_num = from_str::<uint>("5");
2179 let input_num: Option<uint> = from_str("5");
2182 Anyway, with us now converting our input to a number, our code looks like this:
2189 println!("Guess the number!");
2191 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2193 println!("The secret number is: {}", secret_number);
2195 println!("Please input your guess.");
2197 let input = io::stdin().read_line()
2199 .expect("Failed to read line");
2200 let input_num: Option<uint> = from_str(input.as_slice());
2202 println!("You guessed: {}", input_num);
2204 match cmp(input_num, secret_number) {
2205 Less => println!("Too small!"),
2206 Greater => println!("Too big!"),
2207 Equal => println!("You win!"),
2211 fn cmp(a: uint, b: uint) -> Ordering {
2213 else if a > b { Greater }
2222 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2223 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)
2224 src/main.rs:22 match cmp(input_num, secret_number) {
2226 error: aborting due to previous error
2229 Oh yeah! Our `input_num` has the type `Option<uint>`, rather than `uint`. We
2230 need to unwrap the Option. If you remember from before, `match` is a great way
2231 to do that. Try this code:
2238 println!("Guess the number!");
2240 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2242 println!("The secret number is: {}", secret_number);
2244 println!("Please input your guess.");
2246 let input = io::stdin().read_line()
2248 .expect("Failed to read line");
2249 let input_num: Option<uint> = from_str(input.as_slice());
2251 let num = match input_num {
2254 println!("Please input a number!");
2260 println!("You guessed: {}", num);
2262 match cmp(num, secret_number) {
2263 Less => println!("Too small!"),
2264 Greater => println!("Too big!"),
2265 Equal => println!("You win!"),
2269 fn cmp(a: uint, b: uint) -> Ordering {
2271 else if a > b { Greater }
2276 We use a `match` to either give us the `uint` inside of the `Option`, or we
2277 print an error message and return. Let's give this a shot:
2281 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2282 Running `target/guessing_game`
2284 The secret number is: 17
2285 Please input your guess.
2287 Please input a number!
2290 Uh, what? But we did!
2292 ... actually, we didn't. See, when you get a line of input from `stdin()`,
2293 you get all the input. Including the `\n` character from you pressing Enter.
2294 So, `from_str()` sees the string `"5\n"` and says "nope, that's not a number,
2295 there's non-number stuff in there!" Luckily for us, `&str`s have an easy
2296 method we can use defined on them: `trim()`. One small modification, and our
2297 code looks like this:
2304 println!("Guess the number!");
2306 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2308 println!("The secret number is: {}", secret_number);
2310 println!("Please input your guess.");
2312 let input = io::stdin().read_line()
2314 .expect("Failed to read line");
2315 let input_num: Option<uint> = from_str(input.as_slice().trim());
2317 let num = match input_num {
2320 println!("Please input a number!");
2326 println!("You guessed: {}", num);
2328 match cmp(num, secret_number) {
2329 Less => println!("Too small!"),
2330 Greater => println!("Too big!"),
2331 Equal => println!("You win!"),
2335 fn cmp(a: uint, b: uint) -> Ordering {
2337 else if a > b { Greater }
2346 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2347 Running `target/guessing_game`
2349 The secret number is: 58
2350 Please input your guess.
2356 Nice! You can see I even added spaces before my guess, and it still figured
2357 out that I guessed 76. Run the program a few times, and verify that guessing
2358 the number works, as well as guessing a number too small.
2360 The Rust compiler helped us out quite a bit there! This technique is called
2361 "lean on the compiler," and it's often useful when working on some code. Let
2362 the error messages help guide you towards the correct types.
2364 Now we've got most of the game working, but we can only make one guess. Let's
2365 change that by adding loops!
2369 As we already discussed, the `loop` keyword gives us an infinite loop. So
2377 println!("Guess the number!");
2379 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2381 println!("The secret number is: {}", secret_number);
2385 println!("Please input your guess.");
2387 let input = io::stdin().read_line()
2389 .expect("Failed to read line");
2390 let input_num: Option<uint> = from_str(input.as_slice().trim());
2392 let num = match input_num {
2395 println!("Please input a number!");
2401 println!("You guessed: {}", num);
2403 match cmp(num, secret_number) {
2404 Less => println!("Too small!"),
2405 Greater => println!("Too big!"),
2406 Equal => println!("You win!"),
2411 fn cmp(a: uint, b: uint) -> Ordering {
2413 else if a > b { Greater }
2418 And try it out. But wait, didn't we just add an infinite loop? Yup. Remember
2419 that `return`? If we give a non-number answer, we'll `return` and quit. Observe:
2423 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2424 Running `target/guessing_game`
2426 The secret number is: 59
2427 Please input your guess.
2431 Please input your guess.
2435 Please input your guess.
2439 Please input your guess.
2441 Please input a number!
2444 Ha! `quit` actually quits. As does any other non-number input. Well, this is
2445 suboptimal to say the least. First, let's actually quit when you win the game:
2452 println!("Guess the number!");
2454 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2456 println!("The secret number is: {}", secret_number);
2460 println!("Please input your guess.");
2462 let input = io::stdin().read_line()
2464 .expect("Failed to read line");
2465 let input_num: Option<uint> = from_str(input.as_slice().trim());
2467 let num = match input_num {
2470 println!("Please input a number!");
2476 println!("You guessed: {}", num);
2478 match cmp(num, secret_number) {
2479 Less => println!("Too small!"),
2480 Greater => println!("Too big!"),
2482 println!("You win!");
2489 fn cmp(a: uint, b: uint) -> Ordering {
2491 else if a > b { Greater }
2496 By adding the `return` line after the `You win!`, we'll exit the program when
2497 we win. We have just one more tweak to make: when someone inputs a non-number,
2498 we don't want to quit, we just want to ignore it. Change that `return` to
2507 println!("Guess the number!");
2509 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2511 println!("The secret number is: {}", secret_number);
2515 println!("Please input your guess.");
2517 let input = io::stdin().read_line()
2519 .expect("Failed to read line");
2520 let input_num: Option<uint> = from_str(input.as_slice().trim());
2522 let num = match input_num {
2525 println!("Please input a number!");
2531 println!("You guessed: {}", num);
2533 match cmp(num, secret_number) {
2534 Less => println!("Too small!"),
2535 Greater => println!("Too big!"),
2537 println!("You win!");
2544 fn cmp(a: uint, b: uint) -> Ordering {
2546 else if a > b { Greater }
2551 Now we should be good! Let's try:
2555 Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
2556 Running `target/guessing_game`
2558 The secret number is: 61
2559 Please input your guess.
2563 Please input your guess.
2567 Please input your guess.
2569 Please input a number!
2570 Please input your guess.
2576 Awesome! With one tiny last tweak, we have finished the guessing game. Can you
2577 think of what it is? That's right, we don't want to print out the secret number.
2578 It was good for testing, but it kind of ruins the game. Here's our final source:
2585 println!("Guess the number!");
2587 let secret_number = (rand::random::<uint>() % 100u) + 1u;
2591 println!("Please input your guess.");
2593 let input = io::stdin().read_line()
2595 .expect("Failed to read line");
2596 let input_num: Option<uint> = from_str(input.as_slice().trim());
2598 let num = match input_num {
2601 println!("Please input a number!");
2607 println!("You guessed: {}", num);
2609 match cmp(num, secret_number) {
2610 Less => println!("Too small!"),
2611 Greater => println!("Too big!"),
2613 println!("You win!");
2620 fn cmp(a: uint, b: uint) -> Ordering {
2622 else if a > b { Greater }
2629 At this point, you have successfully built the Guessing Game! Congratulations!
2631 You've now learned the basic syntax of Rust. All of this is relatively close to
2632 various other programming languages you have used in the past. These
2633 fundamental syntactical and semantic elements will form the foundation for the
2634 rest of your Rust education.
2636 Now that you're an expert at the basics, it's time to learn about some of
2637 Rust's more unique features.
2639 # Crates and Modules
2641 Rust features a strong module system, but it works a bit differently than in
2642 other programming languages. Rust's module system has two main components:
2643 **crate**s and **module**s.
2645 A crate is Rust's unit of independent compilation. Rust always compiles one
2646 crate at a time, producing either a library or an executable. However, executables
2647 usually depend on libraries, and many libraries depend on other libraries as well.
2648 To support this, crates can depend on other crates.
2650 Each crate contains a hierarchy of modules. This tree starts off with a single
2651 module, called the **crate root**. Within the crate root, we can declare other
2652 modules, which can contain other modules, as deeply as you'd like.
2654 Note that we haven't mentioned anything about files yet. Rust does not impose a
2655 particular relationship between your filesystem structure and your module
2656 structure. That said, there is a conventional approach to how Rust looks for
2657 modules on the file system, but it's also overridable.
2659 Enough talk, let's build something! Let's make a new project called `modules`.
2663 $ cargo new modules --bin
2667 Let's double check our work by compiling:
2671 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2672 Running `target/modules`
2676 Excellent! So, we already have a single crate here: our `src/main.rs` is a crate.
2677 Everything in that file is in the crate root. A crate that generates an executable
2678 defines a `main` function inside its root, as we've done here.
2680 Let's define a new module inside our crate. Edit `src/main.rs` to look
2685 println!("Hello, world!");
2690 println!("Hello, world!");
2695 We now have a module named `hello` inside of our crate root. Modules use
2696 `snake_case` naming, like functions and variable bindings.
2698 Inside the `hello` module, we've defined a `print_hello` function. This will
2699 also print out our hello world message. Modules allow you to split up your
2700 program into nice neat boxes of functionality, grouping common things together,
2701 and keeping different things apart. It's kinda like having a set of shelves:
2702 a place for everything and everything in its place.
2704 To call our `print_hello` function, we use the double colon (`::`):
2707 hello::print_hello();
2710 You've seen this before, with `io::stdin()` and `rand::random()`. Now you know
2711 how to make your own. However, crates and modules have rules about
2712 **visibility**, which controls who exactly may use the functions defined in a
2713 given module. By default, everything in a module is private, which means that
2714 it can only be used by other functions in the same module. This will not
2719 hello::print_hello();
2724 println!("Hello, world!");
2732 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2733 src/main.rs:2:5: 2:23 error: function `print_hello` is private
2734 src/main.rs:2 hello::print_hello();
2738 To make it public, we use the `pub` keyword:
2742 hello::print_hello();
2746 pub fn print_hello() {
2747 println!("Hello, world!");
2752 Usage of the `pub` keyword is sometimes called 'exporting', because
2753 we're making the function available for other modules. This will work:
2757 Compiling modules v0.0.1 (file:///home/you/projects/modules)
2758 Running `target/modules`
2762 Nice! There are more things we can do with modules, including moving them into
2763 their own files. This is enough detail for now.
2767 Traditionally, testing has not been a strong suit of most systems programming
2768 languages. Rust, however, has very basic testing built into the language
2769 itself. While automated testing cannot prove that your code is bug-free, it is
2770 useful for verifying that certain behaviors work as intended.
2772 Here's a very basic test:
2776 fn is_one_equal_to_one() {
2781 You may notice something new: that `#[test]`. Before we get into the mechanics
2782 of testing, let's talk about attributes.
2786 Rust's testing system uses **attribute**s to mark which functions are tests.
2787 Attributes can be placed on any Rust **item**. Remember how most things in
2788 Rust are an expression, but `let` is not? Item declarations are also not
2789 expressions. Here's a list of things that qualify as an item:
2800 You haven't learned about all of these things yet, but that's the list. As
2801 you can see, functions are at the top of it.
2803 Attributes can appear in three ways:
2805 1. A single identifier, the attribute name. `#[test]` is an example of this.
2806 2. An identifier followed by an equals sign (`=`) and a literal. `#[cfg=test]`
2807 is an example of this.
2808 3. An identifier followed by a parenthesized list of sub-attribute arguments.
2809 `#[cfg(unix, target_word_size = "32")]` is an example of this, where one of
2810 the sub-arguments is of the second kind.
2812 There are a number of different kinds of attributes, enough that we won't go
2813 over them all here. Before we talk about the testing-specific attributes, I
2814 want to call out one of the most important kinds of attributes: stability
2817 ## Stability attributes
2819 Rust provides six attributes to indicate the stability level of various
2820 parts of your library. The six levels are:
2822 * deprecated: This item should no longer be used. No guarantee of backwards
2824 * experimental: This item was only recently introduced or is otherwise in a
2825 state of flux. It may change significantly, or even be removed. No guarantee
2826 of backwards-compatibility.
2827 * unstable: This item is still under development, but requires more testing to
2828 be considered stable. No guarantee of backwards-compatibility.
2829 * stable: This item is considered stable, and will not change significantly.
2830 Guarantee of backwards-compatibility.
2831 * frozen: This item is very stable, and is unlikely to change. Guarantee of
2832 backwards-compatibility.
2833 * locked: This item will never change unless a serious bug is found. Guarantee
2834 of backwards-compatibility.
2836 All of Rust's standard library uses these attribute markers to communicate
2837 their relative stability, and you should use them in your code, as well.
2838 There's an associated attribute, `warn`, that allows you to warn when you
2839 import an item marked with certain levels: deprecated, experimental and
2840 unstable. For now, only deprecated warns by default, but this will change once
2841 the standard library has been stabilized.
2843 You can use the `warn` attribute like this:
2849 And later, when you import a crate:
2852 extern crate some_crate;
2855 You'll get a warning if you use something marked unstable.
2857 You may have noticed an exclamation point in the `warn` attribute declaration.
2858 The `!` in this attribute means that this attribute applies to the enclosing
2859 item, rather than to the item that follows the attribute. So this `warn`
2860 attribute declaration applies to the enclosing crate itself, rather than
2861 to whatever item statement follows it:
2864 // applies to the crate we're in
2867 extern crate some_crate;
2869 // applies to the following `fn`.
2878 Let's write a very simple crate in a test-driven manner. You know the drill by
2879 now: make a new project:
2883 $ cargo new testing --bin
2891 Compiling testing v0.0.1 (file:///home/you/projects/testing)
2892 Running `target/testing`
2896 Great. Rust's infrastructure supports tests in two sorts of places, and they're
2897 for two kinds of tests: you include **unit test**s inside of the crate itself,
2898 and you place **integration test**s inside a `tests` directory. "Unit tests"
2899 are small tests that test one focused unit, "integration tests" tests multiple
2900 units in integration. That said, this is a social convention, they're no different
2901 in syntax. Let's make a `tests` directory:
2907 Next, let's create an integration test in `tests/lib.rs`:
2916 It doesn't matter what you name your test functions, though it's nice if
2917 you give them descriptive names. You'll see why in a moment. We then use a
2918 macro, `assert!`, to assert that something is true. In this case, we're giving
2919 it `false`, so this test should fail. Let's try it!
2923 Compiling testing v0.0.1 (file:///home/you/projects/testing)
2924 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
2925 /home/you/projects/testing/src/main.rs:1 fn main() {
2926 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
2927 /home/you/projects/testing/src/main.rs:3 }
2931 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
2939 ---- foo stdout ----
2940 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
2947 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
2949 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
2952 Lots of output! Let's break this down:
2956 Compiling testing v0.0.1 (file:///home/you/projects/testing)
2959 You can run all of your tests with `cargo test`. This runs both your tests in
2960 `tests`, as well as the tests you put inside of your crate.
2963 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
2964 /home/you/projects/testing/src/main.rs:1 fn main() {
2965 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
2966 /home/you/projects/testing/src/main.rs:3 }
2969 Rust has a **lint** called 'warn on dead code' used by default. A lint is a
2970 bit of code that checks your code, and can tell you things about it. In this
2971 case, Rust is warning us that we've written some code that's never used: our
2972 `main` function. Of course, since we're running tests, we don't use `main`.
2973 We'll turn this lint off for just this function soon. For now, just ignore this
2979 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
2982 Wait a minute, zero tests? Didn't we define one? Yup. This output is from
2983 attempting to run the tests in our crate, of which we don't have any.
2984 You'll note that Rust reports on several kinds of tests: passed, failed,
2985 ignored, and measured. The 'measured' tests refer to benchmark tests, which
2986 we'll cover soon enough!
2993 Now we're getting somewhere. Remember when we talked about naming our tests
2994 with good names? This is why. Here, it says 'test foo' because we called our
2995 test 'foo.' If we had given it a good name, it'd be more clear which test
2996 failed, especially as we accumulate more tests.
3001 ---- foo stdout ----
3002 task 'foo' failed at 'assertion failed: false', /home/you/projects/testing/tests/lib.rs:3
3009 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
3011 task '<main>' failed at 'Some tests failed', /home/you/src/rust/src/libtest/lib.rs:242
3014 After all the tests run, Rust will show us any output from our failed tests.
3015 In this instance, Rust tells us that our assertion failed, with false. This was
3018 Whew! Let's fix our test:
3027 And then try to run our tests again:
3031 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3032 /home/you/projects/testing/src/main.rs:1:1: 3:2 warning: code is never used: `main`, #[warn(dead_code)] on by default
3033 /home/you/projects/testing/src/main.rs:1 fn main() {
3034 /home/you/projects/testing/src/main.rs:2 println!("Hello, world");
3035 /home/you/projects/testing/src/main.rs:3 }
3039 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3045 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3048 Nice! Our test passes, as we expected. Let's get rid of that warning for our `main`
3049 function. Change your `src/main.rs` to look like this:
3054 println!("Hello, world");
3058 This attribute combines two things: `cfg` and `not`. The `cfg` attribute allows
3059 you to conditionally compile code based on something. The following item will
3060 only be compiled if the configuration says it's true. And when Cargo compiles
3061 our tests, it sets things up so that `cfg(test)` is true. But we want to only
3062 include `main` when it's _not_ true. So we use `not` to negate things:
3063 `cfg(not(test))` will only compile our code when the `cfg(test)` is false.
3065 With this attribute, we won't get the warning:
3069 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3073 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3079 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3082 Nice. Okay, let's write a real test now. Change your `tests/lib.rs`
3087 fn math_checks_out() {
3088 let result = add_three_times_four(5i);
3090 assert_eq!(32i, result);
3094 And try to run the test:
3098 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3099 /home/you/projects/testing/tests/lib.rs:3:18: 3:38 error: unresolved name `add_three_times_four`.
3100 /home/you/projects/testing/tests/lib.rs:3 let result = add_three_times_four(5i);
3101 ^~~~~~~~~~~~~~~~~~~~
3102 error: aborting due to previous error
3103 Build failed, waiting for other jobs to finish...
3104 Could not compile `testing`.
3106 To learn more, run the command again with --verbose.
3109 Rust can't find this function. That makes sense, as we didn't write it yet!
3111 In order to share this code with our tests, we'll need to make a library crate.
3112 This is also just good software design: as we mentioned before, it's a good idea
3113 to put most of your functionality into a library crate, and have your executable
3114 crate use that library. This allows for code re-use.
3116 To do that, we'll need to make a new module. Make a new file, `src/lib.rs`,
3121 pub fn add_three_times_four(x: int) -> int {
3126 We're calling this file `lib.rs` because it has the same name as our project,
3127 and so it's named this, by convention.
3129 We'll then need to use this crate in our `src/main.rs`:
3132 extern crate testing;
3136 println!("Hello, world");
3140 Finally, let's import this function in our `tests/lib.rs`:
3143 extern crate testing;
3144 use testing::add_three_times_four;
3147 fn math_checks_out() {
3148 let result = add_three_times_four(5i);
3150 assert_eq!(32i, result);
3154 Let's give it a run:
3158 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3162 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3167 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3171 test math_checks_out ... ok
3173 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3176 Great! One test passed. We've got an integration test showing that our public
3177 method works, but maybe we want to test some of the internal logic as well.
3178 While this function is simple, if it were more complicated, you can imagine
3179 we'd need more tests. So let's break it up into two helper functions, and
3180 write some unit tests to test those.
3182 Change your `src/lib.rs` to look like this:
3185 pub fn add_three_times_four(x: int) -> int {
3186 times_four(add_three(x))
3189 fn add_three(x: int) -> int { x + 3 }
3191 fn times_four(x: int) -> int { x * 4 }
3194 If you run `cargo test`, you should get the same output:
3198 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3202 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3207 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3211 test math_checks_out ... ok
3213 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3216 If we tried to write a test for these two new functions, it wouldn't
3220 extern crate testing;
3221 use testing::add_three_times_four;
3222 use testing::add_three;
3225 fn math_checks_out() {
3226 let result = add_three_times_four(5i);
3228 assert_eq!(32i, result);
3232 fn test_add_three() {
3233 let result = add_three(5i);
3235 assert_eq!(8i, result);
3239 We'd get this error:
3242 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3243 /home/you/projects/testing/tests/lib.rs:3:5: 3:24 error: function `add_three` is private
3244 /home/you/projects/testing/tests/lib.rs:3 use testing::add_three;
3248 Right. It's private. So external, integration tests won't work. We need a
3249 unit test. Open up your `src/lib.rs` and add this:
3252 pub fn add_three_times_four(x: int) -> int {
3253 times_four(add_three(x))
3256 fn add_three(x: int) -> int { x + 3 }
3258 fn times_four(x: int) -> int { x * 4 }
3262 use super::add_three;
3263 use super::times_four;
3266 fn test_add_three() {
3267 let result = add_three(5i);
3269 assert_eq!(8i, result);
3273 fn test_times_four() {
3274 let result = times_four(5i);
3276 assert_eq!(20i, result);
3281 Let's give it a shot:
3285 Compiling testing v0.0.1 (file:///home/you/projects/testing)
3288 test test::test_times_four ... ok
3289 test test::test_add_three ... ok
3291 test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
3296 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
3300 test math_checks_out ... ok
3302 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
3305 Cool! We now have two tests of our internal functions. You'll note that there
3306 are three sets of output now: one for `src/main.rs`, one for `src/lib.rs`, and
3307 one for `tests/lib.rs`. There's one interesting thing that we haven't talked
3308 about yet, and that's these lines:
3311 use super::add_three;
3312 use super::times_four;
3315 Because we've made a nested module, we can import functions from the parent
3316 module by using `super`. Sub-modules are allowed to 'see' private functions in
3319 We've now covered the basics of testing. Rust's tools are primitive, but they
3320 work well in the simple cases. There are some Rustaceans working on building
3321 more complicated frameworks on top of all of this, but they're just starting
3326 In systems programming, pointers are an incredibly important topic. Rust has a
3327 very rich set of pointers, and they operate differently than in many other
3328 languages. They are important enough that we have a specific [Pointer
3329 Guide](guide-pointers.html) that goes into pointers in much detail. In fact,
3330 while you're currently reading this guide, which covers the language in broad
3331 overview, there are a number of other guides that put a specific topic under a
3332 microscope. You can find the list of guides on the [documentation index
3333 page](index.html#guides).
3335 In this section, we'll assume that you're familiar with pointers as a general
3336 concept. If you aren't, please read the [introduction to
3337 pointers](guide-pointers.html#an-introduction) section of the Pointer Guide,
3338 and then come back here. We'll wait.
3340 Got the gist? Great. Let's talk about pointers in Rust.
3344 The most primitive form of pointer in Rust is called a **reference**.
3345 References are created using the ampersand (`&`). Here's a simple
3353 `y` is a reference to `x`. To dereference (get the value being referred to
3354 rather than the reference itself) `y`, we use the asterisk (`*`):
3363 Like any `let` binding, references are immutable by default.
3365 You can declare that functions take a reference:
3368 fn add_one(x: &int) -> int { *x + 1 }
3371 assert_eq!(6, add_one(&5));
3375 As you can see, we can make a reference from a literal by applying `&` as well.
3376 Of course, in this simple function, there's not a lot of reason to take `x` by
3377 reference. It's just an example of the syntax.
3379 Because references are immutable, you can have multiple references that
3380 **alias** (point to the same place):
3388 We can make a mutable reference by using `&mut` instead of `&`:
3395 Note that `x` must also be mutable. If it isn't, like this:
3405 6:19 error: cannot borrow immutable local variable `x` as mutable
3410 We don't want a mutable reference to immutable data! This error message uses a
3411 term we haven't talked about yet, 'borrow.' We'll get to that in just a moment.
3413 This simple example actually illustrates a lot of Rust's power: Rust has
3414 prevented us, at compile time, from breaking our own rules. Because Rust's
3415 references check these kinds of rules entirely at compile time, there's no
3416 runtime overhead for this safety. At runtime, these are the same as a raw
3417 machine pointer, like in C or C++. We've just double-checked ahead of time
3418 that we haven't done anything dangerous.
3420 Rust will also prevent us from creating two mutable references that alias.
3429 It gives us this error:
3432 error: cannot borrow `x` as mutable more than once at a time
3435 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3438 note: previous borrow ends here
3447 This is a big error message. Let's dig into it for a moment. There are three
3448 parts: the error and two notes. The error says what we expected, we cannot have
3449 two pointers that point to the same memory.
3451 The two notes give some extra context. Rust's error messages often contain this
3452 kind of extra information when the error is complex. Rust is telling us two
3453 things: first, that the reason we cannot **borrow** `x` as `z` is that we
3454 previously borrowed `x` as `y`. The second note shows where `y`'s borrowing
3459 In order to truly understand this error, we have to learn a few new concepts:
3460 **ownership**, **borrowing**, and **lifetimes**.
3462 ## Ownership, borrowing, and lifetimes
3464 Whenever a resource of some kind is created, something must be responsible
3465 for destroying that resource as well. Given that we're discussing pointers
3466 right now, let's discuss this in the context of memory allocation, though
3467 it applies to other resources as well.
3469 When you allocate heap memory, you need a mechanism to free that memory. Many
3470 languages use a garbage collector to handle deallocation. This is a valid,
3471 time-tested strategy, but it's not without its drawbacks: it adds overhead, and
3472 can lead to unpredictable pauses in execution. Because the programmer does not
3473 have to think as much about deallocation, allocation becomes something
3474 commonplace, leading to more memory usage. And if you need precise control
3475 over when something is deallocated, leaving it up to your runtime can make this
3478 Rust chooses a different path, and that path is called **ownership**. Any
3479 binding that creates a resource is the **owner** of that resource.
3481 Being an owner affords you some privileges:
3483 1. You control when that resource is deallocated.
3484 2. You may lend that resource, immutably, to as many borrowers as you'd like.
3485 3. You may lend that resource, mutably, to a single borrower.
3487 But it also comes with some restrictions:
3489 1. If someone is borrowing your resource (either mutably or immutably), you may
3490 not mutate the resource or mutably lend it to someone.
3491 2. If someone is mutably borrowing your resource, you may not lend it out at
3492 all (mutably or immutably) or access it in any way.
3494 What's up with all this 'lending' and 'borrowing'? When you allocate memory,
3495 you get a pointer to that memory. This pointer allows you to manipulate said
3496 memory. If you are the owner of a pointer, then you may allow another
3497 binding to temporarily borrow that pointer, and then they can manipulate the
3498 memory. The length of time that the borrower is borrowing the pointer
3499 from you is called a **lifetime**.
3501 If two distinct bindings share a pointer, and the memory that pointer points to
3502 is immutable, then there are no problems. But if it's mutable, the result of
3503 changing it can vary unpredictably depending on who happens to access it first,
3504 which is called a **race condition**. To avoid this, if someone wants to mutate
3505 something that they've borrowed from you, you must not have lent out that
3506 pointer to anyone else.
3508 Rust has a sophisticated system called the **borrow checker** to make sure that
3509 everyone plays by these rules. At compile time, it verifies that none of these
3510 rules are broken. If our program compiles successfully, Rust can guarantee it
3511 is free of data races and other memory errors, and there is no runtime overhead
3512 for any of this. The borrow checker works only at compile time. If the borrow
3513 checker did find a problem, it will report an error and your program will
3516 That's a lot to take in. It's also one of the _most_ important concepts in
3517 all of Rust. Let's see this syntax in action:
3521 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3523 // other code here...
3525 } // privilege 1: when x goes out of scope, this memory is deallocated
3527 /// this function borrows an integer. It's given back automatically when the
3528 /// function returns.
3529 fn foo(x: &int) -> &int { x }
3532 let x = 5i; // x is the owner of this integer, which is memory on the stack.
3534 // privilege 2: you may lend that resource, to as many borrowers as you'd like
3538 foo(&x); // functions can borrow too!
3540 let a = &x; // we can do this alllllll day!
3544 let mut x = 5i; // x is the owner of this integer, which is memory on the stack.
3546 let y = &mut x; // privilege 3: you may lend that resource to a single borrower,
3551 If you are a borrower, you get a few privileges as well, but must also obey a
3554 1. If the borrow is immutable, you may read the data the pointer points to.
3555 2. If the borrow is mutable, you may read and write the data the pointer points to.
3556 3. You may lend the pointer to someone else, **BUT**
3557 4. When you do so, they must return it before you can give your own borrow back.
3559 This last requirement can seem odd, but it also makes sense. If you have to
3560 return something, and you've lent it to someone, they need to give it back to
3561 you for you to give it back! If we didn't, then the owner could deallocate
3562 the memory, and the person we've loaned it out to would have a pointer to
3563 invalid memory. This is called a 'dangling pointer.'
3565 Let's re-examine the error that led us to talk about all of this, which was a
3566 violation of the restrictions placed on owners who lend something out mutably.
3578 error: cannot borrow `x` as mutable more than once at a time
3581 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3584 note: previous borrow ends here
3593 This error comes in three parts. Let's go over each in turn.
3596 error: cannot borrow `x` as mutable more than once at a time
3601 This error states the restriction: you cannot lend out something mutable more
3602 than once at the same time. The borrow checker knows the rules!
3605 note: previous borrow of `x` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `x` until the borrow ends
3610 Some compiler errors come with notes to help you fix the error. This error comes
3611 with two notes, and this is the first. This note informs us of exactly where
3612 the first mutable borrow occurred. The error showed us the second. So now we
3613 see both parts of the problem. It also alludes to rule #3, by reminding us that
3614 we can't change `x` until the borrow is over.
3617 note: previous borrow ends here
3626 Here's the second note, which lets us know where the first borrow would be over.
3627 This is useful, because if we wait to try to borrow `x` after this borrow is
3628 over, then everything will work.
3630 For more advanced patterns, please consult the [Lifetime
3631 Guide](guide-lifetimes.html). You'll also learn what this type signature with
3635 pub fn as_maybe_owned(&self) -> MaybeOwned<'a> { ... }
3640 All of our references so far have been to variables we've created on the stack.
3641 In Rust, the simplest way to allocate heap variables is using a *box*. To
3642 create a box, use the `box` keyword:
3648 This allocates an integer `5` on the heap, and creates a binding `x` that
3649 refers to it. The great thing about boxed pointers is that we don't have to
3650 manually free this allocation! If we write
3659 then Rust will automatically free `x` at the end of the block. This isn't
3660 because Rust has a garbage collector -- it doesn't. Instead, when `x` goes out
3661 of scope, Rust `free`s `x`. This Rust code will do the same thing as the
3666 int *x = (int *)malloc(sizeof(int));
3672 This means we get the benefits of manual memory management, but the compiler
3673 ensures that we don't do something wrong. We can't forget to `free` our memory.
3675 Boxes are the sole owner of their contents, so you cannot take a mutable
3676 reference to them and then use the original box:
3682 *x; // you might expect 5, but this is actually an error
3685 This gives us this error:
3688 8:7 error: cannot use `*x` because it was mutably borrowed
3691 6:19 note: borrow of `x` occurs here
3696 As long as `y` is borrowing the contents, we cannot use `x`. After `y` is
3697 done borrowing the value, we can use it again. This works fine:
3704 } // y goes out of scope at the end of the block
3711 Sometimes, you need to allocate something on the heap, but give out multiple
3712 references to the memory. Rust's `Rc<T>` (pronounced 'arr cee tee') and
3713 `Arc<T>` types (again, the `T` is for generics, we'll learn more later) provide
3714 you with this ability. **Rc** stands for 'reference counted,' and **Arc** for
3715 'atomically reference counted.' This is how Rust keeps track of the multiple
3716 owners: every time we make a new reference to the `Rc<T>`, we add one to its
3717 internal 'reference count.' Every time a reference goes out of scope, we
3718 subtract one from the count. When the count is zero, the `Rc<T>` can be safely
3719 deallocated. `Arc<T>` is almost identical to `Rc<T>`, except for one thing: The
3720 'atomically' in 'Arc' means that increasing and decreasing the count uses a
3721 thread-safe mechanism to do so. Why two types? `Rc<T>` is faster, so if you're
3722 not in a multi-threaded scenario, you can have that advantage. Since we haven't
3723 talked about threading yet in Rust, we'll show you `Rc<T>` for the rest of this
3726 To create an `Rc<T>`, use `Rc::new()`:
3731 let x = Rc::new(5i);
3734 To create a second reference, use the `.clone()` method:
3739 let x = Rc::new(5i);
3743 The `Rc<T>` will live as long as any of its references are alive. After they
3744 all go out of scope, the memory will be `free`d.
3746 If you use `Rc<T>` or `Arc<T>`, you have to be careful about introducing
3747 cycles. If you have two `Rc<T>`s that point to each other, the reference counts
3748 will never drop to zero, and you'll have a memory leak. To learn more, check
3749 out [the section on `Rc<T>` and `Arc<T>` in the pointers
3750 guide](guide-pointers.html#rc-and-arc).
3754 We've made use of patterns a few times in the guide: first with `let` bindings,
3755 then with `match` statements. Let's go on a whirlwind tour of all of the things
3758 A quick refresher: you can match against literals directly, and `_` acts as an
3765 1 => println!("one"),
3766 2 => println!("two"),
3767 3 => println!("three"),
3768 _ => println!("anything"),
3772 You can match multiple patterns with `|`:
3778 1 | 2 => println!("one or two"),
3779 3 => println!("three"),
3780 _ => println!("anything"),
3784 You can match a range of values with `...`:
3790 1 ... 5 => println!("one through five"),
3791 _ => println!("anything"),
3795 Ranges are mostly used with integers and single characters.
3797 If you're matching multiple things, via a `|` or a `...`, you can bind
3798 the value to a name with `@`:
3804 x @ 1 ... 5 => println!("got {}", x),
3805 _ => println!("anything"),
3809 If you're matching on an enum which has variants, you can use `..` to
3810 ignore the value in the variant:
3821 Value(..) => println!("Got an int!"),
3822 Missing => println!("No such luck."),
3826 You can introduce **match guards** with `if`:
3837 Value(x) if x > 5 => println!("Got an int bigger than five!"),
3838 Value(..) => println!("Got an int!"),
3839 Missing => println!("No such luck."),
3843 If you're matching on a pointer, you can use the same syntax as you declared it
3850 &x => println!("Got a value: {}", x),
3854 Here, the `x` inside the `match` has type `int`. In other words, the left hand
3855 side of the pattern destructures the value. If we have `&5i`, then in `&x`, `x`
3858 If you want to get a reference, use the `ref` keyword:
3864 ref x => println!("Got a reference to {}", x),
3868 Here, the `x` inside the `match` has the type `&int`. In other words, the `ref`
3869 keyword _creates_ a reference, for use in the pattern. If you need a mutable
3870 reference, `ref mut` will work in the same way:
3876 ref mut x => println!("Got a mutable reference to {}", x),
3880 If you have a struct, you can destructure it inside of a pattern:
3883 # #![allow(non_shorthand_field_patterns)]
3889 let origin = Point { x: 0i, y: 0i };
3892 Point { x: x, y: y } => println!("({},{})", x, y),
3896 If we only care about some of the values, we don't have to give them all names:
3899 # #![allow(non_shorthand_field_patterns)]
3905 let origin = Point { x: 0i, y: 0i };
3908 Point { x: x, .. } => println!("x is {}", x),
3912 Whew! That's a lot of different ways to match things, and they can all be
3913 mixed and matched, depending on what you're doing:
3917 Foo { x: Some(ref name), y: None } => ...
3921 Patterns are very powerful. Make good use of them.
3925 Functions are great, but if you want to call a bunch of them on some data, it
3926 can be awkward. Consider this code:
3932 We would read this left-to right, and so we see 'baz bar foo.' But this isn't the
3933 order that the functions would get called in, that's inside-out: 'foo bar baz.'
3934 Wouldn't it be nice if we could do this instead?
3937 x.foo().bar().baz();
3940 Luckily, as you may have guessed with the leading question, you can! Rust provides
3941 the ability to use this **method call syntax** via the `impl` keyword.
3943 Here's how it works:
3953 fn area(&self) -> f64 {
3954 std::f64::consts::PI * (self.radius * self.radius)
3959 let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
3960 println!("{}", c.area());
3964 This will print `12.566371`.
3966 We've made a struct that represents a circle. We then write an `impl` block,
3967 and inside it, define a method, `area`. Methods take a special first
3968 parameter, `&self`. There are three variants: `self`, `&self`, and `&mut self`.
3969 You can think of this first parameter as being the `x` in `x.foo()`. The three
3970 variants correspond to the three kinds of thing `x` could be: `self` if it's
3971 just a value on the stack, `&self` if it's a reference, and `&mut self` if it's
3972 a mutable reference. We should default to using `&self`, as it's the most
3975 Finally, as you may remember, the value of the area of a circle is `π*r²`.
3976 Because we took the `&self` parameter to `area`, we can use it just like any
3977 other parameter. Because we know it's a `Circle`, we can access the `radius`
3978 just like we would with any other struct. An import of π and some
3979 multiplications later, and we have our area.
3981 You can also define methods that do not take a `self` parameter. Here's a
3982 pattern that's very common in Rust code:
3985 # #![allow(non_shorthand_field_patterns)]
3993 fn new(x: f64, y: f64, radius: f64) -> Circle {
4003 let c = Circle::new(0.0, 0.0, 2.0);
4007 This **static method** builds a new `Circle` for us. Note that static methods
4008 are called with the `Struct::method()` syntax, rather than the `ref.method()`
4013 So far, we've made lots of functions in Rust. But we've given them all names.
4014 Rust also allows us to create anonymous functions too. Rust's anonymous
4015 functions are called **closure**s. By themselves, closures aren't all that
4016 interesting, but when you combine them with functions that take closures as
4017 arguments, really powerful things are possible.
4019 Let's make a closure:
4022 let add_one = |x| { 1i + x };
4024 println!("The sum of 5 plus 1 is {}.", add_one(5i));
4027 We create a closure using the `|...| { ... }` syntax, and then we create a
4028 binding so we can use it later. Note that we call the function using the
4029 binding name and two parentheses, just like we would for a named function.
4031 Let's compare syntax. The two are pretty close:
4034 let add_one = |x: int| -> int { 1i + x };
4035 fn add_one (x: int) -> int { 1i + x }
4038 As you may have noticed, closures infer their argument and return types, so you
4039 don't need to declare one. This is different from named functions, which
4040 default to returning unit (`()`).
4042 There's one big difference between a closure and named functions, and it's in
4043 the name: a closure "closes over its environment." What's that mean? It means
4050 let printer = || { println!("x is: {}", x); };
4052 printer(); // prints "x is: 5"
4056 The `||` syntax means this is an anonymous closure that takes no arguments.
4057 Without it, we'd just have a block of code in `{}`s.
4059 In other words, a closure has access to variables in the scope that it's
4060 defined. The closure borrows any variables that it uses. This will error:
4066 let printer = || { println!("x is: {}", x); };
4068 x = 6i; // error: cannot assign to `x` because it is borrowed
4074 Rust has a second type of closure, called a **proc**. Procs are created
4075 with the `proc` keyword:
4080 let p = proc() { x * x };
4081 println!("{}", p()); // prints 25
4084 Procs have a big difference from closures: they may only be called once. This
4085 will error when we try to compile:
4090 let p = proc() { x * x };
4091 println!("{}", p());
4092 println!("{}", p()); // error: use of moved value `p`
4095 This restriction is important. Procs are allowed to consume values that they
4096 capture, and thus have to be restricted to being called once for soundness
4097 reasons: any value consumed would be invalid on a second call.
4099 Procs are most useful with Rust's concurrency features, and so we'll just leave
4100 it at this for now. We'll talk about them more in the "Tasks" section of the
4103 ## Accepting closures as arguments
4105 Closures are most useful as an argument to another function. Here's an example:
4108 fn twice(x: int, f: |int| -> int) -> int {
4113 let square = |x: int| { x * x };
4115 twice(5i, square); // evaluates to 50
4119 Let's break the example down, starting with `main`:
4122 let square = |x: int| { x * x };
4125 We've seen this before. We make a closure that takes an integer, and returns
4129 twice(5i, square); // evaluates to 50
4132 This line is more interesting. Here, we call our function, `twice`, and we pass
4133 it two arguments: an integer, `5`, and our closure, `square`. This is just like
4134 passing any other two variable bindings to a function, but if you've never
4135 worked with closures before, it can seem a little complex. Just think: "I'm
4136 passing two variables, one is an int, and one is a function."
4138 Next, let's look at how `twice` is defined:
4141 fn twice(x: int, f: |int| -> int) -> int {
4144 `twice` takes two arguments, `x` and `f`. That's why we called it with two
4145 arguments. `x` is an `int`, we've done that a ton of times. `f` is a function,
4146 though, and that function takes an `int` and returns an `int`. Notice
4147 how the `|int| -> int` syntax looks a lot like our definition of `square`
4148 above, if we added the return type in:
4151 let square = |x: int| -> int { x * x };
4155 This function takes an `int` and returns an `int`.
4157 This is the most complicated function signature we've seen yet! Give it a read
4158 a few times until you can see how it works. It takes a teeny bit of practice, and
4161 Finally, `twice` returns an `int` as well.
4163 Okay, let's look at the body of `twice`:
4166 fn twice(x: int, f: |int| -> int) -> int {
4171 Since our closure is named `f`, we can call it just like we called our closures
4172 before. And we pass in our `x` argument to each one. Hence 'twice.'
4174 If you do the math, `(5 * 5) + (5 * 5) == 50`, so that's the output we get.
4176 Play around with this concept until you're comfortable with it. Rust's standard
4177 library uses lots of closures, where appropriate, so you'll be using
4178 this technique a lot.
4180 If we didn't want to give `square` a name, we could also just define it inline.
4181 This example is the same as the previous one:
4184 fn twice(x: int, f: |int| -> int) -> int {
4189 twice(5i, |x: int| { x * x }); // evaluates to 50
4193 A named function's name can be used wherever you'd use a closure. Another
4194 way of writing the previous example:
4197 fn twice(x: int, f: |int| -> int) -> int {
4201 fn square(x: int) -> int { x * x }
4204 twice(5i, square); // evaluates to 50
4208 Doing this is not particularly common, but every once in a while, it's useful.
4210 That's all you need to get the hang of closures! Closures are a little bit
4211 strange at first, but once you're used to using them, you'll miss them in any
4212 language that doesn't have them. Passing functions to other functions is
4213 incredibly powerful. Next, let's look at one of those things: iterators.
4217 Let's talk about loops.
4219 Remember Rust's `for` loop? Here's an example:
4222 for x in range(0i, 10i) {
4223 println!("{:d}", x);
4227 Now that you know more Rust, we can talk in detail about how this works. The
4228 `range` function returns an **iterator**. An iterator is something that we can
4229 call the `.next()` method on repeatedly, and it gives us a sequence of things.
4234 let mut range = range(0i, 10i);
4237 match range.next() {
4246 We make a mutable binding to the return value of `range`, which is our iterator.
4247 We then `loop`, with an inner `match`. This `match` is used on the result of
4248 `range.next()`, which gives us a reference to the next value of the iterator.
4249 `next` returns an `Option<int>`, in this case, which will be `Some(int)` when
4250 we have a value and `None` once we run out. If we get `Some(int)`, we print it
4251 out, and if we get `None`, we `break` out of the loop.
4253 This code sample is basically the same as our `for` loop version. The `for`
4254 loop is just a handy way to write this `loop`/`match`/`break` construct.
4256 `for` loops aren't the only thing that uses iterators, however. Writing your
4257 own iterator involves implementing the `Iterator` trait. While doing that is
4258 outside of the scope of this guide, Rust provides a number of useful iterators
4259 to accomplish various tasks. Before we talk about those, we should talk about a
4260 Rust anti-pattern. And that's `range`.
4262 Yes, we just talked about how `range` is cool. But `range` is also very
4263 primitive. For example, if you needed to iterate over the contents of
4264 a vector, you may be tempted to write this:
4267 let nums = vec![1i, 2i, 3i];
4269 for i in range(0u, nums.len()) {
4270 println!("{}", nums[i]);
4274 This is strictly worse than using an actual iterator. The `.iter()` method on
4275 vectors returns an iterator which iterates through a reference to each element
4276 of the vector in turn. So write this:
4279 let nums = vec![1i, 2i, 3i];
4281 for num in nums.iter() {
4282 println!("{}", num);
4286 There are two reasons for this. First, this more directly expresses what we
4287 mean. We iterate through the entire vector, rather than iterating through
4288 indexes, and then indexing the vector. Second, this version is more efficient:
4289 the first version will have extra bounds checking because it used indexing,
4290 `nums[i]`. But since we yield a reference to each element of the vector in turn
4291 with the iterator, there's no bounds checking in the second example. This is
4292 very common with iterators: we can ignore unnecessary bounds checks, but still
4293 know that we're safe.
4295 There's another detail here that's not 100% clear because of how `println!`
4296 works. `num` is actually of type `&int`. That is, it's a reference to an `int`,
4297 not an `int` itself. `println!` handles the dereferencing for us, so we don't
4298 see it. This code works fine too:
4301 let nums = vec![1i, 2i, 3i];
4303 for num in nums.iter() {
4304 println!("{}", *num);
4308 Now we're explicitly dereferencing `num`. Why does `iter()` give us references?
4309 Well, if it gave us the data itself, we would have to be its owner, which would
4310 involve making a copy of the data and giving us the copy. With references,
4311 we're just borrowing a reference to the data, and so it's just passing
4312 a reference, without needing to do the copy.
4314 So, now that we've established that `range` is often not what you want, let's
4315 talk about what you do want instead.
4317 There are three broad classes of things that are relevant here: iterators,
4318 **iterator adapters**, and **consumers**. Here's some definitions:
4320 * 'iterators' give you a sequence of values.
4321 * 'iterator adapters' operate on an iterator, producing a new iterator with a
4322 different output sequence.
4323 * 'consumers' operate on an iterator, producing some final set of values.
4325 Let's talk about consumers first, since you've already seen an iterator,
4330 A 'consumer' operates on an iterator, returning some kind of value or values.
4331 The most common consumer is `collect()`. This code doesn't quite compile,
4332 but it shows the intention:
4335 let one_to_one_hundred = range(1i, 101i).collect();
4338 As you can see, we call `collect()` on our iterator. `collect()` takes
4339 as many values as the iterator will give it, and returns a collection
4340 of the results. So why won't this compile? Rust can't determine what
4341 type of things you want to collect, and so you need to let it know.
4342 Here's the version that does compile:
4345 let one_to_one_hundred = range(1i, 101i).collect::<Vec<int>>();
4348 If you remember, the `::<>` syntax allows us to give a type hint,
4349 and so we tell it that we want a vector of integers.
4351 `collect()` is the most common consumer, but there are others too. `find()`
4355 let greater_than_forty_two = range(0i, 100i)
4356 .find(|x| *x >= 42);
4358 match greater_than_forty_two {
4359 Some(_) => println!("We got some numbers!"),
4360 None => println!("No numbers found :("),
4364 `find` takes a closure, and works on a reference to each element of an
4365 iterator. This closure returns `true` if the element is the element we're
4366 looking for, and `false` otherwise. Because we might not find a matching
4367 element, `find` returns an `Option` rather than the element itself.
4369 Another important consumer is `fold`. Here's what it looks like:
4372 let sum = range(1i, 4i)
4373 .fold(0i, |sum, x| sum + x);
4376 `fold()` is a consumer that looks like this:
4377 `fold(base, |accumulator, element| ...)`. It takes two arguments: the first
4378 is an element called the "base". The second is a closure that itself takes two
4379 arguments: the first is called the "accumulator," and the second is an
4380 "element." Upon each iteration, the closure is called, and the result is the
4381 value of the accumulator on the next iteration. On the first iteration, the
4382 base is the value of the accumulator.
4384 Okay, that's a bit confusing. Let's examine the values of all of these things
4387 | base | accumulator | element | closure result |
4388 |------|-------------|---------|----------------|
4389 | 0i | 0i | 1i | 1i |
4390 | 0i | 1i | 2i | 3i |
4391 | 0i | 3i | 3i | 6i |
4393 We called `fold()` with these arguments:
4397 .fold(0i, |sum, x| sum + x);
4400 So, `0i` is our base, `sum` is our accumulator, and `x` is our element. On the
4401 first iteration, we set `sum` to `0i`, and `x` is the first element of `nums`,
4402 `1i`. We then add `sum` and `x`, which gives us `0i + 1i = 1i`. On the second
4403 iteration, that value becomes our accumulator, `sum`, and the element is
4404 the second element of the array, `2i`. `1i + 2i = 3i`, and so that becomes
4405 the value of the accumulator for the last iteration. On that iteration,
4406 `x` is the last element, `3i`, and `3i + 3i = 6i`, which is our final
4407 result for our sum. `1 + 2 + 3 = 6`, and that's the result we got.
4409 Whew. `fold` can be a bit strange the first few times you see it, but once it
4410 clicks, you can use it all over the place. Any time you have a list of things,
4411 and you want a single result, `fold` is appropriate.
4413 Consumers are important due to one additional property of iterators we haven't
4414 talked about yet: laziness. Let's talk some more about iterators, and you'll
4415 see why consumers matter.
4419 As we've said before, an iterator is something that we can call the `.next()`
4420 method on repeatedly, and it gives us a sequence of things. Because you need
4421 to call the method, this means that iterators are **lazy**. This code, for
4422 example, does not actually generate the numbers `1-100`, and just creates a
4423 value that represents the sequence:
4426 let nums = range(1i, 100i);
4429 Since we didn't do anything with the range, it didn't generate the sequence.
4430 Once we add the consumer:
4433 let nums = range(1i, 100i).collect::<Vec<int>>();
4436 Now, `collect()` will require that `range()` give it some numbers, and so
4437 it will do the work of generating the sequence.
4439 `range` is one of two basic iterators that you'll see. The other is `iter()`,
4440 which you've used before. `iter()` can turn a vector into a simple iterator
4441 that gives you each element in turn:
4444 let nums = [1i, 2i, 3i];
4446 for num in nums.iter() {
4447 println!("{}", num);
4451 These two basic iterators should serve you well. There are some more
4452 advanced iterators, including ones that are infinite. Like `count`:
4455 std::iter::count(1i, 5i);
4458 This iterator counts up from one, adding five each time. It will give
4459 you a new integer every time, forever. Well, technically, until the
4460 maximum number that an `int` can represent. But since iterators are lazy,
4461 that's okay! You probably don't want to use `collect()` on it, though...
4463 That's enough about iterators. Iterator adapters are the last concept
4464 we need to talk about with regards to iterators. Let's get to it!
4466 ## Iterator adapters
4468 "Iterator adapters" take an iterator and modify it somehow, producing
4469 a new iterator. The simplest one is called `map`:
4472 range(1i, 100i).map(|x| x + 1i);
4475 `map` is called upon another iterator, and produces a new iterator where each
4476 element reference has the closure it's been given as an argument called on it.
4477 So this would give us the numbers from `2-101`. Well, almost! If you
4478 compile the example, you'll get a warning:
4481 2:37 warning: unused result which must be used: iterator adaptors are lazy and
4482 do nothing unless consumed, #[warn(unused_must_use)] on by default
4483 range(1i, 100i).map(|x| x + 1i);
4484 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4487 Laziness strikes again! That closure will never execute. This example
4488 doesn't print any numbers:
4491 range(1i, 100i).map(|x| println!("{}", x));
4494 If you are trying to execute a closure on an iterator for its side effects,
4495 just use `for` instead.
4497 There are tons of interesting iterator adapters. `take(n)` will get the
4498 first `n` items out of an iterator, and return them as a list. Let's
4499 try it out with our infinite iterator from before, `count()`:
4502 for i in std::iter::count(1i, 5i).take(5) {
4517 `filter()` is an adapter that takes a closure as an argument. This closure
4518 returns `true` or `false`. The new iterator `filter()` produces
4519 only the elements that that closure returns `true` for:
4522 for i in range(1i, 100i).filter(|x| x % 2 == 0) {
4527 This will print all of the even numbers between one and a hundred.
4529 You can chain all three things together: start with an iterator, adapt it
4530 a few times, and then consume the result. Check it out:
4534 .filter(|x| x % 2 == 0)
4535 .filter(|x| x % 3 == 0)
4537 .collect::<Vec<int>>();
4540 This will give you a vector containing `6`, `12`, `18`, `24`, and `30`.
4542 This is just a small taste of what iterators, iterator adapters, and consumers
4543 can help you with. There are a number of really useful iterators, and you can
4544 write your own as well. Iterators provide a safe, efficient way to manipulate
4545 all kinds of lists. They're a little unusual at first, but if you play with
4546 them, you'll get hooked. For a full list of the different iterators and
4547 consumers, check out the [iterator module documentation](std/iter/index.html).
4551 Sometimes, when writing a function or data type, we may want it to work for
4552 multiple types of arguments. For example, remember our `OptionalInt` type?
4561 If we wanted to also have an `OptionalFloat64`, we would need a new enum:
4564 enum OptionalFloat64 {
4570 This is really unfortunate. Luckily, Rust has a feature that gives us a better
4571 way: generics. Generics are called **parametric polymorphism** in type theory,
4572 which means that they are types or functions that have multiple forms ("poly"
4573 is multiple, "morph" is form) over a given parameter ("parametric").
4575 Anyway, enough with type theory declarations, let's check out the generic form
4576 of `OptionalInt`. It is actually provided by Rust itself, and looks like this:
4585 The `<T>` part, which you've seen a few times before, indicates that this is
4586 a generic data type. Inside the declaration of our enum, wherever we see a `T`,
4587 we substitute that type for the same type used in the generic. Here's an
4588 example of using `Option<T>`, with some extra type annotations:
4591 let x: Option<int> = Some(5i);
4594 In the type declaration, we say `Option<int>`. Note how similar this looks to
4595 `Option<T>`. So, in this particular `Option`, `T` has the value of `int`. On
4596 the right hand side of the binding, we do make a `Some(T)`, where `T` is `5i`.
4597 Since that's an `int`, the two sides match, and Rust is happy. If they didn't
4598 match, we'd get an error:
4601 let x: Option<f64> = Some(5i);
4602 // error: mismatched types: expected `core::option::Option<f64>`
4603 // but found `core::option::Option<int>` (expected f64 but found int)
4606 That doesn't mean we can't make `Option<T>`s that hold an `f64`! They just have to
4610 let x: Option<int> = Some(5i);
4611 let y: Option<f64> = Some(5.0f64);
4614 This is just fine. One definition, multiple uses.
4616 Generics don't have to only be generic over one type. Consider Rust's built-in
4617 `Result<T, E>` type:
4626 This type is generic over _two_ types: `T` and `E`. By the way, the capital letters
4627 can be any letter you'd like. We could define `Result<T, E>` as:
4636 if we wanted to. Convention says that the first generic parameter should be
4637 `T`, for 'type,' and that we use `E` for 'error.' Rust doesn't care, however.
4639 The `Result<T, E>` type is intended to
4640 be used to return the result of a computation, and to have the ability to
4641 return an error if it didn't work out. Here's an example:
4644 let x: Result<f64, String> = Ok(2.3f64);
4645 let y: Result<f64, String> = Err("There was an error.".to_string());
4648 This particular Result will return an `f64` if there's a success, and a
4649 `String` if there's a failure. Let's write a function that uses `Result<T, E>`:
4652 fn inverse(x: f64) -> Result<f64, String> {
4653 if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4659 We don't want to take the inverse of zero, so we check to make sure that we
4660 weren't passed zero. If we were, then we return an `Err`, with a message. If
4661 it's okay, we return an `Ok`, with the answer.
4663 Why does this matter? Well, remember how `match` does exhaustive matches?
4664 Here's how this function gets used:
4667 # fn inverse(x: f64) -> Result<f64, String> {
4668 # if x == 0.0f64 { return Err("x cannot be zero!".to_string()); }
4671 let x = inverse(25.0f64);
4674 Ok(x) => println!("The inverse of 25 is {}", x),
4675 Err(msg) => println!("Error: {}", msg),
4679 The `match` enforces that we handle the `Err` case. In addition, because the
4680 answer is wrapped up in an `Ok`, we can't just use the result without doing
4684 let x = inverse(25.0f64);
4685 println!("{}", x + 2.0f64); // error: binary operation `+` cannot be applied
4686 // to type `core::result::Result<f64,collections::string::String>`
4689 This function is great, but there's one other problem: it only works for 64 bit
4690 floating point values. What if we wanted to handle 32 bit floating point as
4691 well? We'd have to write this:
4694 fn inverse32(x: f32) -> Result<f32, String> {
4695 if x == 0.0f32 { return Err("x cannot be zero!".to_string()); }
4701 Bummer. What we need is a **generic function**. Luckily, we can write one!
4702 However, it won't _quite_ work yet. Before we get into that, let's talk syntax.
4703 A generic version of `inverse` would look something like this:
4706 fn inverse<T>(x: T) -> Result<T, String> {
4707 if x == 0.0 { return Err("x cannot be zero!".to_string()); }
4713 Just like how we had `Option<T>`, we use a similar syntax for `inverse<T>`.
4714 We can then use `T` inside the rest of the signature: `x` has type `T`, and half
4715 of the `Result` has type `T`. However, if we try to compile that example, we'll get
4719 error: binary operation `==` cannot be applied to type `T`
4722 Because `T` can be _any_ type, it may be a type that doesn't implement `==`,
4723 and therefore, the first line would be wrong. What do we do?
4725 To fix this example, we need to learn about another Rust feature: traits.
4729 Do you remember the `impl` keyword, used to call a function with method
4740 fn area(&self) -> f64 {
4741 std::f64::consts::PI * (self.radius * self.radius)
4746 Traits are similar, except that we define a trait with just the method
4747 signature, then implement the trait for that struct. Like this:
4757 fn area(&self) -> f64;
4760 impl HasArea for Circle {
4761 fn area(&self) -> f64 {
4762 std::f64::consts::PI * (self.radius * self.radius)
4767 As you can see, the `trait` block looks very similar to the `impl` block,
4768 but we don't define a body, just a type signature. When we `impl` a trait,
4769 we use `impl Trait for Item`, rather than just `impl Item`.
4771 So what's the big deal? Remember the error we were getting with our generic
4775 error: binary operation `==` cannot be applied to type `T`
4778 We can use traits to constrain our generics. Consider this function, which
4779 does not compile, and gives us a similar error:
4782 fn print_area<T>(shape: T) {
4783 println!("This shape has an area of {}", shape.area());
4790 error: type `T` does not implement any method in scope named `area`
4793 Because `T` can be any type, we can't be sure that it implements the `area`
4794 method. But we can add a **trait constraint** to our generic `T`, ensuring
4799 # fn area(&self) -> f64;
4801 fn print_area<T: HasArea>(shape: T) {
4802 println!("This shape has an area of {}", shape.area());
4806 The syntax `<T: HasArea>` means `any type that implements the HasArea trait`.
4807 Because traits define function type signatures, we can be sure that any type
4808 which implements `HasArea` will have an `.area()` method.
4810 Here's an extended example of how this works:
4814 fn area(&self) -> f64;
4823 impl HasArea for Circle {
4824 fn area(&self) -> f64 {
4825 std::f64::consts::PI * (self.radius * self.radius)
4835 impl HasArea for Square {
4836 fn area(&self) -> f64 {
4837 self.side * self.side
4841 fn print_area<T: HasArea>(shape: T) {
4842 println!("This shape has an area of {}", shape.area());
4863 This program outputs:
4866 This shape has an area of 3.141593
4867 This shape has an area of 1
4870 As you can see, `print_area` is now generic, but also ensures that we
4871 have passed in the correct types. If we pass in an incorrect type:
4877 We get a compile-time error:
4880 error: failed to find an implementation of trait main::HasArea for int
4883 So far, we've only added trait implementations to structs, but you can
4884 implement a trait for any type. So technically, we _could_ implement
4885 `HasArea` for `int`:
4889 fn area(&self) -> f64;
4892 impl HasArea for int {
4893 fn area(&self) -> f64 {
4894 println!("this is silly");
4903 It is considered poor style to implement methods on such primitive types, even
4904 though it is possible.
4906 This may seem like the Wild West, but there are two other restrictions around
4907 implementing traits that prevent this from getting out of hand. First, traits
4908 must be `use`d in any scope where you wish to use the trait's method. So for
4909 example, this does not work:
4913 use std::f64::consts;
4916 fn area(&self) -> f64;
4925 impl HasArea for Circle {
4926 fn area(&self) -> f64 {
4927 consts::PI * (self.radius * self.radius)
4933 let c = shapes::Circle {
4939 println!("{}", c.area());
4943 Now that we've moved the structs and traits into their own module, we get an
4947 error: type `shapes::Circle` does not implement any method in scope named `area`
4950 If we add a `use` line right above `main` and make the right things public,
4954 use shapes::HasArea;
4957 use std::f64::consts;
4960 fn area(&self) -> f64;
4969 impl HasArea for Circle {
4970 fn area(&self) -> f64 {
4971 consts::PI * (self.radius * self.radius)
4978 let c = shapes::Circle {
4984 println!("{}", c.area());
4988 This means that even if someone does something bad like add methods to `int`,
4989 it won't affect you, unless you `use` that trait.
4991 There's one more restriction on implementing traits. Either the trait or the
4992 type you're writing the `impl` for must be inside your crate. So, we could
4993 implement the `HasArea` type for `int`, because `HasArea` is in our crate. But
4994 if we tried to implement `Float`, a trait provided by Rust, for `int`, we could
4995 not, because both the trait and the type aren't in our crate.
4997 One last thing about traits: generic functions with a trait bound use
4998 **monomorphization** ("mono": one, "morph": form), so they are statically
4999 dispatched. What's that mean? Well, let's take a look at `print_area` again:
5002 fn print_area<T: HasArea>(shape: T) {
5003 println!("This shape has an area of {}", shape.area());
5007 let c = Circle { ... };
5009 let s = Square { ... };
5016 When we use this trait with `Circle` and `Square`, Rust ends up generating
5017 two different functions with the concrete type, and replacing the call sites with
5018 calls to the concrete implementations. In other words, you get something like
5022 fn __print_area_circle(shape: Circle) {
5023 println!("This shape has an area of {}", shape.area());
5026 fn __print_area_square(shape: Square) {
5027 println!("This shape has an area of {}", shape.area());
5031 let c = Circle { ... };
5033 let s = Square { ... };
5035 __print_area_circle(c);
5036 __print_area_square(s);
5040 The names don't actually change to this, it's just for illustration. But
5041 as you can see, there's no overhead of deciding which version to call here,
5042 hence 'statically dispatched.' The downside is that we have two copies of
5043 the same function, so our binary is a little bit larger.
5047 Concurrency and parallelism are topics that are of increasing interest to a
5048 broad subsection of software developers. Modern computers are often multi-core,
5049 to the point that even embedded devices like cell phones have more than one
5050 processor. Rust's semantics lend themselves very nicely to solving a number of
5051 issues that programmers have with concurrency. Many concurrency errors that are
5052 runtime errors in other languages are compile-time errors in Rust.
5054 Rust's concurrency primitive is called a **task**. Tasks are lightweight, and
5055 do not share memory in an unsafe manner, preferring message passing to
5056 communicate. It's worth noting that tasks are implemented as a library, and
5057 not part of the language. This means that in the future, other concurrency
5058 libraries can be written for Rust to help in specific scenarios. Here's an
5059 example of creating a task:
5063 println!("Hello from a task!");
5067 The `spawn` function takes a proc as an argument, and runs that proc in a new
5068 task. A proc takes ownership of its entire environment, and so any variables
5069 that you use inside the proc will not be usable afterward:
5072 let mut x = vec![1i, 2i, 3i];
5075 println!("The value of x[0] is: {}", x[0]);
5078 println!("The value of x[0] is: {}", x[0]); // error: use of moved value: `x`
5081 `x` is now owned by the proc, and so we can't use it anymore. Many other
5082 languages would let us do this, but it's not safe to do so. Rust's borrow
5083 checker catches the error.
5085 If tasks were only able to capture these values, they wouldn't be very useful.
5086 Luckily, tasks can communicate with each other through **channel**s. Channels
5090 let (tx, rx) = channel();
5093 tx.send("Hello from a task!".to_string());
5096 let message = rx.recv();
5097 println!("{}", message);
5100 The `channel()` function returns two endpoints: a `Receiver<T>` and a
5101 `Sender<T>`. You can use the `.send()` method on the `Sender<T>` end, and
5102 receive the message on the `Receiver<T>` side with the `recv()` method. This
5103 method blocks until it gets a message. There's a similar method, `.try_recv()`,
5104 which returns an `Result<T, TryRecvError>` and does not block.
5106 If you want to send messages to the task as well, create two channels!
5109 let (tx1, rx1) = channel();
5110 let (tx2, rx2) = channel();
5113 tx1.send("Hello from a task!".to_string());
5114 let message = rx2.recv();
5115 println!("{}", message);
5118 let message = rx1.recv();
5119 println!("{}", message);
5121 tx2.send("Goodbye from main!".to_string());
5124 The proc has one sending end and one receiving end, and the main task has one
5125 of each as well. Now they can talk back and forth in whatever way they wish.
5127 Notice as well that because `Sender` and `Receiver` are generic, while you can
5128 pass any kind of information through the channel, the ends are strongly typed.
5129 If you try to pass a string, and then an integer, Rust will complain.
5133 With these basic primitives, many different concurrency patterns can be
5134 developed. Rust includes some of these types in its standard library. For
5135 example, if you wish to compute some value in the background, `Future` is
5136 a useful thing to use:
5139 use std::sync::Future;
5141 let mut delayed_value = Future::spawn(proc() {
5142 // just return anything for examples' sake
5146 println!("value = {}", delayed_value.get());
5149 Calling `Future::spawn` works just like `spawn()`: it takes a proc. In this
5150 case, though, you don't need to mess with the channel: just have the proc
5153 `Future::spawn` will return a value which we can bind with `let`. It needs
5154 to be mutable, because once the value is computed, it saves a copy of the
5155 value, and if it were immutable, it couldn't update itself.
5157 The proc will go on processing in the background, and when we need the final
5158 value, we can call `get()` on it. This will block until the result is done,
5159 but if it's finished computing in the background, we'll just get the value
5162 ## Success and failure
5164 Tasks don't always succeed, they can also fail. A task that wishes to fail
5165 can call the `fail!` macro, passing a message:
5173 If a task fails, it is not possible for it to recover. However, it can
5174 notify other tasks that it has failed. We can do this with `task::try`:
5180 let result = task::try(proc() {
5189 This task will randomly fail or succeed. `task::try` returns a `Result`
5190 type, so we can handle the response like any other computation that may
5195 One of Rust's most advanced features is its system of **macro**s. While
5196 functions allow you to provide abstractions over values and operations, macros
5197 allow you to provide abstractions over syntax. Do you wish Rust had the ability
5198 to do something that it can't currently do? You may be able to write a macro
5199 to extend Rust's capabilities.
5201 You've already used one macro extensively: `println!`. When we invoke
5202 a Rust macro, we need to use the exclamation mark (`!`). There's two reasons
5203 that this is true: the first is that it makes it clear when you're using a
5204 macro. The second is that macros allow for flexible syntax, and so Rust must
5205 be able to tell where a macro starts and ends. The `!(...)` helps with this.
5207 Let's talk some more about `println!`. We could have implemented `println!` as
5208 a function, but it would be worse. Why? Well, what macros allow you to do
5209 is write code that generates more code. So when we call `println!` like this:
5213 println!("x is: {}", x);
5216 The `println!` macro does a few things:
5218 1. It parses the string to find any `{}`s
5219 2. It checks that the number of `{}`s matches the number of other arguments.
5220 3. It generates a bunch of Rust code, taking this in mind.
5222 What this means is that you get type checking at compile time, because
5223 Rust will generate code that takes all of the types into account. If
5224 `println!` was a function, it could still do this type checking, but it
5225 would happen at run time rather than compile time.
5227 We can check this out using a special flag to `rustc`. This code, in a file
5233 println!("x is: {}", x);
5237 Can have its macros expanded like this: `rustc print.rs --pretty=expanded`, will
5238 give us this huge result:
5244 #[phase(plugin, link)]
5245 extern crate "std" as std;
5246 extern crate "native" as rt;
5248 use std::prelude::*;
5255 static __STATIC_FMTSTR: [&'static str, ..1u] = ["x is: "];
5257 &[::std::fmt::argument(::std::fmt::secret_show, __arg0)];
5260 ::std::fmt::Arguments::new(__STATIC_FMTSTR, __args_vec)
5262 ::std::io::stdio::println_args(&__args)
5268 Whew! This isn't too terrible. You can see that we still `let x = 5i`,
5269 but then things get a little bit hairy. Three more bindings get set: a
5270 static format string, an argument vector, and the arguments. We then
5271 invoke the `println_args` function with the generated arguments.
5273 This is the code that Rust actually compiles. You can see all of the extra
5274 information that's here. We get all of the type safety and options that it
5275 provides, but at compile time, and without needing to type all of this out.
5276 This is how macros are powerful. Without them, you would need to type all of
5277 this by hand to get a type checked `println`.
5279 For more on macros, please consult [the Macros Guide](guide-macros.html).
5280 Macros are a very advanced and still slightly experimental feature, but don't
5281 require a deep understanding to call, since they look just like functions. The
5282 Guide can help you if you want to write your own.
5286 Finally, there's one more Rust concept that you should be aware of: `unsafe`.
5287 There are two circumstances where Rust's safety provisions don't work well.
5288 The first is when interfacing with C code, and the second is when building
5289 certain kinds of abstractions.
5291 Rust has support for FFI (which you can read about in the [FFI
5292 Guide](guide-ffi.html)), but can't guarantee that the C code will be safe.
5293 Therefore, Rust marks such functions with the `unsafe`
5294 keyword, which indicates that the function may not behave properly.
5296 Second, if you'd like to create some sort of shared-memory data structure, Rust
5297 won't allow it, because memory must be owned by a single owner. However, if
5298 you're planning on making access to that shared memory safe, such as with a
5299 mutex, _you_ know that it's safe, but Rust can't know. Writing an `unsafe`
5300 block allows you to ask the compiler to trust you. In this case, the _internal_
5301 implementation of the mutex is considered unsafe, but the _external_ interface
5302 we present is safe. This allows it to be effectively used in normal Rust, while
5303 being able to implement functionality that the compiler can't double check for
5306 Doesn't an escape hatch undermine the safety of the entire system? Well, if
5307 Rust code segfaults, it _must_ be because of unsafe code somewhere. By
5308 annotating exactly where that is, you have a significantly smaller area to
5311 We haven't even talked about any examples here, and that's because I want to
5312 emphasize that you should not be writing unsafe code unless you know exactly
5313 what you're doing. The vast majority of Rust developers will only interact with
5314 it when doing FFI, and advanced library authors may use it to build certain
5315 kinds of abstraction.
5319 We covered a lot of ground here. When you've mastered everything in this Guide,
5320 you will have a firm grasp of basic Rust development. There's a whole lot more
5321 out there, we've just covered the surface. There's tons of topics that you can
5322 dig deeper into, and we've built specialized guides for many of them. To learn
5323 more, dig into the [full documentation