% Rust Language Tutorial
This is a tutorial for the Rust programming language. It assumes the reader is familiar with the basic concepts of programming, and has programmed in one or more other languages before. It will often make comparisons to other languages in the C family. The tutorial covers the whole language, though not with the depth and precision of the language reference.
Rust is a systems programming language with a focus on type safety, memory safety, concurrency and performance. It is intended for writing large, high-performance applications while preventing several classes of errors commonly found in languages like C++. Rust has a sophisticated memory model that makes possible many of the efficient data structures used in C++, while disallowing invalid memory accesses that would otherwise cause segmentation faults. Like other systems languages, it is statically typed and compiled ahead of time.
As a multi-paradigm language, Rust supports writing code in procedural, functional and object-oriented styles. Some of its nice high-level features include:
- Pattern matching and algebraic data types (enums). Common in functional languages, pattern matching on ADTs provides a compact and expressive way to encode program logic.
- Task-based concurrency. Rust uses lightweight tasks that do not share memory.
- Higher-order functions. Rust functions may take closures as arguments or return closures as return values. Closures in Rust are very powerful and used pervasively.
- Trait polymorphism. Rust's type system features a unique combination of Java-style interfaces and Haskell-style typeclasses called traits.
- Parametric polymorphism (generics). Functions and types can be parameterized over type variables with optional type constraints.
- Type inference. Type annotations on local variable declarations can be omitted.
As a curly-brace language in the tradition of C, C++, and JavaScript, Rust looks a lot like other languages you may be familiar with.
fn boring_old_factorial(n: int) -> int {
let mut result = 1, i = 1;
while i <= n {
result *= i;
i += 1;
}
return result;
}
Several differences from C stand out. Types do not come before, but
after variable names (preceded by a colon). For local variables
(introduced with let), types are optional, and will be inferred when
left off. Constructs like while and if do not require parentheses
around the condition (though they allow them).
You should, however, not conclude that Rust is simply an evolution of C. As will become clear in the rest of this tutorial, it goes in quite a different direction, with efficient, strongly-typed and memory-safe support for many high-level idioms.
Throughout the tutorial, words that indicate language keywords or
identifiers defined in the example code are displayed in code font.
Code snippets are indented, and also shown in a monospaced font. Not
all snippets constitute whole programs. For brevity, we'll often show
fragments of programs that don't compile on their own. To try them
out, you might have to wrap them in fn main() { ... }, and make sure
they don't contain references to things that aren't actually defined.
Warning: Rust is a language under heavy development. Notes about potential changes to the language, implementation deficiencies, and other caveats appear offset in blockquotes.
The Rust compiler currently must be built from a tarball. We hope to be distributing binary packages for various operating systems in the future.
The Rust compiler is slightly unusual in that it is written in Rust and therefore must be built by a precompiled "snapshot" version of itself (made in an earlier state of development). As such, source builds require that:
- You are connected to the internet, to fetch snapshots.
- You can at least execute snapshot binaries of one of the forms we
offer them in. Currently we build and test snapshots on:
- Windows (7, server 2008 r2) x86 only
- Linux (various distributions) x86 and x86-64
- OSX 10.6 ("Snow Leopard") or 10.7 ("Lion") x86 and x86-64
You may find other platforms work, but these are our "tier 1" supported build environments that are most likely to work. Further platforms will be added to the list in the future via cross-compilation.
To build from source you will also need the following prerequisite packages:
- g++ 4.4 or clang++ 3.x
- python 2.6 or later
- perl 5.0 or later
- gnu make 3.81 or later
- curl
Assuming you're on a relatively modern *nix system and have met the prerequisites, something along these lines should work. Building from source on Windows requires some extra steps: please see the getting started page on the Rust wiki.
$ wget http://dl.rust-lang.org/dist/rust-0.3.tar.gz
$ tar -xzf rust-0.3.tar.gz
$ cd rust-0.3
$ ./configure
$ make && make install
You may need to use sudo make install if you do not normally have
permission to modify the destination directory. The install locations
can be adjusted by passing a --prefix argument to
configure. Various other options are also supported, pass --help
for more information on them.
When complete, make install will place the following programs into
/usr/local/bin:
rustc, the Rust compilerrustdoc, the API-documentation toolcargo, the Rust package manager
Rust program files are, by convention, given the extension .rs. Say
we have a file hello.rs containing this program:
fn main() {
io::println("hello world!");
}
If the Rust compiler was installed successfully, running rustc hello.rs will produce a binary called hello (or hello.exe).
If you modify the program to make it invalid (for example, by changing
io::println to some nonexistent function), and then compile it,
you'll see an error message like this:
hello.rs:2:4: 2:16 error: unresolved name: io::print_it
hello.rs:2 io::print_it("hello world!");
^~~~~~~~~~~~
The Rust compiler tries to provide useful information when it runs into an error.
In its simplest form, a Rust program is a .rs file with some
types and functions defined in it. If it has a main function, it can
be compiled to an executable. Rust does not allow code that's not a
declaration to appear at the top level of the file—all statements must
live inside a function.
Rust programs can also be compiled as libraries, and included in other
programs. The extern mod std directive that appears at the top of a lot of
examples imports the standard library. This is described in more
detail later on.
There are Vim highlighting and indentation scripts in the Rust source
distribution under src/etc/vim/, and an emacs mode under
src/etc/emacs/. There is a package for Sublime Text 2 at
github.com/dbp/sublime-rust, also
available through package control.
Other editors are not provided for yet. If you end up writing a Rust mode for your favorite editor, let us know so that we can link to it.
Assuming you've programmed in any C-family language (C++, Java,
JavaScript, C#, or PHP), Rust will feel familiar. The main surface
difference to be aware of is that the bodies of if statements and of
while loops have to be wrapped in brackets. Single-statement,
bracket-less bodies are not allowed.
Accounting for these differences, the surface syntax of Rust
statements and expressions is C-like. Function calls are written
myfunc(arg1, arg2), operators have mostly the same name and
precedence that they have in C, comments look the same, and constructs
like if and while are available:
# fn it_works() {}
# fn abort() {}
fn main() {
while true {
/* Ensure that basic math works. */
if 2*20 > 30 {
// Everything is OK.
it_works();
} else {
abort();
}
break;
}
}
Though it isn't apparent in all code, there is a fundamental difference between Rust's syntax and its predecessors in this family of languages. Many constructs that are statements in C are expressions in Rust. This allows Rust to be more expressive. For example, you might write a piece of code like this:
# let item = "salad";
let price;
if item == "salad" {
price = 3.50;
} else if item == "muffin" {
price = 2.25;
} else {
price = 2.00;
}
But, in Rust, you don't have to repeat the name price:
# let item = "salad";
let price = if item == "salad" { 3.50 }
else if item == "muffin" { 2.25 }
else { 2.00 };
Both pieces of code are exactly equivalent—they assign a value to price
depending on the condition that holds. Note that the semicolons are omitted
from the second snippet. This is important; the lack of a semicolon after the
last statement in a braced block gives the whole block the value of that last
expression.
Put another way, the semicolon in Rust ignores the value of an expression.
Thus, if the branches of the if had looked like { 4; }, the above example
would simply assign nil (void) to price. But without the semicolon, each
branch has a different value, and price gets the value of the branch that
was taken.
This feature also works for function bodies. This function returns a boolean:
fn is_four(x: int) -> bool { x == 4 }
In short, everything that's not a declaration (let for variables,
fn for functions, et cetera) is an expression.
If all those things are expressions, you might conclude that you have
to add a terminating semicolon after every statement, even ones that
are not traditionally terminated with a semicolon in C (like while).
That is not the case, though. Expressions that end in a block only
need a semicolon if that block contains a trailing expression. while
loops do not allow trailing expressions, and if statements tend to
only have a trailing expression when you want to use their value for
something—in which case you'll have embedded it in a bigger statement,
like the let x = ... example above.
Rust identifiers follow the same rules as C; they start with an alphabetic character or an underscore, and after that may contain any sequence of alphabetic characters, numbers, or underscores. The preferred style is to begin function, variable, and module names with a lowercase letter, using underscores where they help readability, while beginning types with a capital letter.
The double-colon (::) is used as a module separator, so
io::println means 'the thing named println in the module
named io.
The let keyword, as we've seen, introduces a local variable. Local
variables are immutable by default: let mut can be used to introduce
a local variable that can be reassigned. Global constants can be
defined with const:
const REPEAT: int = 5;
fn main() {
let hi = "Hi!";
let mut count = 0;
while count < REPEAT {
io::println(hi);
count += 1;
}
}
Local variables may shadow earlier declarations, making the earlier variables inaccessible.
let my_favorite_value: float = 57.8;
let my_favorite_value: int = my_favorite_value as int;
The basic types are written like this:
()
: Nil, the type that has only a single value.
bool
: Boolean type, with values true and false.
int
: A machine-pointer-sized integer.
uint
: A machine-pointer-sized unsigned integer.
i8, i16, i32, i64
: Signed integers with a specific size (in bits).
u8, u16, u32, u64
: Unsigned integers with a specific size.
float
: The largest floating-point type efficiently supported on the target
machine.
f32, f64
: Floating-point types with a specific size.
char
: A Unicode character (32 bits).
These can be combined in composite types, which will be described in
more detail later on (the Ts here stand for any other type):
[T * N]
: Vector (like an array in other languages) with N elements.
[mut T * N]
: Mutable vector with N elements.
(T1, T2)
: Tuple type. Any arity above 1 is supported.
@T, ~T, &T
: Pointer types. See Boxes and pointers for an explanation of what @, ~, and & mean.
Some types can only be manipulated by pointer, never directly. For instance,
you cannot refer to a string (str); instead you refer to a pointer to a
string (@str, ~str, or &str). These dynamically-sized types consist
of:
fn(arg1: T1, arg2: T2) -> T3
: Function types.
str
: String type (in UTF-8).
[T]
: Vector with unknown size (also called a slice).
[mut T]
: Mutable vector with unknown size.
Types can be given names with type declarations:
type MonsterSize = uint;
This will provide a synonym, MonsterSize, for unsigned integers. It will not
actually create a new, incompatible type—MonsterSize and uint can be used
interchangeably, and using one where the other is expected is not a type
error. Read about single-variant enums
further on if you need to create a type name that's not just a
synonym.
The -> bool in the is_four example is the way a function's return
type is written. For functions that do not return a meaningful value,
you can optionally say -> (), but usually the return annotation is simply
left off, as in the fn main() { ... } examples we've seen earlier.
Every argument to a function must have its type declared (for example,
x: int). Inside the function, type inference will be able to
automatically deduce the type of most locals (generic functions, which
we'll come back to later, will occasionally need additional
annotation). Locals can be written either with or without a type
annotation:
// The type of this vector will be inferred based on its use.
let x = [];
# vec::map(x, fn&(&&_y:int) -> int { _y });
// Explicitly say this is a vector of zero integers.
let y: [int * 0] = [];
Integers can be written in decimal (144), hexadecimal (0x90), and
binary (0b10010000) base.
If you write an integer literal without a suffix (3, -500, etc.),
the Rust compiler will try to infer its type based on type annotations
and function signatures in the surrounding program. In the absence of any type
annotations at all, Rust will assume that an unsuffixed integer literal has
type int. It's also possible to avoid any type ambiguity by writing integer
literals with a suffix. For example:
let x = 50;
log(error, x); // x is an int
let y = 100u;
log(error, y); // y is an uint
Note that, in Rust, no implicit conversion between integer types
happens. If you are adding one to a variable of type uint, saying
+= 1u8 will give you a type error.
Floating point numbers are written 0.0, 1e6, or 2.1e-4. Without
a suffix, the literal is assumed to be of type float. Suffixes f (32-bit)
and l (64-bit) can be used to create literals of a specific type.
The nil literal is written just like the type: (). The keywords
true and false produce the boolean literals.
Character literals are written between single quotes, as in 'x'. Just as in
C, Rust understands a number of character escapes, using the backslash
character, \n, \r, and \t being the most common.
String literals allow the same escape sequences. They are written
between double quotes ("hello"). Rust strings may contain newlines.
Rust's set of operators contains very few surprises. Arithmetic is done with
*, /, %, +, and - (multiply, divide, remainder, plus, minus). - is
also a unary prefix operator that does negation. As in C, the bit operators
>>, <<, &, |, and ^ are also supported.
Note that, if applied to an integer value, ! flips all the bits (like ~ in
C).
The comparison operators are the traditional ==, !=, <, >,
<=, and >=. Short-circuiting (lazy) boolean operators are written
&& (and) and || (or).
For type casting, Rust uses the binary as operator. It takes an
expression on the left side and a type on the right side and will,
if a meaningful conversion exists, convert the result of the
expression to the given type.
let x: float = 4.0;
let y: uint = x as uint;
assert y == 4u;
The main difference with C is that ++ and -- are missing, and that
the logical bitwise operators have higher precedence — in C, x & 2 > 0
comes out as x & (2 > 0), in Rust, it means (x & 2) > 0, which is
more likely to be what you expect (unless you are a C veteran).
Syntax extensions are special forms that are not built into the language,
but are instead provided by the libraries. To make it clear to the reader when
a syntax extension is being used, the names of all syntax extensions end with
!. The standard library defines a few syntax extensions, the most useful of
which is fmt!, a sprintf-style text formatter that is expanded at compile
time.
io::println(fmt!("%s is %d", ~"the answer", 42));
fmt! supports most of the directives that printf supports, but
will give you a compile-time error when the types of the directives
don't match the types of the arguments.
You can define your own syntax extensions with the macro system, which is out of scope of this tutorial.
We've seen if pass by a few times already. To recap, braces are
compulsory, an optional else clause can be appended, and multiple
if/else constructs can be chained together:
if false {
io::println(~"that's odd");
} else if true {
io::println(~"right");
} else {
io::println(~"neither true nor false");
}
The condition given to an if construct must be of type boolean (no
implicit conversion happens). If the arms return a value, this value
must be of the same type for every arm in which control reaches the
end of the block:
fn signum(x: int) -> int {
if x < 0 { -1 }
else if x > 0 { 1 }
else { return 0 }
}
Rust's match construct is a generalized, cleaned-up version of C's
switch construct. You provide it with a value and a number of arms,
each labelled with a pattern, and the code will attempt to match each pattern
in order. For the first one that matches, the arm is executed.
# let my_number = 1;
match my_number {
0 => io::println("zero"),
1 | 2 => io::println("one or two"),
3..10 => io::println("three to ten"),
_ => io::println("something else")
}
There is no 'falling through' between arms, as in C—only one arm is
executed, and it doesn't have to explicitly break out of the
construct when it is finished.
The part to the left of the arrow => is called the pattern. Literals are
valid patterns and will match only their own value. The pipe operator
(|) can be used to assign multiple patterns to a single arm. Ranges
of numeric literal patterns can be expressed with two dots, as in M..N. The
underscore (_) is a wildcard pattern that matches everything.
The patterns in an match arm are followed by a fat arrow, =>, then an
expression to evaluate. Each case is separated by commas. It's often
convenient to use a block expression for a case, in which case the
commas are optional.
# let my_number = 1;
match my_number {
0 => {
io::println("zero")
}
_ => {
io::println("something else")
}
}
match constructs must be exhaustive: they must have an arm covering every
possible case. For example, if the arm with the wildcard pattern was left off
in the above example, the typechecker would reject it.
A powerful application of pattern matching is destructuring, where
you use the matching to get at the contents of data types. Remember
that (float, float) is a tuple of two floats:
use float::consts::pi;
fn angle(vector: (float, float)) -> float {
match vector {
(0f, y) if y < 0f => 1.5 * pi,
(0f, y) => 0.5 * pi,
(x, y) => float::atan(y / x)
}
}
A variable name in a pattern matches everything, and binds that name
to the value of the matched thing inside of the arm block. Thus, (0f, y) matches any tuple whose first element is zero, and binds y to
the second element. (x, y) matches any tuple, and binds both
elements to a variable.
Any match arm can have a guard clause (written if EXPR), which is
an expression of type bool that determines, after the pattern is
found to match, whether the arm is taken or not. The variables bound
by the pattern are available in this guard expression.
You've already seen simple let bindings. let is also a little fancier: it
is possible to use destructuring patterns in it. For example, you can say this
to extract the fields from a tuple:
# fn get_tuple_of_two_ints() -> (int, int) { (1, 1) }
let (a, b) = get_tuple_of_two_ints();
This will introduce two new variables, a and b, bound to the
content of the tuple.
You may only use irrefutable patterns—patterns that can never fail to match—in let bindings. Other types of patterns, such as literals, are not allowed.
while produces a loop that runs as long as its given condition
(which must have type bool) evaluates to true. Inside a loop, the
keyword break can be used to abort the loop, and again can be used
to abort the current iteration and continue with the next.
let mut cake_amount = 8;
while cake_amount > 0 {
cake_amount -= 1;
}
loop is the preferred way of writing while true:
let mut x = 5;
loop {
x += x - 3;
if x % 5 == 0 { break; }
io::println(int::str(x));
}
This code prints out a weird sequence of numbers and stops as soon as it finds one that can be divided by five.
For more involved iteration, such as going over the elements of a collection, Rust uses higher-order functions. We'll come back to those in a moment.
Like all other static declarations, such as type, functions can be
declared both at the top level and inside other functions (or modules,
which we'll come back to later).
We've already seen several function definitions. They are introduced
with the fn keyword, the type of arguments are specified following
colons and the return type follows the arrow.
fn repeat(string: &str, count: int) -> ~str {
let mut result = ~"";
for count.times {
result += string;
}
return result;
}
The return keyword immediately returns from the body of a function. It
is optionally followed by an expression to return. A function can
also return a value by having its top level block produce an
expression.
# const copernicus: int = 0;
fn int_to_str(i: int) -> ~str {
if i == copernicus {
return ~"tube sock";
} else {
return ~"violin";
}
}
# const copernicus: int = 0;
fn int_to_str(i: int) -> ~str {
if i == copernicus { ~"tube sock" }
else { ~"violin" }
}
Functions that do not return a value are said to return nil, (),
and both the return type and the return value may be omitted from
the definition. The following two functions are equivalent.
fn do_nothing_the_hard_way() -> () { return (); }
fn do_nothing_the_easy_way() { }
The core datatypes of Rust are structs, enums (tagged unions, algebraic data types), and tuples. They are immutable by default.
struct Point { x: float, y: float }
enum Shape {
Circle(Point, float),
Rectangle(Point, Point)
}
Rust struct types must be declared before they are used using the struct
syntax: struct Name { field1: T1, field2: T2 [, ...] }, where T1, T2,
... denote types. To construct a struct, use the same syntax, but leave off
the struct; for example: Point { x: 1.0, y: 2.0 }.
Structs are quite similar to C structs and are even laid out the same way in
memory (so you can read from a Rust struct in C, and vice-versa). The dot
operator is used to access struct fields (mypoint.x).
Fields that you want to mutate must be explicitly marked mut.
struct Stack {
content: ~[int],
mut head: uint
}
With a value of such a type, you can do mystack.head += 1. If mut were
omitted from the type, such an assignment would result in a type error.
Structs can be destructured in match patterns. The basic syntax is
Name {fieldname: pattern, ...}:
# struct Point { x: float, y: float }
# let mypoint = Point { x: 0.0, y: 0.0 };
match mypoint {
Point { x: 0.0, y: y } => { io::println(y.to_str()); }
Point { x: x, y: y } => { io::println(x.to_str() + " " + y.to_str()); }
}
In general, the field names of a struct do not have to appear in the same
order they appear in the type. When you are not interested in all
the fields of a struct, a struct pattern may end with , _ (as in
Name {field1, _}) to indicate that you're ignoring all other fields.
Enums are datatypes that have several alternate representations. For example, consider the type shown earlier:
# struct Point { x: float, y: float }
enum Shape {
Circle(Point, float),
Rectangle(Point, Point)
}
A value of this type is either a Circle, in which case it contains a point struct and a float, or a Rectangle, in which case it contains two point records. The run-time representation of such a value includes an identifier of the actual form that it holds, much like the 'tagged union' pattern in C, but with better ergonomics.
The above declaration will define a type shape that can be used to
refer to such shapes, and two functions, circle and rectangle,
which can be used to construct values of the type (taking arguments of
the specified types). So circle({x: 0f, y: 0f}, 10f) is the way to
create a new circle.
Enum variants need not have type parameters. This, for example, is equivalent to a C enum:
enum direction {
north,
east,
south,
west
}
This will define north, east, south, and west as constants,
all of which have type direction.
When an enum is C-like, that is, when none of the variants have parameters, it is possible to explicitly set the discriminator values to an integer value:
enum color {
red = 0xff0000,
green = 0x00ff00,
blue = 0x0000ff
}
If an explicit discriminator is not specified for a variant, the value
defaults to the value of the previous variant plus one. If the first
variant does not have a discriminator, it defaults to 0. For example,
the value of north is 0, east is 1, etc.
When an enum is C-like the as cast operator can be used to get the
discriminator's value.
There is a special case for enums with a single variant. These are used to define new types in such a way that the new name is not just a synonym for an existing type, but its own distinct type. If you say:
enum gizmo_id = int;
That is a shorthand for this:
enum gizmo_id { gizmo_id(int) }
Enum types like this can have their content extracted with the
dereference (*) unary operator:
# enum gizmo_id = int;
let my_gizmo_id = gizmo_id(10);
let id_int: int = *my_gizmo_id;
For enum types with multiple variants, destructuring is the only way to
get at their contents. All variant constructors can be used as
patterns, as in this definition of area:
# type point = {x: float, y: float};
# enum shape { circle(point, float), rectangle(point, point) }
fn area(sh: shape) -> float {
match sh {
circle(_, size) => float::consts::pi * size * size,
rectangle({x, y}, {x: x2, y: y2}) => (x2 - x) * (y2 - y)
}
}
Another example, matching nullary enum variants:
# type point = {x: float, y: float};
# enum direction { north, east, south, west }
fn point_from_direction(dir: direction) -> point {
match dir {
north => {x: 0f, y: 1f},
east => {x: 1f, y: 0f},
south => {x: 0f, y: -1f},
west => {x: -1f, y: 0f}
}
}
Tuples in Rust behave exactly like records, except that their fields
do not have names (and can thus not be accessed with dot notation).
Tuples can have any arity except for 0 or 1 (though you may consider
nil, (), as the empty tuple if you like).
let mytup: (int, int, float) = (10, 20, 30.0);
match mytup {
(a, b, c) => log(info, a + b + (c as int))
}
At this junction let's take a detour to explain the concepts involved in Rust's memory model. Rust has a very particular approach to memory management that plays a significant role in shaping the "feel" of the language. Understanding the memory landscape will illuminate several of Rust's unique features as we encounter them.
Rust has three competing goals that inform its view of memory:
- Memory safety: memory that is managed by and is accessible to the Rust language must be guaranteed to be valid; under normal circumstances it must be impossible for Rust to trigger a segmentation fault or leak memory
- Performance: high-performance low-level code must be able to employ a number of allocation strategies; low-performance high-level code must be able to employ a single, garbage-collection-based, heap allocation strategy
- Concurrency: Rust must maintain memory safety guarantees, even for code running in parallel
Most languages that offer strong memory safety guarantees rely upon a garbage-collected heap to manage all of the objects. This approach is straightforward both in concept and in implementation, but has significant costs. Languages that take this approach tend to aggressively pursue ways to ameliorate allocation costs (think the Java Virtual Machine). Rust supports this strategy with shared boxes: memory allocated on the heap that may be referred to (shared) by multiple variables.
By comparison, languages like C++ offer very precise control over where objects are allocated. In particular, it is common to put them directly on the stack, avoiding expensive heap allocation. In Rust this is possible as well, and the compiler will use a clever pointer lifetime analysis to ensure that no variable can refer to stack objects after they are destroyed.
Memory safety in a concurrent environment involves avoiding race conditions between two threads of execution accessing the same memory. Even high-level languages often require programmers to correctly employ locking to ensure that a program is free of races.
Rust starts from the position that memory cannot be shared between tasks. Experience in other languages has proven that isolating each task's heap from the others is a reliable strategy and one that is easy for programmers to reason about. Heap isolation has the additional benefit that garbage collection must only be done per-heap. Rust never "stops the world" to garbage-collect memory.
Complete isolation of heaps between tasks implies that any data transferred between tasks must be copied. While this is a fine and useful way to implement communication between tasks, it is also very inefficient for large data structures. Because of this, Rust also employs a global exchange heap. Objects allocated in the exchange heap have ownership semantics, meaning that there is only a single variable that refers to them. For this reason, they are referred to as unique boxes. All tasks may allocate objects on the exchange heap, then transfer ownership of those objects to other tasks, avoiding expensive copies.
Rust has three "realms" in which objects can be allocated: the stack,
the local heap, and the exchange heap. These realms have corresponding
pointer types: the borrowed pointer (&T), the shared box (@T),
and the unique box (~T). These three sigils will appear
repeatedly as we explore the language. Learning the appropriate role
of each is key to using Rust effectively.
In contrast to a lot of modern languages, aggregate types like records
and enums are not represented as pointers to allocated memory in
Rust. They are, as in C and C++, represented directly. This means that
if you let x = {x: 1f, y: 1f};, you are creating a record on the
stack. If you then copy it into a data structure, the whole record is
copied, not just a pointer.
For small records like point, this is usually more efficient than
allocating memory and going through a pointer. But for big records, or
records with mutable fields, it can be useful to have a single copy on
the heap, and refer to that through a pointer.
Rust supports several types of pointers. The safe pointer types are
@T for shared boxes allocated on the local heap, ~T, for
uniquely-owned boxes allocated on the exchange heap, and &T, for
borrowed pointers, which may point to any memory, and whose lifetimes
are governed by the call stack.
Rust also has an unsafe pointer, written *T, which is a completely
unchecked pointer type only used in unsafe code (and thus, in typical
Rust code, very rarely).
All pointer types can be dereferenced with the * unary operator.
Shared boxes are pointers to heap-allocated, garbage collected memory.
Creating a shared box is done by simply applying the unary @
operator to an expression. The result of the expression will be boxed,
resulting in a box of the right type. Copying a shared box, as happens
during assignment, only copies a pointer, never the contents of the
box.
let x: @int = @10; // New box, refcount of 1
let y = x; // Copy the pointer, increase refcount
// When x and y go out of scope, refcount goes to 0, box is freed
Shared boxes never cross task boundaries.
Note: shared boxes are currently reclaimed through reference counting and cycle collection, but we will switch to a tracing garbage collector.
In contrast to shared boxes, unique boxes have a single owner and thus two unique boxes may not refer to the same memory. All unique boxes across all tasks are allocated on a single exchange heap, where their uniquely owned nature allows them to be passed between tasks.
Because unique boxes are uniquely owned, copying them involves allocating a new unique box and duplicating the contents. Copying unique boxes is expensive so the compiler will complain if you do.
let x = ~10;
let y = x; // error: copying a non-implicitly copyable type
If you really want to copy a unique box you must say so explicitly.
let x = ~10;
let y = copy x;
This is where the 'move' (<-) operator comes in. It is similar to
=, but it de-initializes its source. Thus, the unique box can move
from x to y, without violating the constraint that it only has a
single owner (if you used assignment instead of the move operator, the
box would, in principle, be copied).
let x = ~10;
let y <- x;
Note: this discussion of copying vs moving does not account for the "last use" rules that automatically promote copy operations to moves. This is an evolving area of the language that will continue to change.
Unique boxes, when they do not contain any shared boxes, can be sent to other tasks. The sending task will give up ownership of the box, and won't be able to access it afterwards. The receiving task will become the sole owner of the box.
Rust borrowed pointers are a general purpose reference/pointer type, similar to the C++ reference type, but guaranteed to point to valid memory. In contrast to unique pointers, where the holder of a unique pointer is the owner of the pointed-to memory, borrowed pointers never imply ownership. Pointers may be borrowed from any type, in which case the pointer is guaranteed not to outlive the value it points to.
# fn work_with_foo_by_pointer(f: &~str) { }
let foo = ~"foo";
work_with_foo_by_pointer(&foo);
The following shows an example of what is not possible with borrowed
pointers. If you were able to write this then the pointer to foo
would outlive foo itself.
let foo_ptr;
{
let foo = ~"foo";
foo_ptr = &foo;
}
Note: borrowed pointers are a new addition to the language. They are not used extensively yet but are expected to become the pointer type used in many common situations, in particular for by-reference argument passing. Rust's current solution for passing arguments by reference is argument modes.
All pointer types have a mutable variant, written @mut T or ~mut T. Given such a pointer, you can write to its contents by combining
the dereference operator with a mutating action.
fn increase_contents(pt: @mut int) {
*pt += 1;
}
Vectors are a contiguous section of memory containing zero or more values of the same type. Like other types in Rust, vectors can be stored on the stack, the local heap, or the exchange heap.
enum crayon {
almond, antique_brass, apricot,
aquamarine, asparagus, atomic_tangerine,
banana_mania, beaver, bittersweet
}
// A stack vector of crayons
let stack_crayons: &[crayon] = &[almond, antique_brass, apricot];
// A local heap (shared) vector of crayons
let local_crayons: @[crayon] = @[aquamarine, asparagus, atomic_tangerine];
// An exchange heap (unique) vector of crayons
let exchange_crayons: ~[crayon] = ~[banana_mania, beaver, bittersweet];
Note: Until recently Rust only had unique vectors, using the unadorned
[]syntax for literals. This syntax is still supported but is deprecated. In the future it will probably represent some "reasonable default" vector type.Unique vectors are the currently-recommended vector type for general use as they are the most tested and well-supported by existing libraries. There will be a gradual shift toward using more stack and local vectors in the coming releases.
Vector literals are enclosed in square brackets and dereferencing is also done with square brackets (zero-based):
# enum crayon { almond, antique_brass, apricot,
# aquamarine, asparagus, atomic_tangerine,
# banana_mania, beaver, bittersweet };
# fn draw_scene(c: crayon) { }
let crayons = ~[banana_mania, beaver, bittersweet];
match crayons[0] {
bittersweet => draw_scene(crayons[0]),
_ => ()
}
By default, vectors are immutable—you can not replace their elements.
The type written as ~[mut T] is a vector with mutable
elements. Mutable vector literals are written ~[mut] (empty) or ~[mut 1, 2, 3] (with elements).
# enum crayon { almond, antique_brass, apricot,
# aquamarine, asparagus, atomic_tangerine,
# banana_mania, beaver, bittersweet };
let crayons = ~[mut banana_mania, beaver, bittersweet];
crayons[0] = atomic_tangerine;
The + operator means concatenation when applied to vector types.
# enum crayon { almond, antique_brass, apricot,
# aquamarine, asparagus, atomic_tangerine,
# banana_mania, beaver, bittersweet };
let my_crayons = ~[almond, antique_brass, apricot];
let your_crayons = ~[banana_mania, beaver, bittersweet];
let our_crayons = my_crayons + your_crayons;
The += operator also works as expected, provided the assignee
lives in a mutable slot.
# enum crayon { almond, antique_brass, apricot,
# aquamarine, asparagus, atomic_tangerine,
# banana_mania, beaver, bittersweet };
let mut my_crayons = ~[almond, antique_brass, apricot];
let your_crayons = ~[banana_mania, beaver, bittersweet];
my_crayons += your_crayons;
Both vectors and strings support a number of useful methods. While we haven't covered methods yet, most vector functionality is provided by methods, so let's have a brief look at a few common ones.
# use io::println;
# enum crayon {
# almond, antique_brass, apricot,
# aquamarine, asparagus, atomic_tangerine,
# banana_mania, beaver, bittersweet
# }
# fn unwrap_crayon(c: crayon) -> int { 0 }
# fn eat_crayon_wax(i: int) { }
# fn store_crayon_in_nasal_cavity(i: uint, c: crayon) { }
# fn crayon_to_str(c: crayon) -> ~str { ~"" }
let crayons = ~[almond, antique_brass, apricot];
// Check the length of the vector
assert crayons.len() == 3;
assert !crayons.is_empty();
// Iterate over a vector
for crayons.each |crayon| {
let delicious_crayon_wax = unwrap_crayon(crayon);
eat_crayon_wax(delicious_crayon_wax);
}
// Map vector elements
let crayon_names = crayons.map(crayon_to_str);
let favorite_crayon_name = crayon_names[0];
// Remove whitespace from before and after the string
let new_favorite_crayon_name = favorite_crayon_name.trim();
if favorite_crayon_name.len() > 5 {
// Create a substring
println(favorite_crayon_name.substr(0, 5));
}
Named functions, like those we've seen so far, may not refer to local variables declared outside the function - they do not "close over their environment". For example, you couldn't write the following:
let foo = 10;
fn bar() -> int {
return foo; // `bar` cannot refer to `foo`
}
Rust also supports closures, functions that can access variables in the enclosing scope.
# use println = io::println;
fn call_closure_with_ten(b: fn(int)) { b(10); }
let captured_var = 20;
let closure = |arg| println(fmt!("captured_var=%d, arg=%d", captured_var, arg));
call_closure_with_ten(closure);
Closures begin with the argument list between bars and are followed by a single expression. The types of the arguments are generally omitted, as is the return type, because the compiler can almost always infer them. In the rare case where the compiler needs assistance though, the arguments and return types may be annotated.
# type mygoodness = fn(~str) -> ~str; type what_the = int;
let bloop = |well, oh: mygoodness| -> what_the { fail oh(well) };
There are several forms of closure, each with its own role. The most
common, called a stack closure, has type fn& and can directly
access local variables in the enclosing scope.
let mut max = 0;
(~[1, 2, 3]).map(|x| if x > max { max = x });
Stack closures are very efficient because their environment is allocated on the call stack and refers by pointer to captured locals. To ensure that stack closures never outlive the local variables to which they refer, they can only be used in argument position and cannot be stored in structures nor returned from functions. Despite the limitations stack closures are used pervasively in Rust code.
When you need to store a closure in a data structure, a stack closure
will not do, since the compiler will refuse to let you store it. For
this purpose, Rust provides a type of closure that has an arbitrary
lifetime, written fn@ (boxed closure, analogous to the @ pointer
type described earlier).
A boxed closure does not directly access its environment, but merely copies out the values that it closes over into a private data structure. This means that it can not assign to these variables, and will not 'see' updates to them.
This code creates a closure that adds a given string to its argument, returns it from a function, and then calls it:
extern mod std;
fn mk_appender(suffix: ~str) -> fn@(~str) -> ~str {
return fn@(s: ~str) -> ~str { s + suffix };
}
fn main() {
let shout = mk_appender(~"!");
io::println(shout(~"hey ho, let's go"));
}
This example uses the long closure syntax, fn@(s: ~str) ...,
making the fact that we are declaring a box closure explicit. In
practice boxed closures are usually defined with the short closure
syntax introduced earlier, in which case the compiler will infer
the type of closure. Thus our boxed closure example could also
be written:
fn mk_appender(suffix: ~str) -> fn@(~str) -> ~str {
return |s| s + suffix;
}
Unique closures, written fn~ in analogy to the ~ pointer type,
hold on to things that can safely be sent between
processes. They copy the values they close over, much like boxed
closures, but they also 'own' them—meaning no other code can access
them. Unique closures are used in concurrent code, particularly
for spawning tasks.
A nice property of Rust closures is that you can pass any kind of
closure (as long as the arguments and return types match) to functions
that expect a fn(). Thus, when writing a higher-order function that
wants to do nothing with its function argument beyond calling it, you
should almost always specify the type of that argument as fn(), so
that callers have the flexibility to pass whatever they want.
fn call_twice(f: fn()) { f(); f(); }
call_twice(|| { ~"I am an inferred stack closure"; } );
call_twice(fn&() { ~"I am also a stack closure"; } );
call_twice(fn@() { ~"I am a boxed closure"; });
call_twice(fn~() { ~"I am a unique closure"; });
fn bare_function() { ~"I am a plain function"; }
call_twice(bare_function);
Closures in Rust are frequently used in combination with higher-order
functions to simulate control structures like if and
loop. Consider this function that iterates over a vector of
integers, applying an operator to each:
fn each(v: ~[int], op: fn(int)) {
let mut n = 0;
while n < v.len() {
op(v[n]);
n += 1;
}
}
As a caller, if we use a closure to provide the final operator argument, we can write it in a way that has a pleasant, block-like structure.
# fn each(v: ~[int], op: fn(int)) {}
# fn do_some_work(i: int) { }
each(~[1, 2, 3], |n| {
debug!("%i", n);
do_some_work(n);
});
This is such a useful pattern that Rust has a special form of function call that can be written more like a built-in control structure:
# fn each(v: ~[int], op: fn(int)) {}
# fn do_some_work(i: int) { }
do each(~[1, 2, 3]) |n| {
debug!("%i", n);
do_some_work(n);
}
The call is prefixed with the keyword do and, instead of writing the
final closure inside the argument list it is moved outside of the
parenthesis where it looks visually more like a typical block of
code. The do expression is purely syntactic sugar for a call that
takes a final closure argument.
do is often used for task spawning.
use task::spawn;
do spawn() || {
debug!("I'm a task, whatever");
}
That's nice, but look at all those bars and parentheses - that's two empty argument lists back to back. Wouldn't it be great if they weren't there?
# use task::spawn;
do spawn {
debug!("Kablam!");
}
Empty argument lists can be omitted from do expressions.
Most iteration in Rust is done with for loops. Like do,
for is a nice syntax for doing control flow with closures.
Additionally, within a for loop, break, again, and return
work just as they do with while and loop.
Consider again our each function, this time improved to
break early when the iteratee returns false:
fn each(v: ~[int], op: fn(int) -> bool) {
let mut n = 0;
while n < v.len() {
if !op(v[n]) {
break;
}
n += 1;
}
}
And using this function to iterate over a vector:
# use each = vec::each;
# use println = io::println;
each(~[2, 4, 8, 5, 16], |n| {
if n % 2 != 0 {
println(~"found odd number!");
false
} else { true }
});
With for, functions like each can be treated more
like builtin looping structures. When calling each
in a for loop, instead of returning false to break
out of the loop, you just write break. To skip ahead
to the next iteration, write again.
# use each = vec::each;
# use println = io::println;
for each(~[2, 4, 8, 5, 16]) |n| {
if n % 2 != 0 {
println(~"found odd number!");
break;
}
}
As an added bonus, you can use the return keyword, which is not
normally allowed in closures, in a block that appears as the body of a
for loop — this will cause a return to happen from the outer
function, not just the loop body.
# use each = vec::each;
fn contains(v: ~[int], elt: int) -> bool {
for each(v) |x| {
if (x == elt) { return true; }
}
false
}
for syntax only works with stack closures.
Throughout this tutorial, we've been defining functions that act only on single data types. It's a burden to define such functions again and again for every type they apply to. Thus, Rust allows functions and datatypes to have type parameters.
fn map<T, U>(vector: &[T], function: fn(T) -> U) -> ~[U] {
let mut accumulator = ~[];
for vector.each |element| {
vec::push(accumulator, function(element));
}
return accumulator;
}
When defined with type parameters, this function can be applied to any
type of vector, as long as the type of function's argument and the
type of the vector's content agree with each other.
Inside a generic function, the names of the type parameters (capitalized by convention) stand for opaque types. You can't look inside them, but you can pass them around.
Generic type, struct, and enum declarations follow the same pattern:
struct Stack<T> {
elements: ~[mut T]
}
enum Maybe<T> {
Just(T),
Nothing
}
These declarations produce valid types like Stack<u8> and Maybe<int>.
Perhaps surprisingly, the 'copy' (duplicate) operation is not defined for all Rust types. Resource types (classes with destructors) cannot be copied, and neither can any type whose copying would require copying a resource (such as records or unique boxes containing a resource).
This complicates handling of generic functions. If you have a type
parameter T, can you copy values of that type? In Rust, you can't,
unless you explicitly declare that type parameter to have copyable
'kind'. A kind is a type of type.
// This does not compile
fn head_bad<T>(v: ~[T]) -> T { v[0] }
// This does
fn head<T: Copy>(v: ~[T]) -> T { v[0] }
When instantiating a generic function, you can only instantiate it
with types that fit its kinds. So you could not apply head to a
resource type. Rust has several kinds that can be used as type bounds:
Copy- Copyable types. All types are copyable unless they are classes with destructors or otherwise contain classes with destructors.Send- Sendable types. All types are sendable unless they contain shared boxes, closures, or other local-heap-allocated types.Const- Constant types. These are types that do not contain mutable fields nor shared boxes.
Note: Rust type kinds are syntactically very similar to traits when used as type bounds, and can be conveniently thought of as built-in traits. In the future type kinds will actually be traits that the compiler has special knowledge about.
Traits are Rust's take on value polymorphism—the thing that object-oriented languages tend to solve with methods and inheritance. For example, writing a function that can operate on multiple types of collections.
Note: This feature is very new, and will need a few extensions to be applicable to more advanced use cases.
A trait consists of a set of methods. A method is a function that
can be applied to a self value and a number of arguments, using the
dot notation: self.foo(arg1, arg2).
For example, we could declare the trait to_str for things that
can be converted to a string, with a single method of the same name:
trait to_str {
fn to_str() -> ~str;
}
To actually implement a trait for a given type, the impl form
is used. This defines implementations of to_str for the int and
~str types.
# trait to_str { fn to_str() -> ~str; }
impl int: to_str {
fn to_str() -> ~str { int::to_str(self, 10u) }
}
impl ~str: to_str {
fn to_str() -> ~str { self }
}
Given these, we may call 1.to_str() to get ~"1", or
(~"foo").to_str() to get ~"foo" again. This is basically a form of
static overloading—when the Rust compiler sees the to_str method
call, it looks for an implementation that matches the type with a
method that matches the name, and simply calls that.
The useful thing about value polymorphism is that it does not have to be static. If object-oriented languages only let you call a method on an object when they knew exactly which sub-type it had, that would not get you very far. To be able to call methods on types that aren't known at compile time, it is possible to specify 'bounds' for type parameters.
# trait to_str { fn to_str() -> ~str; }
fn comma_sep<T: to_str>(elts: ~[T]) -> ~str {
let mut result = ~"", first = true;
for elts.each |elt| {
if first { first = false; }
else { result += ~", "; }
result += elt.to_str();
}
return result;
}
The syntax for this is similar to the syntax for specifying that a
parameter type has to be copyable (which is, in principle, another
kind of bound). By declaring T as conforming to the to_str
trait, it becomes possible to call methods from that trait on
values of that type inside the function. It will also cause a
compile-time error when anyone tries to call comma_sep on an array
whose element type does not have a to_str implementation in scope.
Traits may contain type parameters. A trait for generalized sequence types is:
trait seq<T> {
fn len() -> uint;
fn iter(fn(T));
}
impl<T> ~[T]: seq<T> {
fn len() -> uint { vec::len(self) }
fn iter(b: fn(T)) {
for self.each |elt| { b(elt); }
}
}
The implementation has to explicitly declare the type
parameter that it binds, T, before using it to specify its trait type. Rust requires this declaration because the impl could also, for example, specify an implementation of seq<int>. The trait type -- appearing after the colon in the impl -- refers to a type, rather than defining one.
The type parameters bound by a trait are in scope in each of the
method declarations. So, re-declaring the type parameter
T as an explicit type parameter for len -- in either the trait or
the impl -- would be a compile-time error.
In a trait, self is a special type that you can think of as a
type parameter. An implementation of the trait for any given type
T replaces the self type parameter with T. The following
trait describes types that support an equality operation:
trait eq {
fn equals(&&other: self) -> bool;
}
impl int: eq {
fn equals(&&other: int) -> bool { other == self }
}
Notice that equals takes an int argument, rather than a self argument, in
an implementation for type int.
The above allows us to define functions that polymorphically act on values of an unknown type that conforms to a given trait. However, consider this function:
# type circle = int; type rectangle = int;
# trait drawable { fn draw(); }
# impl int: drawable { fn draw() {} }
# fn new_circle() -> int { 1 }
fn draw_all<T: drawable>(shapes: ~[T]) {
for shapes.each |shape| { shape.draw(); }
}
# let c: circle = new_circle();
# draw_all(~[c]);
You can call that on an array of circles, or an array of squares
(assuming those have suitable drawable traits defined), but not
on an array containing both circles and squares.
When this is needed, a trait name can be used as a type, causing the function to be written simply like this:
# trait drawable { fn draw(); }
fn draw_all(shapes: ~[drawable]) {
for shapes.each |shape| { shape.draw(); }
}
There is no type parameter anymore (since there isn't a single type
that we're calling the function on). Instead, the drawable type is
used to refer to a type that is a reference-counted box containing a
value for which a drawable implementation exists, combined with
information on where to find the methods for this implementation. This
is very similar to the 'vtables' used in most object-oriented
languages.
To construct such a value, you use the as operator to cast a value
to a trait type:
# type circle = int; type rectangle = int;
# trait drawable { fn draw(); }
# impl int: drawable { fn draw() {} }
# fn new_circle() -> int { 1 }
# fn new_rectangle() -> int { 2 }
# fn draw_all(shapes: ~[drawable]) {}
let c: circle = new_circle();
let r: rectangle = new_rectangle();
draw_all(~[c as drawable, r as drawable]);
This will store the value into a box, along with information about the
implementation (which is looked up in the scope of the cast). The
drawable type simply refers to such boxes, and calling methods on it
always works, no matter what implementations are in scope.
Note that the allocation of a box is somewhat more expensive than simply using a type parameter and passing in the value as-is, and much more expensive than statically resolved method calls.
If you only intend to use an implementation for static overloading, and there is no trait available that it conforms to, you are free to leave off the type after the colon. However, this is only possible when you are defining an implementation in the same module as the receiver type, and the receiver type is a named type (i.e., an enum or a class); single-variant enums are a common choice.
The Rust namespace is divided into modules. Each source file starts with its own module.
The mod keyword can be used to open a new, local module. In the
example below, chicken lives in the module farm, so, unless you
explicitly import it, you must refer to it by its long name,
farm::chicken.
mod farm {
fn chicken() -> ~str { ~"cluck cluck" }
fn cow() -> ~str { ~"mooo" }
}
fn main() {
io::println(farm::chicken());
}
Modules can be nested to arbitrary depth.
The unit of independent compilation in Rust is the crate. Libraries tend to be packaged as crates, and your own programs may consist of one or more crates.
When compiling a single .rs file, the file acts as the whole crate.
You can compile it with the --lib compiler switch to create a shared
library, or without, provided that your file contains a fn main
somewhere, to create an executable.
It is also possible to include multiple files in a crate. For this
purpose, you create a .rc crate file, which references any number of
.rs code files. A crate file could look like this:
#[link(name = "farm", vers = "2.5", author = "mjh")];
#[crate_type = "lib"];
mod cow;
mod chicken;
mod horse;
Compiling this file will cause rustc to look for files named
cow.rs, chicken.rs, horse.rs in the same directory as the .rc
file, compile them all together, and, depending on the presence of the
crate_type = "lib" attribute, output a shared library or an executable.
(If the line #[crate_type = "lib"]; was omitted, rustc would create an
executable.)
The #[link(...)] part provides meta information about the module,
which other crates can use to load the right module. More about that
later.
To have a nested directory structure for your source files, you can
nest mods in your .rc file:
mod poultry {
mod chicken;
mod turkey;
}
The compiler will now look for poultry/chicken.rs and
poultry/turkey.rs, and export their content in poultry::chicken
and poultry::turkey. You can also provide a poultry.rs to add
content to the poultry module itself.
Having compiled a crate that contains the #[crate_type = "lib"]
attribute, you can use it in another crate with a use
directive. We've already seen extern mod std in several of the
examples, which loads in the standard library.
use directives can appear in a crate file, or at the top level of a
single-file .rs crate. They will cause the compiler to search its
library search path (which you can extend with -L switch) for a Rust
crate library with the right name.
It is possible to provide more specific information when using an external crate.
use myfarm (name = "farm", vers = "2.7");
When a comma-separated list of name/value pairs is given after use,
these are matched against the attributes provided in the link
attribute of the crate file, and a crate is only used when the two
match. A name value can be given to override the name used to search
for the crate. So the above would import the farm crate under the
local name myfarm.
Our example crate declared this set of link attributes:
#[link(name = "farm", vers = "2.5", author = "mjh")];
The version does not match the one provided in the use directive, so
unless the compiler can find another crate with the right version
somewhere, it will complain that no matching crate was found.
A set of basic library routines, mostly related to built-in datatypes and the task system, are always implicitly linked and included in any Rust program.
This library is documented here.
Now for something that you can actually compile yourself. We have these two files:
// mylib.rs
#[link(name = "mylib", vers = "1.0")];
fn world() -> ~str { ~"world" }
// main.rs
extern mod std;
use mylib;
fn main() { io::println(~"hello " + mylib::world()); }
Now compile and run like this (adjust to your platform if necessary):
> rustc --lib mylib.rs
> rustc main.rs -L .
> ./main
"hello world"
When using identifiers from other modules, it can get tiresome to qualify them with the full module path every time (especially when that path is several modules deep). Rust allows you to import identifiers at the top of a file, module, or block.
extern mod std;
use io::println;
fn main() {
println(~"that was easy");
}
It is also possible to import just the name of a module (use std::list;, then use list::find), to import all identifiers exported
by a given module (use io::*), or to import a specific set
of identifiers (use math::{min, max, pi}).
You can rename an identifier when importing using the = operator:
use prnt = io::println;
By default, a module exports everything that it defines. This can be
restricted with export directives at the top of the module or file.
mod enc {
export encrypt, decrypt;
const super_secret_number: int = 10;
fn encrypt(n: int) -> int { n + super_secret_number }
fn decrypt(n: int) -> int { n - super_secret_number }
}
This defines a rock-solid encryption algorithm. Code outside of the
module can refer to the enc::encrypt and enc::decrypt identifiers
just fine, but it does not have access to enc::super_secret_number.
Rust uses three different namespaces: one for modules, one for types, and one for values. This means that this code is valid:
mod buffalo {
type buffalo = int;
fn buffalo<buffalo>(+buffalo: buffalo) -> buffalo { buffalo }
}
fn main() {
let buffalo: buffalo::buffalo = 1;
buffalo::buffalo::<buffalo::buffalo>(buffalo::buffalo(buffalo));
}
You don't want to write things like that, but it is very practical
to not have to worry about name clashes between types, values, and
modules. This allows us to have a module core::str, for example, even
though str is a built-in type name.
The resolution process in Rust simply goes up the chain of contexts, looking for the name in each context. Nested functions and modules create new contexts inside their parent function or module. A file that's part of a bigger crate will have that crate's context as its parent context.
Identifiers can shadow each other. In this program, x is of type
int:
type t = ~str;
fn main() {
type t = int;
let x: t;
}
An use directive will only import into the namespaces for which
identifiers are actually found. Consider this example:
type bar = uint;
mod foo { fn bar() {} }
mod baz {
use foo::bar;
const x: bar = 20u;
}
When resolving the type name bar in the const definition, the
resolver will first look at the module context for baz. This has an
import named bar, but that's a function, not a type, So it continues
to the top level and finds a type named bar defined there.
Normally, multiple definitions of the same identifier in a scope are
disallowed. Local variables defined with let are an exception to
this—multiple let directives can redefine the same variable in a
single scope. When resolving the name of such a variable, the most
recent definition is used.
fn main() {
let x = 10;
let x = x + 10;
assert x == 20;
}
This makes it possible to rebind a variable without actually mutating it, which is mostly useful for destructuring (which can rebind, but not assign).
Rust supports a system of lightweight tasks, similar to what is found in Erlang or other actor systems. Rust tasks communicate via messages and do not share data. However, it is possible to send data without copying it by making use of the exchange heap, which allow the sending task to release ownership of a value, so that the receiving task can keep on using it.
Note: As Rust evolves, we expect the task API to grow and change somewhat. The tutorial documents the API as it exists today.
Spawning a task is done using the various spawn functions in the
module task. Let's begin with the simplest one, task::spawn():
use task::spawn;
use io::println;
let some_value = 22;
do spawn {
println(~"This executes in the child task.");
println(fmt!("%d", some_value));
}
The argument to task::spawn() is a unique
closure of type fn~(), meaning that it takes no
arguments and generates no return value. The effect of task::spawn()
is to fire up a child task that will execute the closure in parallel
with the creator.
Now that we have spawned a child task, it would be nice if we could
communicate with it. This is done using pipes. Pipes are simply a
pair of endpoints, with one for sending messages and another for
receiving messages. The easiest way to create a pipe is to use
pipes::stream. Imagine we wish to perform two expensive
computations in parallel. We might write something like:
use task::spawn;
use pipes::{stream, Port, Chan};
let (chan, port) = stream();
do spawn {
let result = some_expensive_computation();
chan.send(result);
}
some_other_expensive_computation();
let result = port.recv();
# fn some_expensive_computation() -> int { 42 }
# fn some_other_expensive_computation() {}
Let's walk through this code line-by-line. The first line creates a stream for sending and receiving integers:
# use pipes::stream;
let (chan, port) = stream();
This port is where we will receive the message from the child task once it is complete. The channel will be used by the child to send a message to the port. The next statement actually spawns the child:
# use task::{spawn};
# use comm::{Port, Chan};
# fn some_expensive_computation() -> int { 42 }
# let port = Port();
# let chan = port.chan();
do spawn {
let result = some_expensive_computation();
chan.send(result);
}
This child will perform the expensive computation send the result
over the channel. (Under the hood, chan was captured by the
closure that forms the body of the child task. This capture is
allowed because channels are sendable.)
Finally, the parent continues by performing some other expensive computation and then waiting for the child's result to arrive on the port:
# use pipes::{stream, Port, Chan};
# fn some_other_expensive_computation() {}
# let (chan, port) = stream::<int>();
# chan.send(0);
some_other_expensive_computation();
let result = port.recv();
A very common thing to do is to spawn a child task where the parent
and child both need to exchange messages with each other. The
function std::comm::DuplexStream() supports this pattern. We'll
look briefly at how it is used.
To see how spawn_conversation() works, we will create a child task
that receives uint messages, converts them to a string, and sends
the string in response. The child terminates when 0 is received.
Here is the function that implements the child task:
# use std::comm::DuplexStream;
# use pipes::{Port, Chan};
fn stringifier(channel: DuplexStream<~str, uint>) {
let mut value: uint;
loop {
value = channel.recv();
channel.send(uint::to_str(value, 10u));
if value == 0u { break; }
}
}
The implementation of DuplexStream supports both sending and
receiving. The stringifier function takes a DuplexStream that can
send strings (the first type parameter) and receive uint messages
(the second type parameter). The body itself simply loops, reading
from the channel and then sending its response back. The actual
response itself is simply the strified version of the received value,
uint::to_str(value).
Here is the code for the parent task:
# use std::comm::DuplexStream;
# use pipes::{Port, Chan};
# use task::spawn;
# fn stringifier(channel: DuplexStream<~str, uint>) {
# let mut value: uint;
# loop {
# value = channel.recv();
# channel.send(uint::to_str(value, 10u));
# if value == 0u { break; }
# }
# }
# fn main() {
let (from_child, to_child) = DuplexStream();
do spawn || {
stringifier(to_child);
};
from_child.send(22u);
assert from_child.recv() == ~"22";
from_child.send(23u);
from_child.send(0u);
assert from_child.recv() == ~"23";
assert from_child.recv() == ~"0";
# }
The parent task first calls DuplexStream to create a pair of bidirectional endpoints. It then uses task::spawn to create the child task, which captures one end of the communication channel. As a result, both parent
and child can send and receive data to and from the other.
The Rust language has a facility for testing built into the language.
Tests can be interspersed with other code, and annotated with the
#[test] attribute.
# // FIXME: xfailed because test_twice is a #[test] function it's not
# // getting compiled
extern mod std;
fn twice(x: int) -> int { x + x }
#[test]
fn test_twice() {
let mut i = -100;
while i < 100 {
assert twice(i) == 2 * i;
i += 1;
}
}
When you compile the program normally, the test_twice function will
not be included. To compile and run such tests, compile with the
--test flag, and then run the result:
> rustc --test twice.rs
> ./twice
running 1 tests
test test_twice ... ok
result: ok. 1 passed; 0 failed; 0 ignored
Or, if we change the file to fail, for example by replacing x + x
with x + 1:
running 1 tests
test test_twice ... FAILED
failures:
test_twice
result: FAILED. 0 passed; 1 failed; 0 ignored
You can pass a command-line argument to a program compiled with
--test to run only the tests whose name matches the given string. If
we had, for example, test functions test_twice, test_once_1, and
test_once_2, running our program with ./twice test_once would run
the latter two, and running it with ./twice test_once_2 would run
only the last.
To indicate that a test is supposed to fail instead of pass, you can
give it a #[should_fail] attribute.
extern mod std;
fn divide(a: float, b: float) -> float {
if b == 0f { fail; }
a / b
}
#[test]
#[should_fail]
fn divide_by_zero() { divide(1f, 0f); }
# fn main() { }
To disable a test completely, add an #[ignore] attribute. Running a
test runner (the program compiled with --test) with an --ignored
command-line flag will cause it to also run the tests labelled as
ignored.
A program compiled as a test runner will have the configuration flag
test defined, so that you can add code that won't be included in a
normal compile with the #[cfg(test)] attribute (see conditional
compilation).