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f7d2d587
编写于
4月 19, 2014
作者:
M
mdinger
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src/doc/tutorial.md
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@@ -1723,59 +1723,72 @@ let x = 3;
fn fun() -> () { println!("{}", x); }
~~~~
Rust also supports _closures_, functions that can access variables in
the enclosing scope. Compare
`x`
in these:
A _closure_ does support accessing the enclosing scope; below we will create
2 _closures_ (nameless functions). Compare how
`||`
replaces
`()`
and how
they try to access
`x`
:
~~~~
~~~~
{.ignore}
let x = 3;
// `fun` is an invalid definition
fn fun () -> () { println!("{}", x) }; // cannot
reach
enclosing scope
let closure = || -> () { println!("{}", x) }; // can
reach
enclosing scope
fn fun () -> () { println!("{}", x) }; // cannot
capture
enclosing scope
let closure = || -> () { println!("{}", x) }; // can
capture
enclosing scope
fun(); // Still won't work
closure(); // Prints: 3
~~~~
// `fun_arg` is an invalid definition
fn fun_arg (arg: int) -> () { println!("{}", arg + x) }; // cannot capture enclosing scope
let closure_arg = |arg: int| -> () { println!("{}", arg + x) }; // Can capture enclosing scope
// ^
// Requires a type because the implementation needs to know which `+` to use.
// In the future, the implementation may not need the help.
Closures can be utilized in this fashion:
fun(); // Still won't work
closure(); // Prints: 3
fun_arg(7); // Still won't work
closure_arg(7); // Prints: 10
~~~~
// Create a nameless function and assign it to `closure`. It's sole
// argument is a yet unknown `x` to be supplied by the caller.
let closure = |x| -> () { println!("{}", x) };
// Define `call_closure_with_ten` to take one argument and return null `()`.
// `fun` is a function which takes one `int` argument `|int|` and also returns
// null `()`. `|int|` defines the `fun` to be of type _closure_
fn call_closure_with_ten(fun: |int| -> ()) -> () { fun(10); }
Closures begin with the argument list between vertical bars and are followed by
a single expression. Remember that a block,
`{ <expr1>; <expr2>; ... }`
, is
considered a single expression: it evaluates to the result of the last
expression it contains if that expression is not followed by a semicolon,
otherwise the block evaluates to
`()`
.
// The caller supplies `10` to the closure
// which prints out the value
call_closure_with_ten(closure);
~~~~
Since a closure is an expression, the compiler can usually infer the argument and
return types; so they are often omitted. This is in contrast to a function which
is a declaration and _not_ an expression. Declarations require the types to be
specified and carry no inference. Compare:
This can be simplified by removing null arguments:
~~~~
{.ignore}
// `fun` cannot infer the type of `x` so it must be provided because it is a function.
fn fun (x: int) -> () { println!("{}", x) };
let closure = |x | -> () { println!("{}", x) };
fun(10); // Prints 10
closure(20); // Prints 20
fun("String"); // Error: wrong type
// Error: This type is different from when `x` was originally evaluated
closure("String");
~~~~
let closure = |x| println!("{}", x);
fn call_closure_with_ten(fun: |int|) { fun(10); }
call_closure_with_ten(closure);
The null arguments
`()`
are typically dropped so the end result
is more compact.
~~~~
let closure = |x| { println!("{}", x) };
Closures begin with the argument list between vertical bars and are followed by
a single expression. Remember that a block,
`{ <expr1>; <expr2>; ... }`
, is
considered a single expression: it evaluates to the result of the last
expression it contains if that expression is not followed by a semicolon,
otherwise the block evaluates to
`()`
.
closure(20); // Prints 20
~~~~
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.
Here, in the rare case where the compiler needs assistance,
the arguments and return types may be annotated.
~~~~
let square = |x: int| -> uint { (x * x) as uint };
println!("{}", square(20)); // 400
println!("{}", square(-20)); // 400
~~~~
There are several forms of closure, each with its own role. The most
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