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b3d7aa39
编写于
4月 30, 2014
作者:
M
mdinger
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Merge pull request #1 from pnkfelix/clarify-omit-unit-vs-inference
Suggested revisions to PR 13676.
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f79571f5
636f7d23
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src/doc/tutorial.md
src/doc/tutorial.md
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src/doc/tutorial.md
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b3d7aa39
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@@ -1731,13 +1731,13 @@ they try to access `x`:
let x = 3;
// `fun` is an invalid definition
fn fun () -> () { println!("{}", x) } // cannot capture enclosing scope
let closure = || -> () { println!("{}", x) }; // can capture enclosing scope
fn fun () -> () { println!("{}", x) } // cannot capture
from
enclosing scope
let closure = || -> () { println!("{}", x) }; // can capture
from
enclosing scope
// `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
//
^
fn fun_arg (arg: int) -> () { println!("{}", arg + x) } // cannot capture
let closure_arg = |arg: int| -> () { println!("{}", arg + x) }; // can capture
// ^
// Requires a type because the implementation needs to know which `+` to use.
// In the future, the implementation may not need the help.
...
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@@ -1752,43 +1752,68 @@ 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
`()`
.
otherwise the block evaluates to
`()`
, the unit value
.
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:
In general, return types and all argument types must be specified
explicitly for function definitions. (As previously mentioned in the
[
Functions section
](
#functions
)
, omitting the return type from a
function declaration is synonymous with an explicit declaration of
return type unit,
`()`
.)
~~~~
{.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) };
fn fun (x: int) { println!("{}", x) } // this is same as saying `-> ()`
fn square(x: int) -> uint { (x * x) as uint } // other return types are explicit
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");
// Error: mismatched types: expected `()` but found `uint`
fn badfun(x: int) { (x * x) as uint }
~~~~
The null arguments
`()`
are typically dropped so the end result
is more compact.
On the other hand, the compiler can usually infer both the argument
and return types for a closure expression; therefore they are often
omitted, since both a human reader and the compiler can deduce the
types from the immediate context. This is in contrast to function
declarations, which require types to be specified and are not subject
to type inference. Compare:
~~~~
{.ignore}
// `fun` as a function declaration cannot infer the type of `x`, so it must be provided
fn fun (x: int) { println!("{}", x) }
let closure = |x | { println!("{}", x) }; // infers `x: int`, return type `()`
// For closures, omitting a return type is *not* synonymous with `-> ()`
let add_3 = |y | { 3i + y }; // infers `y: int`, return type `int`.
fun(10); // Prints 10
closure(20); // Prints 20
closure(add_3(30)); // Prints 33
fun("String"); // Error: mismatched types
// Error: mismatched types
// inference already assigned `closure` the type `|int| -> ()`
closure("String");
~~~~
let closure = |x| { println!("{}", x) };
closure(20); // Prints 20
In cases where the compiler needs assistance, the arguments and return
types may be annotated on closures, using the same notation as shown
earlier. In the example below, since different types provide an
implementation for the operator
`*`
, the argument type for the
`x`
parameter must be explicitly provided.
~~~~
{.ignore}
// Error: the type of `x` must be known to be used with `x * x`
let square = |x | -> uint { (x * x) as uint };
~~~~
Here, in the rare case where the compiler needs assistance
,
the arguments and return types may be annotat
ed.
In the corrected version, the argument type is explicitly annotated
,
while the return type can still be inferr
ed.
~~~~
let square = |x: int| -> uint { (x * x) as uint };
let square_explicit = |x: int| -> uint { (x * x) as uint };
let square_infer = |x: int| { (x * x) as uint };
println!("{}", square(20)); // 400
println!("{}", square
(-20));
// 400
println!("{}", square
_explicit
(20)); // 400
println!("{}", square
_infer(-20));
// 400
~~~~
There are several forms of closure, each with its own role. The most
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