Actually this makes more sense, due to the following analogy:
// given this:
let x = ~"foo";
let v;
// ensures that `v` is of type `&str`
v: &str = s;
// ensures that `v` is of type `&str` by declaring it
let w: &str = s;
Patrick Walton <[email protected]> wrote:
>
> On 3/23/13 10:52 PM, Chris Peterson wrote:
>> To reduce the number of rust keywords, I offer the lowly colon as
>> alternative syntax for the `as` keyword.
>
> I'd like to use the colon for type ascription (i.e. an *assertion* that
> a value has some type), which is different from a cast. This would be
> useful to help along the typechecker in cases in which it doesn't have
> enough information to determine the result of an expression. For example:
>
> trait Number {
> fn make_number() -> Self;
> fn incr(self) -> Self;
> }
>
> impl Number for int {
> fn make_number() -> int { 1 }
> fn incr(self) -> int { self + 1 }
> }
>
> impl Number for uint {
> fn make_number() -> uint { 2 }
> fn incr(self) -> uint { self + 1 }
> }
>
> fn main() {
> println(Number::make_number().incr().to_str());
> }
>
> This results in an error because the typechecker can't tell whether 2 or
> 3 should be printed. But type ascription would solve it:
>
> fn main() {
> println(Number::make_number():int.incr().to_str()); // 2
> }
>
> However, a cast to int with `as` would convert both uint and int to int
> and therefore it wouldn't help the typechecker enough.
>
> Patrick
--
-- Kang Seonghoon | Software Engineer, iPlateia Inc. | http://mearie.org/
-- Opinions expressed in this email do not necessarily represent the
views of my employer.
--
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