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
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