[rust-dev] Proposal to replace the `as` keyword with syntax

Patrick Walton pwalton at mozilla.com
Sun Mar 24 01:09:57 PDT 2013

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() {

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.


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