Re:Coroutines are now in Clang trunk, libc++
by int_19h on Friday May 26, 2017 @02:01PM
(#14425986)
The extra syntax in this case is all about static typing (and in the case of Rust, exacerbated by having lifetimes as part of the type).
> But Rust's type system has no good way to implicitly figure out the full type of a lambda by its signature, so if you want to pass one around you need to specify its whole type signature twice, once in the expression itself and then again in the signature of the function that will receive or return it.
Rust has no problem figuring out the type of the lambda. But the type of the lambda is anonymous, and unique to that particular lambda. That type is guaranteed to implement the Fn trait, so that's what you specify when you need to return it. You don't need to specify it twice, however - I'm not sure how that follows from the snippet above?
And C++ has the same exact problem: C++ lambdas also each have their own unique type. However, C++ doesn't have traits at all. In C++, if you want to return a lambda, you'd use std::function, which is roughly analogous to Rust's Box<> for functions in this case - you can't just return the lambda directly (well, you can, but only in a context where the return type is implicit and can be inferred from usage - e.g. when returning from another lambda).
Re:Coroutines are now in Clang trunk, libc++
by comex on Friday May 26, 2017 @06:10PM
(#14427564)
In C++14 and later you can return a lambda by using auto as a return type, which compared to impl Trait is shorter but less strongly typed (so more prone to confusing errors).
> But Rust's type system has no good way to implicitly figure out the full type of a lambda by its signature, so if you want to pass one around you need to specify its whole type signature twice, once in the expression itself and then again in the signature of the function that will receive or return it.
Rust has no problem figuring out the type of the lambda. But the type of the lambda is anonymous, and unique to that particular lambda. That type is guaranteed to implement the Fn trait, so that's what you specify when you need to return it. You don't need to specify it twice, however - I'm not sure how that follows from the snippet above?
And C++ has the same exact problem: C++ lambdas also each have their own unique type. However, C++ doesn't have traits at all. In C++, if you want to return a lambda, you'd use std::function, which is roughly analogous to Rust's Box<> for functions in this case - you can't just return the lambda directly (well, you can, but only in a context where the return type is implicit and can be inferred from usage - e.g. when returning from another lambda).