Map methods on constant sized arrays return dynamically sized arrays.

[mstewartgallus]

Consider the following code samples:
(1)

    let mut positions = [Zero::zero (), ..ObjectAmount];
    for uint::range (0, positions.len ()) |ii| {
         positions[ii] = do objects[ii].position
    }

(2)

   let positions = do objects.map |object| { object.position }

Code example number (2) looks much prettier than code example (1) but is incorrect, as it returns a dynamically sized array. The code returns a dynamically sized array because the objects array is coerced to a dynamically sized array, and then the map method is called on the dynamic array. Can some method of making code sample number (2) happen be implemented? Right now, the behaviour is unintuitive, and annoying.

[catamorphism]

I'm not sure this is practical without some sort of dependent types, but in any case, nominating for milestone 1 (well-defined) so we can decide.

[mstewartgallus]

Maybe some kind of Array kind could be added?

For example,

pub trait <T> MapUtil: Array<T> {
    fn map<S, U: Array<T>>(Self) -> U;
}

[thestinger]

The map method on vectors is marked as deprecated in vec.rs and will be replaced by the iterator form of map.

There's currently no way to implement methods on fixed-size arrays, so the issue here is #7622.