I've been reading more closely into the impl/iface code and I think I've
found another problem. The current `self` type is actually more than a
type---rather it's a kind of type constructor. I think we should change
it to be just a type, because we do not have other type constructors in
our system and in general things do not hold together. Let me explain
briefly what I mean.
Currently, the type `self` refers to "the type implemented the current
iface". However, the self type is parameterized just as the containing
iface. So, technically, you could have an iface like:
iface foo<T> {
fn bar<S>(x: self<S>);
}
Now this would correspond to an impl like:
impl of foo<T> for [T] {
fn bar<S>(x: [S]) { ... }
}
Here, the type `self<S>` was transformed to `[S]`. This is a fairly
complex transformation, actually, as must "reverse-link" the parameter T
from its use in `[T]` to the appearance in `foo<T>` and so forth. In
fact, this transformation is not especially well-defined: there may be
multiple types or no types which are equally valid.
To see why there could be multiple types, consider something like
`impl<X> of foo<T> for either<T,X>`. What type does `self<S>` have? It
would be valid for any `either<S,?>`.
To see why there might be no types, consider something like `impl of
foo<uint> for uint`. What type does `self<S>` have? Or, similarly,
consider:
fn some_func<T: foo<uint>>(t: T) {
t.bar(...);
}
What is the type of the parameter expected by `bar`?
In other languages, there is support for parameterizing a function or
type by a "type constructor" (which is essentially what `self` is). I
don't think we need those capabilities, though of course we could add
them later.
I would rather see the `self` type be like any other type parameter:
non-parameterized. In that case, the iface shown above would be
invalid. This does naturally mean that our type system is less
expressive. However, its behavior is well-defined in all cases, which
seems like a nice property to have.
Niko
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