Placeholder types

[dabrahams (Dave Abrahams)]

@xAlien95

This is a typo, right? It looks like a fusion between Swift and Kotlin

The Rorschach language™ strikes again!

@xAlien95

Putting aside the _ placeholder, this is something already mentioned in the "medium priority list" of the Generics Manifesto. You can find it under the Parameterized extensions subsection:

Thanks for pointing that out, that is indeed the same kind of constraint. Considering that it's framed as being just for extensions I'm not sure I we can say it's mentioned there, but it's definitely related.

@xAlien95

Your syntax would be probably be spelled in the following way:

struct<Key: Comparable> Wrapper<T> where T == Dictionary<Key, _> { ... }

with Key being available exclusively in that declaration scope.

IMO that syntax runs against the spirit of changes we made long ago, moving parts of a generic declaration that don't describe structure to the back. For example, IIRC, it used to be possible to write

struct X<A, B: P where B.C == A, D>

and where clauses on generic functions used to appear before the return type.

In the example you gave, Key is not a part of the structure of the type being declared, and putting it at the head of the declaration gives it too much weight.

@Alejandro is tackling parameterized extensions (here are some insights with his current implementation), so if this proposal gets accepted and a proposal about parameterized extensions comes out, your suggestion would be more appropriate to be mentioned in the future directions section of that proposal, I think.

Good point that these things should be tied together. IMO it would be a mistake to tackle these things as though they were separate features unless there was some real implementation difficulty, though. We have enough contextual inconsistencies already regarding what can be expressed in the generics system.