[Pitch] Enable pack iteration

[Karl]

@Slava_Pestov

You can use the same trick to “lift” the local type to a named type parameter that works with existentials; define a generic function (local or top-level) that takes a ‘T’ with the requirements you want (which must be satisfied by the requirements of the existential/pack element); then you call the function with this value, and now inside the function you refer to ‘T’.

Aye, but it can be annoying to have to do that. For instance, within the loop body, I may want to write things like:

let value: ElementType

if thing { 
  value = ...
} else {
  value = ...
}

// or:

var values = [ElementType]()

There are tricks, but it doesn't feel as harmonious when you're forced to adapt your regular coding style to gaps in the language. This kind of code works in other generic contexts, but in these contexts you need to use a more awkward formulation due to not being able to give the element type a name.

The inability to introduce local generic types has been a hole in the language for a long time, and it significantly affects the ergonomics of both variadic generics and existentials. That is why, a long time ago, I suggested that this feature would actually be a good first step towards introducing variadic generics, and I mention it at every opportunity since then :

@Karl

But I do think there is significant overlap between existentials (and especially collections of existentials) and variadic generics, and the language is currently not well-equipped to deal with either of them comfortably because we don't have a concept of introducing new generic type parameters in the middle of a function. That's why, in the previous thread, I suggested that adding this would be a good step towards variadic generics

So as nice as this feature is (and it is very nice), I kind of do need to express some slight disappointment that we're not truly tackling local generic types while we do it. Instead, we're carrying on the trend of making generic type parameters unutterable - and in doing so, needlessly restricting the kinds of patterns developers are able to express.

I'm fully okay with that being a future direction, but I think we should acknowledge that pack iteration is not "finished" until we can name the generic types bound on each iteration of the loop.