Re: [ghc-steering-committee] Modern Scoped Type Variables #448: recommendation (mostly) accept

Of course, I think we all understand that all these extensions are opt-in, and the behaviour of GHC would not change, for existing modules, would not change if this proposal was accepted.

I think we should express an opinion about the intended direction. Are we advising that ExtendedForallScope is a dead end, because we want TypeAbstraction?

One way to think of it is this: if Haskell had type-abstraction from Day 1, would we ever have introduced ExtendedForAllScope? I'm sure we would not. Even at the time we introduced it, I remember we were concerned that it was a very strange (non-nested) scoping construct. We bind term variables in patterns on the LHS; it makes sense to do the same for type variables. So yes, in that sense the story is that we recommend TypeAbstraction for introducing a scoped type variable. Thus, instead of ``` id :: forall a. a -> a id x = (x :: a) ``` we would have ``` id :: forall a. a -> a id @b x = (x::b) -- I have used a different name only for illustrative purposes; could also be 'a'. ``` We have to repeat that type-variable pattern in each equation for the function. But we also have to repeat the term variables, and we just take that for granted. But as a "recommendation", it's a pretty weak one. You are still free to use either ExtendedForAllScope or TypeAbstraction or both at once in a particular module (subject to point 5 of 5.2). So it's a bit like let-vs-where, or H98 data decls vs GADTs. Do we even need a firm "recommendation"? Simon On Fri, 1 Apr 2022 at 08:27, Simon Marlow wrote:

In the spirit of not accepting proposals that lead to language forks, it would be great to get some clarification on the concerns that Arnaud raises here:

On Tue, 29 Mar 2022 at 16:02, Spiwack, Arnaud wrote:

...

With the caveat that this proposal introduces quite a few extensions. And at this point, I'm still not quite sure what Richard recommends is the set of extensions that I should use (and I'm slightly dismayed that I believe that it will be a set of cardinal more than 1). I think this reflects a vision of extensions as switches to customise the behaviour of GHC. This vision, as I've stated before, is very alien to me: I see extensions as staging areas for features to become an integral part of Haskell. So I don't know what to think of all these extensions. I'm definitely not against splitting -XScopedTypeVariables into smaller components, if it is done so that they are reassembled in a different way in an alternative extension that would now be the recommended default (or at least is to become the next recommended default).

I think we should express an opinion about the intended direction. Are we advising that ExtendedForallScope is a dead end, because we want TypeAbstraction?

Cheers Simon

...

Finally, there are Sections 6 to 8. These are entirely new. Though they are working towards the new principles (well, as far as I can tell, Section 6 doesn't contribute to the principles, but it is a stepping stone for both Sections 7 and 8). These sections are concerned with adding local let-bindings of type variables, in particular inside types and patterns.

By the way, Section 7 proposes two syntaxes for let binders in patterns, and I *strongly* prefer the second syntax, which reads something like `f (let b = Bool) (True :: Bool) = …`.

Anyway, these are new, I feel that they are a bit out of place in a proposal that is about tidying up the existing designs. That being said, they are here, and they seem like fairly uncontroversial to me, (except, probably the syntax `(let b = _)` to bind a variable to a type to be filled by the compiler). I'm fine with accepting these, though they may require a bit more scrutiny than the rest.

Best, Arnaud

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