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

[Spiwack, Arnaud]

Dear all,

Let me bring to your attention the Modern Scoped Type Variables proposal,
by our own Richard https://github.com/ghc-proposals/ghc-proposals/pull/448 .

The proposal is a touch intimidating, because the text is large. But most
of it comes from other, already accepted, proposals that this proposal is
tidying up and tying together.

What this proposal tries to achieve is to make a consistent text about all
the recent changes to binding type variables.

The proposal adds new principles to the `principles.rst` files which inform
the changes proposed, of these, I have the following comments:
- The Visibility Orthogonality Principle doesn't seem to have a very clear
definition. It may be a sign that it's not something that is so important
- The Explicit Binding Principle says that we need to be able to enforce
that all type variables have a binding site. I think it's a bit strong: I
would like Haskell to let me write a binding site for every type variable,
but not necessarily to error out when that doesn't happen. That being said,
I'm happy with warnings to help me along the way, but I don't think that
this Explicit Binding Principle should be phrased in a way that requires
adding extensions to control this behaviour.
- The Contiguous Scoping Principle, which states that a binder binds in one
contiguous region sounds dubious to me. I don't see a particular reason for
this to be.

---

In the proposed changes themselves: up to Section 5, the proposed changes
are mostly existing accepted proposals, cleaned up with what was learnt
since they were written, as well as to adhere to the new principles.

These are mostly uncontroversial and the specification makes sense to me.
I'm in favour of accepting all that.

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).

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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