[ghc-steering-committee] GHC2021: a final plea for ScopedTypeVariables

Alejandro Serrano Mena trupill at gmail.com
Wed Dec 9 13:43:44 UTC 2020

On 9 Dec 2020 at 14:40:13, Joachim Breitner <mail at joachim-breitner.de>

> Hi,
> Am Mittwoch, den 09.12.2020, 12:08 +0000 schrieb Simon Peyton Jones via
> ghc-steering-committee:
> I agree with this.  Not having scoped type variables (in any form) is
> a huge blemish.  The existing design may not be perfect, but it’s
> worked rather well for a long time.
> The same is true for ViewPatterns, which are even more harmless (clear
> syntactic opt-in; no spooky effect at a distance when type signature
> and definition are far apart).
> There we argued that the fact that we don’t think it’s perfect, and
> some wiki page with an alternative design exists, was enough to plea
> against it.
> Here, the situation seems equivalent (we know of better designs, namely
> binding type variables on the LHS of the equation).
> Can we justify why we go one way here and another way there? Is it
> because ViewPatterns are just mere convenience, and ScopedTypeVariables
> are absolutely needed for some things?

This is what I think at least (I also voted ‘yes’ for ViewPatterns, but I
wouldn’t mind not having them). No ViewPatterns, I just need one extra
‘case’ to stay in GHC2021.

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