[Haskell-cafe] Counterintuitive ScopedTypeVariables behaviour

Richard Eisenberg rae at richarde.dev
Tue Sep 8 18:16:28 UTC 2020

> On Sep 7, 2020, at 8:04 AM, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> On the
> other hand I'm still somewhat concerned that the meaning of the syntax
> can vary depending on whether the type variable is in scope or not.

Yes. This has been a spot of discomfort for some time (at least for me, and I know I'm not completely alone).

A recent accepted, unimplemented proposal (https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0285-no-implicit-binds.rst) describes -XNoPatternSignatureBinds, which disallows not-in-scope variables from appearing in pattern signatures. Instead, the variable must come into scope some other way. That "other way" depends on proposals not yet completely formed/accepted, but once the dust has settled, I think you'll get what you want with -XNoPatternSignatureBinds.

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