[Haskell-cafe] ghc-9 needs type declaration where ghc-8 didn't (with RankNTypes)

[Doug Burke]

The 9.0 upgrade may help:
https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0

On Thu, Oct 1, 2020 at 12:20 PM Johannes Waldmann <
johannes.waldmann at htwk-leipzig.de> wrote:

> Dear Cafe,
>
> and thanks, Georgi, for answering my previous question
>
> here's another difference I noticed
> (based on real code, but reduced to a minimal test case)
>
> GHCi, version 9.0.0.20200925: https://www.haskell.org/ghc/  :? for help
> Prelude> :set -XRankNTypes
> Prelude> let f :: (forall a . a -> ()) -> () ; f = g ; g x = undefined
>
> <interactive>:2:43: error:
>     • Couldn't match type ‘p0’ with ‘forall a. a -> ()’
>       Expected: (forall a. a -> ()) -> ()
>         Actual: p0 -> ()
>       Cannot instantiate unification variable ‘p0’
>       with a type involving polytypes: forall a. a -> ()
>     • In the expression: g
>       In an equation for ‘f’: f = g
>
> The declaration is accepted by ghc-8.
> For ghc-9, I have to give a type signature for g (copy that of f),
> or replace `f = g` with  `f x = g x`.
>
> - J.W.
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