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  • Glasgow Haskell Compiler
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  • #17213

Closed
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Opened Sep 19, 2019 by Krzysztof Gogolewski@monoidalDeveloper

Rank-n-types without -XRankNTypes

Consider two modules:

{-# LANGUAGE RankNTypes #-}
module X where

foo :: (forall a. a -> a) -> Int
foo = foo
module Y where
import X

g = foo

Note that Y does not use RankNTypes.

Currently, g is accepted. However, the inferred type of g is (forall a. a -> a) -> Int, which cannot be written. We should not allow a value if writing its type requires an additional extension.

Reporting on behalf of Simon PJ.

Edited Sep 19, 2019 by Krzysztof Gogolewski
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Reference: ghc/ghc#17213