How to reuse a type variable in an inner type declaration

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I think you can do this in general with ScopedTypeVariables which GHC supports. This certainly compiles:

{-# LANGUAGE ScopedTypeVariables #-}
foo :: forall a. (a -> a) -> a -> a
foo f arg = bar arg
  where
    bar :: a -> a
    bar a = f a

Note the “forall a.”

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