Polymorphic implicit parameters

Roman Cheplyaka roma at ro-che.info
Wed Mar 20 09:58:04 CET 2013

I'm curious as to why GHC doesn't accept the following (contrived)

  {-# LANGUAGE ImplicitParams, RankNTypes #-}

  f :: (?g :: (forall a . a -> a)) => a -> a
  f = ?g

The error message is:

  Illegal polymorphic or qualified type: forall a. a -> a
  In the type signature for `f':
    f :: ?g :: (forall a. a -> a) => a -> a

It's not a big deal since one can wrap the polymorphism in a newtype,
but it's somewhat unexpected.


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