This does what you want, I think:
{-# LANGUAGE ExistentialQuantification #-}
module Exist where
data Showable = forall a. (Show a) => Showable a
instance Show Showable where
showsPrec p (Showable a) = showsPrec p a
show (Showable a) = show a
-- You have to use the default implementation of showList
-- because a list could be heterogeneous
data T a = forall b. (Show b) => T b a
extShow :: T a -> Showable
extShow (T b _) = Showable b
-- ryan
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