てすと

Test.hs

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE RankNTypes #-}
module Test where

data X = X String 
       deriving Show
                
type Y = Int

class Z a where
  zed :: a
instance Z X where
  zed = X "zed"
instance Z Y where
  zed = 0
  
data D = D { x :: X
           , y :: Y
           , z :: forall a. (Show a, Z a) => a
           }

{-
*Test> :load "/home/cutsea110/devel/haskell/metadata/Test.hs"
[1 of 1] Compiling Test             ( /home/cutsea110/devel/haskell/metadata/Test.hs, interpreted )

/home/cutsea110/devel/haskell/metadata/Test.hs:18:17:
    Can't make a derived instance of `Show D':
      Constructor `D' must have a Haskell-98 type
      Possible fix: use a standalone deriving declaration instead
    In the data type declaration for `D'
Failed, modules loaded: none.
Prelude> :t D (X "Hello") 123 zed
D (X "Hello") 123 zed :: D
-}