Instances.hs

module Instances where

import Data.Typeable

instance (Typeable a, Typeable b) => Show (a -> b) where
    showsPrec _ _ = showParen True $ showString "_ :: " . showsTypeRep (typeRep p)
        where p = Proxy :: Proxy (a -> b)

traceback

  test\Instances.hs: line 6, column 73:
    Error: * Could not deduce (Typeable a0) arising from a use of `typeRep'
  from the context: (Typeable a, Typeable b)
    bound by the instance declaration at test\Instances.hs:5:10-50
  The type variable `a0' is ambiguous
* In the first argument of `showsTypeRep', namely `(typeRep p)'
  In the second argument of `(.)', namely `showsTypeRep (typeRep p)'
  In the second argument of `($)', namely
    `showString "_ :: " . showsTypeRep (typeRep p)'