Haskell RankNType example

NoRankN.hs

module RankN where

data Foo = Foo Int | Bar Double

mangle :: Num a => (a -> a) -> Foo -> Foo
mangle f (Foo i) = Foo (f i)
mangle f (Bar d) = Bar (f d)

{-
NoRankN.hs:6:25:
    Could not deduce (a ~ Int)
    from the context (Num a)
      bound by the type signature for
                 mangle :: Num a => (a -> a) -> Foo -> Foo
      at NoRankN.hs:5:11-41
      ‘a’ is a rigid type variable bound by
          the type signature for mangle :: Num a => (a -> a) -> Foo -> Foo
          at NoRankN.hs:5:11
    Relevant bindings include
      f :: a -> a (bound at NoRankN.hs:6:8)
      mangle :: (a -> a) -> Foo -> Foo (bound at NoRankN.hs:6:1)
    In the first argument of ‘Foo’, namely ‘(f i)’
    In the expression: Foo (f i)

-}

RankN.hs

{-# LANGUAGE RankNTypes #-}
module RankN where

data Foo = Foo Int | Bar Double

mangle :: (forall a . Num a => a -> a) -> Foo -> Foo
mangle f (Foo i) = Foo (f i)
mangle f (Bar d) = Bar (f d)