a.hs

f :: a -> String
f a = "Cool"

g :: (a -> String) -> String
g h = h 1

main = do
  putStrLn (f 1)
  putStrLn (g f)

b.sh

runhaskell a.hs 

a.hs:5:9:
    No instance for (Num a) arising from the literal ‘1’
    Possible fix:
      add (Num a) to the context of
        the type signature for g :: (a -> String) -> String
    In the first argument of ‘h’, namely ‘1’
    In the expression: h 1
    In an equation for ‘g’: g h = h 1