Example code for http://stackoverflow.com/questions/1865351/ambiguous-type-variable

AmbiguousTypeVariable.hs

{-# LANGUAGE DeriveDataTypeable #-}

import Data.Data
import Data.Generics.PlateData

data Child1 a = Child1 a Int
              deriving (Data, Show, Typeable)

data Child2 a = Child2 a String
              deriving (Data, Show, Typeable)

data Child3 a = Child3 a
                deriving (Data, Show, Typeable)

data Parent a = Leaf (Child1 a) (Child2 a) (Child3 a)
              | Tree [ Parent a ]
              deriving (Data, Show, Typeable)

modify :: (Parent Bool -> Parent Bool) -> String -> String
modify f s1 = show $ f tree
  where tree = Tree [ Leaf (Child1 False 42) (Child2 False s1) (Child3 False),
                    Tree [ Leaf (Child1 True 0) (Child2 True "x") (Child3 True) ] ]

doInt :: Child1 l -> Child1 l
doInt (Child1 l n) = Child1 l (n + 1)

doString :: Child2 l -> Child2 l
doString (Child2 l (_:s)) = Child2 l ('j' : s)

replace :: Data l => Parent l -> Parent l
replace = transformBi doInt
        . transformBi doString

main :: IO ()
main = putStrLn $ modify replace "hello"