SYB

gistfile1.hs

{-# LANGUAGE DeriveDataTypeable #-}

module A where

import Data.Typeable (Typeable)
import Data.Generics (everywhere, mkT, Data)

data T = Tree [U] deriving (Show,Data,Typeable)

data U = I Int | T T deriving (Show,Data,Typeable)

plus1 :: U -> U
plus1 (I n) = I (n + 1)
plus1 x     = x

tree :: T
tree = Tree [I 1, T (Tree [I 2])]

tree2 :: T
tree2 = everywhere (mkT plus1) tree
{-
> tree2
Tree [I 2,T (Tree [I 3])]
-}