gistfile1.hs

module Main where
data Exp a = Const a
           | Plus (Exp a) (Exp a)
           
type ExpAlgebra a r = (a -> r
                      ,r -> r -> r)

foldExp :: ExpAlgebra a r -> Exp a -> r
foldExp (c, p) = f
  where f (Const a)    = c a
        f (Plus  l r)  = p (f l) (f r)
        
test = Const 4 `Plus` Const 8

calc  = foldExp (id, (+))

pprint :: Show a => Exp a -> String
pprint = foldExp (show,\ l r -> "(" ++ l ++ " + " ++ r ++ ")")