gistfile1.hs

module AST where

data E a
  = Var a
  | Add (E a) (E a)
  | Sub (E a) (E a)
  | Mul (E a) (E a)
  | Div (E a) (E a)
    deriving (Eq,Show)

instance Functor E where
  fmap f e = case e of
    Var a   -> Var (f a)
    Add a b -> Add (fmap f a) (fmap f b)
    Sub a b -> Sub (fmap f a) (fmap f b)
    Mul a b -> Mul (fmap f a) (fmap f b)
    Div a b -> Div (fmap f a) (fmap f b)

instance Monad E where
  return = Var
  Var a >>= f = f a
  Add a b >>= f = Add (a >>= f) (b >>= f)
  Sub a b >>= f = Sub (a >>= f) (b >>= f)
  Mul a b >>= f = Mul (a >>= f) (b >>= f)
  Div a b >>= f = Div (a >>= f) (b >>= f)


do { x <- Add (Var 3) (Var 1); y <- Mul (Var x) (Var 8); return y }
Add (Mul (Var 3) (Var 8)) (Mul (Var 1) (Var 8))