Free for expressions

gistfile1.txt

import Control.Monad.Free

data ExprF a k = Lit a | Add k k

instance Functor (ExprF a) where
  fmap f (Lit x) = Lit x
  fmap f (Add x y) = Add (f x) (f y)

type Expr a = Free (ExprF a) ()

lit :: a -> Expr a
lit x = liftF $ Lit x

add :: Expr a -> Expr a -> Expr a
add x y = do
  x' <- x
  y' <- y
  liftF $ Add x' y'

test :: Expr Int
test = lit 1 `add` lit 2 `add` lit 3