Staging without quotation

gistfile1.hs

import Control.Applicative

-- NOTE: in a lazy language we only need 'data Expr a = Expr a'
data Expr a = Expr (() -> a)

instance Functor Expr where
    fmap f e = Expr $ λ() -> f (eval e)

instance Applicative Expr where
    pure x  = Expr $ λ() -> x
    f <*> x = Expr $ λ() -> (eval f) (eval x)

eval :: Expr a -> a
eval (Expr e) = e ()

main = do
    putStrLn $ show $ eval $ (+) <$> pure 3 <*> pure 4
    putStrLn $ show $ eval $ pure (λx -> x + x) <*> pure 3