monad in 5 min

monad.hs

-- Import this for fromJust
import Data.Maybe

data Expr = Val Int | Div Expr Expr

-- Example expressions
one = Val 1
two = Div (Val 6) (Div (Val 3) (Val 1))
three = Div (Val 6) (Val 2)

-- Naive implementation of division using Expr, doesn't handle division by 0.
-- eval :: Expr -> Int
-- eval (Val n) = n
-- eval (Div x y) = eval x `div` eval y

-- Safe division
safediv :: Int -> Int -> Maybe Int
safediv n m = if m == 0 then
                Nothing
              else
                Just (n `div` m)

-- Verbose eval implementaion
-- eval :: Expr -> Maybe Int
-- eval (Val n) = Just n
-- eval (Div x y) = case eval x of
--                     Nothing -> Nothing
--                     Just n -> case eval y of
--                         Nothing -> Nothing
--                         Just m -> safediv n m

-- Little better with Maybe Monad
-- eval :: Expr -> Maybe Int
-- eval (Val n) = return n
-- eval (Div x y) = eval x >>= (\n ->
--                     eval y >>= (\m ->
--                         safediv n m))

-- Combine with do
eval :: Expr -> Maybe Int
eval (Val n) = return n
eval (Div x y) = do n <- eval x
                    m <- eval y
                    safediv n m

main = putStrLn $ show $ fromJust $ eval one