Maybe and IO Monads

Code

-- Maybe: handle nullability
safeDiv :: Double -> Double -> Maybe Double
safeDiv _ 0 = Nothing
safeDiv x y = Just (x / y)

-- Chain Maybes with >>= (bind)
compute :: Double -> Double -> Double -> Maybe Double
compute a b c = do
  x <- safeDiv a b
  y <- safeDiv x c
  return (y + 1)

-- Equivalent without do:
-- compute a b c = safeDiv a b >>= \x -> safeDiv x c >>= \y -> Just (y + 1)

-- IO monad
main :: IO ()
main = do
  putStrLn "Enter two numbers:"
  a <- readLn
  b <- readLn
  case safeDiv a b of
    Just r  -> putStrLn $ "Result: " ++ show r
    Nothing -> putStrLn "Cannot divide by zero"

-- Either for error messages
safeDivEither :: Double -> Double -> Either String Double
safeDivEither _ 0 = Left "Division by zero"
safeDivEither x y = Right (x / y)

Explanation

Maybe represents nullable values (Just x | Nothing). >>= (bind) chains computations that may fail — short-circuits on Nothing. do notation is sugar for >>= chains. IO isolates side effects — you can't escape IO in pure code. Either adds error info (Left err | Right val). The type system ensures safety: you can't forget to handle Nothing.

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