Monads모나드

class Functor m => Monad m where
  return :: a -> m a            -- 순수값을 모나드로 감쌈
  (>>=) :: m a -> (a -> m b) -> m b  -- bind (순차 합성)

-- 모나드 법칙:
-- 왼쪽 항등: return a >>= f  ≡  f a
-- 오른쪽 항등: m >>= return  ≡  m
-- 결합법칙: (m >>= f) >>= g  ≡  m >>= (\x -> f x >>= g)

-- 실패 가능한 계산 체이닝
safeDiv :: Int -> Int -> Maybe Int
safeDiv _ 0 = Nothing
safeDiv a b = Just (a `div` b)

safeLog :: Int -> Maybe Double
safeLog n
  | n <= 0    = Nothing
  | otherwise = Just (log (fromIntegral n))

-- do 표기법으로 체이닝
compute :: Int -> Int -> Maybe Double
compute a b = do
  x <- safeDiv a b    -- Nothing이면 전체 Nothing
  y <- safeLog x
  return y

-- 명시적 bind
compute a b =
  safeDiv a b >>= \x ->
  safeLog x   >>= \y ->
  return y

main :: IO ()
main = do
  putStrLn "이름을 입력하세요:"
  name <- getLine
  putStrLn ("안녕하세요, " ++ name ++ "!")

-- IO는 실제로 부수 효과가 아닌
-- "실행할 액션의 설명"을 값으로 다룸

import Control.Monad.State

type Stack = [Int]

pop :: State Stack Int
pop = do
  (x:xs) <- get
  put xs
  return x

push :: Int -> State Stack ()
push x = modify (x:)

stackOps :: State Stack Int
stackOps = do
  push 3
  push 5
  a <- pop
  b <- pop
  return (a + b)

runState stackOps []  -- (8, [])