[英]Haskell Change state in IO function
我正在嘗試更新 IO 函數中的記錄。 我曾嘗試使用 state-monad,但沒有成功。 我在某處看到一條評論,說可以用 state-monad 解決類似的問題。 是否可以使用 State-monad 來執行此操作,還是必須使用 monad 轉換器? 如果是這樣,我將不勝感激關於如何做到這一點的簡短解釋。
{-# LANGUAGE GADTs #-}
data CarState = CS {color :: String }
initState :: CarState
initState = CS {color = "white"}
data Car where
ChangeColor :: String -> Car
func :: Car -> CarState -> IO ()
func (ChangeColor col) cs = do
putStrLn ("car is " ++ show(color cs))
-- Change state here somehow
-- colorChange "green"
putStrLn ("car is now" ++ show(color cs))
main :: IO ()
main = func (ChangeColor "green") initState
colorChange :: String -> MyState CarState String
colorChange col = do
cs <- get
put $ cs {color = col}
return col
data MyState s a = St (s -> (a,s))
runSt :: MyState s a -> s -> (a,s)
runSt (St f) s = f s
evalSt :: MyState s a -> s -> a
evalSt act = fst . runSt act
get :: MyState s s
get = St $ \s -> (s,s)
put :: s -> MyState s ()
put s = St $ \_ -> ((),s)
instance Monad (MyState s) where
return x = St $ \s -> (x,s)
(St m) >>= k = St $ \s_1 -> let (a, s_2) = m s_1
St k_m = k a
in k_m s_2
instance Functor (MyState s) where
fmap f (St g) = St $ \s0 -> let (a, s1) = g s0
in (f a, s1)
instance Applicative (MyState s) where
pure a = St (\s -> (a,s))
(<*>) (St sa) (St sb) = St (\s0 -> let (fn, s1) = sa s0
(a, s2) = sb s1
in (fn a, s2))
在 Haskell 中,您永遠無法通過任何機制就地改變數據。 在所有。 1
在 Haskell 中對狀態更新進行建模的方式是生成一個新值,該值與舊值相同,除了“更新”位。
例如,您嘗試執行的操作將表示為:
func :: Car -> CarState -> CarState
func (ChangeColor c) cs = cs { color = c }
1
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