这是我试图弄清楚的 State Monad 代码
data State a = State (Int -> (a, Int))
instance Monad State where
return x = State (\c -> (x, c))
State m >>= f = State (\c ->
case m c of { (a, acount) ->
case f a of State b -> b acount})
getState = State (\c -> (c, c))
putState count = State (\_ -> ((), count))
instance Show State where -- doesn't work
show (State a) = show a -- doesn't work
我正在尝试将 State 作为 Show 的实例，以便我可以看到getState and putState count在 ghci 提示符下。
任何教程或 State Monad 材料的链接也很好。
这是一个Show可以帮助了解发生了什么情况的实例：
instance Show a => Show (State a) where
show (State f) = show [show i ++ " => " ++ show (f i) | i <- [0..3]]
然后你可以这样做：
*Main> getState
["0 => (0,0)","1 => (1,1)","2 => (2,2)","3 => (3,3)"]
*Main> putState 1
["0 => ((),1)","1 => ((),1)","2 => ((),1)","3 => ((),1)"]
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