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State monad implementation + example
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import Control.Monad | |
------------------------------------------------------------------------------- | |
-- State Monad Implementation | |
------------------------------------------------------------------------------- | |
newtype State s a = State { runState :: s -> (a,s) } | |
instance Monad (State s) where | |
return a = State $ \s -> (a, s) | |
State act >>= k = State $ \s -> | |
let (a, s') = act s | |
in runState (k a) s' | |
get :: State s s | |
get = State $ \s -> (s, s) | |
put :: s -> State s () | |
put s = State $ \_ -> ((), s) | |
modify :: (s -> s) -> State s () | |
modify f = get >>= \x -> put (f x) | |
evalState :: State s a -> s -> a | |
evalState act = fst . runState act | |
execState :: State s a -> s -> s | |
execState act = snd . runState act | |
------------------------------------------------------------------------------- | |
-- Example | |
------------------------------------------------------------------------------- | |
type Stack = [Int] | |
empty :: Stack | |
empty = [] | |
pop :: State Stack Int | |
pop = State $ \(x:xs) -> (x,xs) | |
push :: Int -> State Stack () | |
push a = State $ \xs -> ((),a:xs) | |
tos :: State Stack Int | |
tos = State $ \(x:xs) -> (x,x:xs) | |
stackManip :: State Stack Int | |
stackManip = do | |
push 10 | |
push 20 | |
a <- pop | |
b <- pop | |
push (a+b) | |
tos | |
main :: IO () | |
main = do | |
let res = evalState stackManip empty | |
print res |
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