CodeDmitry · January 28, 2017 16:28 · Jan 13, 2015
diff --git a/state.hs b/state.hs
@@ -0,0 +1,61 @@
+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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