stacks

Author: xtangle

haskell/playground/monads_more/stack.hs

@@ -0,0 +1,45 @@
+import           Control.Monad       (when)
+import           Control.Monad.State
+
+type Stack = [Int]
+
+pop :: State Stack Int
+pop = state $ \(x:xs) -> (x, xs)
+
+push :: Int -> State Stack ()
+push a = state $ \xs -> ((), a : xs)
+
+-- try:
+-- runState stackManip [5,8,2,1]
+stackManip :: State Stack Int
+stackManip = do
+    push 3
+    pop
+    pop
+
+-- try:
+-- runState stackStuff [9,0,2,1,0]
+stackStuff :: State Stack ()
+stackStuff = do
+    a <- pop
+    if a == 5
+        then push 5
+        else do
+            push 3
+            push 8
+
+-- try:
+-- runState moreStack [100,0,2,1,0]
+moreStack :: State Stack ()
+moreStack = do
+    a <- stackManip
+    when (a == 100) stackStuff
+
+-- try:
+-- runState stackyStack [1,2,3]
+stackyStack :: State Stack ()
+stackyStack = do
+    stackNow <- get
+    if stackNow == [1, 2, 3]
+        then put [8, 3, 1]
+        else put [9, 2, 1]