Data commit

Task/Stack/Haskell/stack-2.hs

@@ -0,0 +1,22 @@
+import Control.Monad.State
+
+type Stack a b = State [a] b
+
+push :: a -> Stack a ()
+push = modify . (:)
+
+pop :: Stack a a
+pop = do
+    nonEmpty
+    x <- peek
+    modify tail
+    return x
+
+empty :: Stack a Bool
+empty = gets null
+
+peek :: Stack a a
+peek = nonEmpty >> gets head
+
+nonEmpty :: Stack a ()
+nonEmpty = empty >>= flip when (fail "Stack empty")