Data commit

Task/Stack/Haskell/stack-1.hs

@@ -0,0 +1,18 @@
+type Stack a = [a]
+
+create :: Stack a
+create = []
+
+push :: a -> Stack a -> Stack a
+push = (:)
+
+pop :: Stack a -> (a, Stack a)
+pop []     = error "Stack empty"
+pop (x:xs) = (x,xs)
+
+empty :: Stack a -> Bool
+empty = null
+
+peek :: Stack a -> a
+peek []    = error "Stack empty"
+peek (x:_) = x

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")