Initial data commit

Task/History-variables/Haskell/history-variables.hs

@@ -0,0 +1,32 @@
+import Data.IORef
+
+newtype HVar a = HVar (IORef [a])
+
+newHVar :: a -> IO (HVar a)
+newHVar value = fmap HVar (newIORef [value])
+
+readHVar :: HVar a -> IO a
+readHVar (HVar ref) = fmap head (readIORef ref)
+
+writeHVar :: a -> HVar a -> IO ()
+writeHVar value (HVar ref) = modifyIORef ref (value:)
+
+undoHVar :: HVar a -> IO ()
+undoHVar (HVar ref) = do
+    (_ : history) <- readIORef ref
+    writeIORef ref history
+
+getHistory :: HVar a -> IO [a]
+getHistory (HVar ref) = readIORef ref
+
+-- Testing
+main :: IO ()
+main = do
+    var <- newHVar 0
+    writeHVar 1 var
+    writeHVar 2 var
+    writeHVar 3 var
+    getHistory var >>= print
+    undoHVar var
+    undoHVar var
+    undoHVar var