Monadic RPN calculator. Monadic composition.

category-theory/monadic-composition.hs

@@ -0,0 +1,17 @@
+import Control.Monad
+
+f = (+1) . (*100)  
+a1 = f 4  
+
+g = (\x -> return (x+1)) <=< (\x -> return (x*100))  
+a2 = Just 4 >>= g
+
+f2 = foldr (.) id [(+1),(*100),(+1)]  
+b1 = f2 1
+
+-- composition for the knight's quest
+inMany :: Int -> KnightPos -> [KnightPos]  
+inMany x start = return start >>= foldr (<=<) return (replicate x moveKnight)
+
+canReachIn :: Int -> KnightPos -> KnightPos -> Bool  
+canReachIn x start end = end `elem` inMany x start

category-theory/rpn-monadic.hs

@@ -0,0 +1,31 @@
+import Control.Monad
+import Data.List  
+
+readMaybe :: (Read a) => String -> Maybe a  
+readMaybe st = case reads st of [(x,"")] -> Just x  
+                                _ -> Nothing
+
+a1 = readMaybe "1" :: Maybe Int  
+a2 = readMaybe "GO TO HELL" :: Maybe Int
+
+foldingFunction :: [Double] -> String -> Maybe [Double]  
+foldingFunction (x:y:ys) "*" = return ((x * y):ys)  
+foldingFunction (x:y:ys) "+" = return ((x + y):ys)  
+foldingFunction (x:y:ys) "-" = return ((y - x):ys)  
+foldingFunction xs numberString = liftM (:xs) (readMaybe numberString)
+
+b1 = foldingFunction [3,2] "*"  
+b2 = foldingFunction [3,2] "-"  
+b3 = foldingFunction [] "*"  
+b4 = foldingFunction [] "1"  
+b5 = foldingFunction [] "1 wawawawa"
+
+solveRPN :: String -> Maybe Double  
+solveRPN st = do  
+    [result] <- foldM foldingFunction [] (words st)  
+    return result
+
+c1 = solveRPN "1 2 * 4 +"  
+c2 = solveRPN "1 2 * 4 + 5 *"  
+c3 = solveRPN "1 2 * 4"  
+c4 = solveRPN "1 8 wharglbllargh"