Add RPN.hs

RPN.hs

@@ -0,0 +1,48 @@
+module RPN (
+    solve
+) where 
+
+import Data.Char
+
+data Expr a = Plus | Minus | Prod | Const a deriving (Show)
+
+solve :: (Num a) => [Expr a] -> Maybe a
+solve = solve' []
+
+solve' :: (Num a) => [a] -> [Expr a] -> Maybe a
+solve' acc (Const x:xs)  = solve' (x:acc) xs
+solve' (x:y:acc) (op:xs) = solve' (val:acc) xs
+    where val = case op of Plus  -> x + y
+                           Minus -> x - y
+                           Prod  -> x * y
+solve' [res] [] = Just res
+solve' _ _ = Nothing
+
+
+parse :: String -> [Expr Int]
+parse "" = []
+parse (x:xs)
+    | x == '+'  = Plus  : parse xs
+    | x == '-'  = Minus : parse xs
+    | x == '*'  = Prod  : parse xs
+    | x == ' '  = parse xs
+    | x >= '0' && x <= '9' = case parseInt(x:xs) of
+        (val, str) -> Const val : parse str
+    | otherwise = []
+
+parseInt :: String -> (Int, String)
+parseInt = parseInt' 0
+
+parseInt' :: Int -> String -> (Int, String)
+parseInt' acc "" = (acc, "")
+parseInt' acc (x:xs)
+    | x >= '0' && x <= '9' = parseInt' (10 * acc + curDigit) xs
+    | otherwise = (acc, x:xs)
+        where curDigit = Data.Char.ord(x) - Data.Char.ord('0')
+
+parse2 :: String -> [Expr Int]
+parse2 = map stringToExpr . words
+    where stringToExpr "+" = Plus
+          stringToExpr "-" = Minus
+          stringToExpr "*" = Prod
+          stringToExpr num = Const (read num :: Int)