# Haskell Quiz/Numeric Maze/Solution Ninju

### From HaskellWiki

I haven't yet added any optimization, because I wanted to keep the program as simple (and therefore readable) as possible, but I might add some later.

module Main where import System.Environment import Data.List main :: IO () main = do args <- getArgs if length args == 2 then do let [a,b] = map read args putStrLn $ show (solve a b) else putStrLn "Usage: solve START TARGET" return () data Operation = AddTwo Integer | Double Integer | Halve Integer valid :: Operation -> Bool valid (Halve x) = x `mod` 2 == 0 valid _ = True apply :: Operation -> Integer apply (AddTwo x) = x + 2 apply (Double x) = x * 2 apply (Halve x) = x `div` 2 solve :: Integer -> Integer -> [Integer] solve a b = solve' [[a]] b where solve' paths target = case find ((== target) . last) paths of Just path -> path Nothing -> solve' (concatMap buildPathsFrom paths) target buildPathsFrom path = [ path ++ [apply (op (last path))] | op <- [AddTwo, Double, Halve], valid (op (last path)) ]