# Haskell Quiz/Numeric Maze/Solution Ninju

From HaskellWiki

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