New interpreters from RosettaCode (#3087)

samples/Haskell/maze-solving.hs

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+#!/usr/bin/runhaskell
+
+import Data.Maybe
+
+-- given two points, returns the average of them
+average :: (Int, Int) -> (Int, Int) -> (Int, Int)
+average (x, y) (x', y') = ((x + x') `div` 2, (y + y') `div` 2)
+
+-- given a maze and a tuple of position and wall position, returns
+-- true if the wall position is not blocked (first position is unused)
+notBlocked :: [String] -> ((Int, Int), (Int, Int)) -> Bool
+notBlocked maze (_, (x, y)) = ' ' == (maze !! y) !! x
+
+-- given a list, a position, and an element, returns a new list
+-- with the new element substituted at the position
+-- (it seems such a function should exist in the standard library;
+-- I must be missing it)
+substitute :: [a] -> Int -> a -> [a]
+substitute orig pos el =
+  let (before, after) = splitAt pos orig
+  in before ++ [el] ++ tail after
+
+-- given a maze and a position, draw a '*' at that position in the maze
+draw :: [String] -> (Int, Int) -> [String]
+draw maze (x,y) = substitute maze y $ substitute row x '*'
+  where row = maze !! y
+
+-- given a maze, a previous position, and a list of tuples of potential
+-- new positions and their wall positions, returns the solved maze, or
+-- None if it cannot be solved
+tryMoves :: [String] -> (Int, Int) -> [((Int, Int), (Int, Int))] -> Maybe [String]
+tryMoves _ _ [] = Nothing
+tryMoves maze prevPos ((newPos,wallPos):more) =
+  case solve' maze newPos prevPos
+       of Nothing -> tryMoves maze prevPos more
+          Just maze' -> Just $ foldl draw maze' [newPos, wallPos]
+
+-- given a maze, a new position, and a previous position, returns
+-- the solved maze, or None if it cannot be solved
+-- (assumes goal is upper-left corner of maze)
+solve' :: [String] -> (Int, Int) -> (Int, Int) -> Maybe [String]
+solve' maze (2, 1) _ = Just maze
+solve' maze pos@(x, y) prevPos =
+  let newPositions = [(x, y - 2), (x + 4, y), (x, y + 2), (x - 4, y)]
+      notPrev pos' = pos' /= prevPos
+      newPositions' = filter notPrev newPositions
+      wallPositions = map (average pos) newPositions'
+      zipped = zip newPositions' wallPositions
+      legalMoves = filter (notBlocked maze) zipped
+  in tryMoves maze pos legalMoves
+
+-- given a maze, returns a solved maze, or None if it cannot be solved
+-- (starts at lower right corner and goes to upper left corner)
+solve :: [String] -> Maybe [String]
+solve maze = solve' (draw maze start) start (-1, -1)
+  where startx = length (head maze) - 3
+        starty = length maze - 2
+        start = (startx, starty)
+
+-- takes unsolved maze on standard input, prints solved maze on standard output
+main = interact main'
+  where main' x = unlines $ fromMaybe ["can't solve"] $ solve $ lines x