Cracker Barrel Peg Board Puzzle in Haskell
I first wrote a program to solve the Cracker Barrel peg board puzzle (15 holes arranged in a triangle with 14 golf tees) many years ago as youth using the BASIC language. I wish I still had the source to that, because I’m pretty sure this Haskell version would kick its butt :)
I’m still trying to get my head around Haskell, so I expect there are many possible improvements to this program, but even so, I’m pleased with how Haskell allows me to express logic.
-- Solve the Cracker Barrel Peg Board Puzzle module Main where type Pos = (Int, Int) type Move = (Pos, Pos) type Board = [ Pos ] isOccupied b p = elem p b isEmpty b p = not (isOccupied b p) isPos (r,c) = elem r [0..4] && elem c [0..r] -- Possible moves for one position positionMoves b p = [ (p, dst) | (neighbor, dst) isPos p1 && isPos p2) [ ((r + or `div` 2, c + oc `div` 2),(r + or, c + oc)) | (or, oc) <- [ (-2,0), (0,2), (2,2), (2,0), (0,-2), (-2,-2) ] ] -- Possible moves for all positions on the board possibleMoves b = concat [ positionMoves b pos | pos (pos /= src) && (pos /= neighbor) -- Make moves until the goal position is met play b p moves = if null nextMoves then if length b == 1 && head b == p then reverse moves else  else tryMoves nextMoves where nextMoves = possibleMoves b tryMoves  =  tryMoves (m:ms) = let result = play (move b m) p (m:moves) in if null result then tryMoves ms else result -- Compute the initial empty position to know the goal, then solve the puzzle solve b = let emptyPos = head [ (r,c) | r <- [0..4], c <- [0..r], isEmpty b (r,c) ] in play b emptyPos  -- A sample board with the topmost hole empty board :: Board board = [ (1,0), (1,1), (2,0), (2,1), (2,2), (3,0), (3,1), (3,2), (3,3), (4,0), (4,1), (4,2), (4,3), (4,4) ] main = print (solve board)