Hi all, I've recently started using Haskell and am looking for some feedback
on code I've written; I sometimes feel that maybe I'm not doing things the
best way possible that Haskell allows, and maybe missing out on obvious
improvements in brevity or elegance. So here's a tic-tac-toe solver I wrote
that does basic min-max search of the entire tree (not efficient
algorithmically, but it's tictactoe so a blank board can be fully solved in
a few seconds). Also, if people have suggestions on how to change the
program to actually output the moves it would make, please let me know -
right now it just responds with 1 for a win for X, 0 for a draw, and -1 for
a win for O. And there are probably bugs!
Thanks in advance
import Data.List
data Box = Blank | X | O deriving (Eq, Show)
-- usage: call score with the board rows concatenated: score [Blank, Blank,
Blank, Blank, Blank, Blank, Blank, Blank, Blank]
-- or score [Blank, X, O, Blank, X, O, Blank, Blank, Blank]
-- score is 1 if X wins, 0 for draw, -1 for lose
score :: [Box] -> Int
score g | haveline X g = 1
| haveline O g = -1
| gridfull g = 0
| isxmove g = maximum (map score (makeallmoves X g))
| otherwise = minimum (map score (makeallmoves O g))
tolines :: [Box] -> [[Box]]
tolines [a1, a2, a3, b1, b2, b3, c1, c2, c3] = [ [a1,a2,a3], [b1,b2,b3],
[c1,c2,c3],
[a1,b1,c1], [a2,b2,c2],
[a3,b3,c3],
[a1,b2,c3], [a3,b2,c1] ]
haveline :: Box->[Box] -> Bool
haveline b g = any ([b,b,b]==) (tolines g)
gridfull :: [Box] -> Bool
gridfull g = not $ any (Blank==) g
isxmove :: [Box] -> Bool
isxmove g = let movecount = sum $ map (\b -> if b==Blank then 0 else 1) g
in mod movecount 2 == 0
isomove = not . isxmove
fl :: Box->[Box]->[Box]->[[Box]]
fl b xs (Blank:ys) = [xs ++ b:ys]
fl _ _ _ = []
makeallmoves :: Box->[Box]-> [[Box]]
makeallmoves b g = concat $ zipWith (fl b) (inits g) (tails g)
