第10回オフラインリアルタイムどう書くの問題をHaskellで書いた

第10回オフラインリアルタイムどう書くの問題をHaskellで書いた

移動場所の記録をWriterモナドで上手く書けそうな気もしますが、モナドの理解不足。。。
あとlet文、if文が多いのもイマイチか。。。
テスト処理ももっと簡潔に書きたい。

honeycomb.hs

import Control.Applicative

honeycomb = [
                "   T   ",
                "  k U  ",
                " j H V ",
                "i S I W",
                " R B J ",
                "h G C X",
                " Q A K ",
                "g F D Y",
                " P E L ",
                "f O M Z",
                " e N a ",
                "  d b  ",
                "   c   "
            ]

honeycombW = length $ honeycomb !! 0
honeycombH = length honeycomb

directions c | c == '0' = (0,-2)
             | c == '1' = (1,-1)
             | c == '2' = (1,1)
             | c == '3' = (0,2)
             | c == '4' = (-1,1)
             | c == '5' = (-1,-1)

nextPosition :: (Int, Int) -> Char -> (Int, Int)
nextPosition (x, y) d = let (dx, dy) = directions d 
                        in (x + dx, y + dy)

validPosition :: (Int, Int) -> Bool
validPosition (x, y) = and [x >= 0, x < honeycombW, y >= 0, y < honeycombH, honeycomb !! y !! x /= ' ']

move :: ((Int, Int), String) -> Char -> ((Int, Int), String)
move ((x, y), path) d = let (x', y') = nextPosition (x, y) d
                        in if validPosition (x', y')
                            then ((x', y'), ((honeycomb !! y') !! x'):path)
                            else ((x, y), '!':path)

solve str = reverse $ snd $ foldl move ((3,6), "A") str

main = do
    tests <- map words <$> lines <$> getContents
    mapM (\t -> print $ runTest solve t) tests
    where
        runTest f t = f (head t) == last t 

test.txt

135004  ACDABHS
1   AC
33333120    AENc!!b!M
0   AB
2   AD
3   AE
4   AF
5   AG
4532120 AFQPOEMD
051455  ABSHSj!
23334551    ADMb!cdeO
22033251    ADLKLa!ML
50511302122 AGSjkTHTU!VW
000051  ABHT!!!
1310105 ACDKJW!V
50002103140 AGSk!HU!IVIU
3112045 AEDKYXKC
02021245535 ABCIJW!JIHBS
014204  ABIBCIB
255230  ADAGAEA
443501  AFPefgQ
022321  ABCKLZ!
554452  AGRh!!Q
051024  ABSHTUH
524002  AGAFGSB
54002441132 AGQRjSRhRSGA
11010554312 ACJV!!UTkSHI
23405300554 ADMNEFOFGRi!
555353201   AGRih!gPQG
22424105    ADLMabaLD
11340202125 ACJKDCKJX!!J
4524451 AFQFPf!P
44434234050 AFPf!!e!!Pgh
00554040132 ABHk!j!i!jRG
3440403 AEOePfgf
111130  ACJW!XW
21133343125 ADKXYZ!a!Z!L
353511  AEFOPFA
22204115220 ADLZYLY!KY!X
03013541    ABABICBGB
101344  ACIVJCA
2432541 ADENbNdN
45332242015 AFQPedc!!NME
215453  ADKCAGF
45540523454 AFQh!i!RQg!!
42434302545 AFEOd!!ONOef