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