Monkey Grid Puzzle

import Data.Char (ord)
import qualified Data.Set as S
import qualified Data.Vector as V

type Coord = (Int,Int)
type Done = S.Set Coord
type Todo = [Coord]

main = putStrLn $ show n

n = loop 0 [(0,0)] S.empty

loop :: Int -> Todo -> Done -> Int
loop n [] _ = n
loop n (t:ts) d = loop (n + count t) (also t d ++ ts) (S.insert t d)

count :: Coord -> Int
count (0,0) = 1
count (0,_) = 4
count (_,0) = 4
count (x,y) = if x == y then 4 else 8

also :: Coord -> Done -> Todo
also c d = filter (\k -> S.notMember k d) $
           filter isAccessible $
           filter inQuadrant $
           next c

isAccessible :: Coord -> Bool
isAccessible (x,y) = sumDigits <= 19
  where
    sumDigits = sums V.! x + sums V.! y

inQuadrant :: Coord -> Bool
inQuadrant (x,y) = y <= x

next :: Coord -> Todo
next (x,y) = [(x+1,y), (x,y+1)]

sums :: V.Vector Int
sums = V.generate 300 sum1
  where
    sum1 n = sum $ map val (show n)
    val x = ord x - ord '0'