Small brute Sudoku solver. The input is a flattened 9x9 table as a string where "0" denotes an empty cell. O/O2 compilation is advised for good performance. The solver may return multiple solutions (though a proper Sudoku table should be unambiguous).

bruteSudoku.hs

import Data.Array.Unboxed
import Data.Array.Base (unsafeAt)
import Data.List.Split
import Data.List
 
type Table = UArray Int Char
 
solve :: String -> [String]
solve = map elems . go 0 . listArray (0, 80) where
 
    rows   = chunksOf 9 [0..80]
    cols   = transpose rows
    blocks = chunksOf 9 . concat . concat . transpose . map (chunksOf 3) $ rows
    pairs  = [(x, delete x g) | g <- rows ++ cols ++ blocks, x <- g]
    neighs = accumArray union [] (0, 80) pairs :: Array Int [Int]
 
    go 81 table = [table :: Table]
    go i  table = case unsafeAt table i of
        '0' -> concat [go (i + 1) (table // [(i, x)]) | x <- ['1'..'9'],
            notElem x . map (unsafeAt table) $ unsafeAt neighs i]
        _ -> go (i + 1) table
     
 
table = concat [
    "480006902",
    "002008001",
    "900370060",
    "840010200",
    "003704100",
    "001060049",
    "020085007",
    "700900600",
    "609200018"]
 
 
main = print $ solve table

--487156932362498751915372864846519273593724186271863549124685397738941625659237418