haskell - Need a basecase for returning position in a list -

haskell - Need a basecase for returning position in a list -

data sudoku = sudoku [[maybe int]] blank :: sudoku -> pos blank (sudoku rs) = [(y,x) | (r,y) <- rs `zip` [0..8], (c,x) <- r `zip` [0..8], c == nothing] !! 0

blank returns first position nothing. if not nothing, homecoming reasonable, not error! how that?

output:

main> blank illustration (0,2) main> blank sud *** exception: prelude.(!!): index big

(sud contains no nothing)

example = sudoku [ [just 3, 6, nothing, nothing, 7, 0, 2, nothing, nothing] , [nothing, 5, nothing, nothing, nothing, nothing, 1, 8, nothing] , [nothing, nothing, 9, 2, nothing, 4, 7, nothing, nothing] , [nothing, nothing, nothing, nothing, 1, 3, nothing, 2, 8 ] , [just 4, nothing, nothing, 5, nothing, 2, nothing, nothing, 9 ] , [just 2, 7, nothing, 4, 6, nothing, nothing, nothing, nothing] , [nothing, nothing, 5, 3, nothing, 8, 9, nothing, nothing] , [nothing, 8, 3, nothing, nothing, nothing, nothing, 6, nothing] , [nothing, nothing, 7, 6, 9, nothing, nothing, 4, 3 ] ]

(!! 0) head, head works on non-empty lists, need check whether case:

blank :: sudoku -> pos blank (sudoku rs) = case [(y,x) | (r,y) <- rs `zip` [0..8] , (c,x) <- r `zip` [0..8] , c == nothing] of (pos:_) -> pos; _ -> (-1, -1)

this of course of study much ill-advised; should homecoming position wrapped in maybe (i.e. either just (y,x) or nothing) type of function changes blank :: sudoku -> maybe pos. utilize built-in function listtomaybe data.maybe:

prelude> :m +data.maybe prelude data.maybe> listtomaybe [1..5] 1 prelude data.maybe> listtomaybe [] nil

so that

blankmaybe :: sudoku -> maybe pos blankmaybe (sudoku rs) = listtomaybe [(y,x) | (r,y) <- rs `zip` [0..8] , (c,x) <- r `zip` [0..8] , c == nothing]

or utilize take 1 instead of head, blankhead :: sudoku -> [pos]; blankhead (sudoku rs) = take 1 [(y,x) | ..... ]. using maybe best choice, type corresponds our intent here.

list haskell sudoku