Question

I’ve been trying to make a function in Haskell that has as an input a list that consists of lists and (it might sound pretty simple) I want this function to check if the input is a correct table with (m,n) dimensions. If it’s not, it will return (0,0). For example, if I input:

[[1,7,2,1],[2,2,7,8],[3,2,0,1]]

I’ll get:

(3,4)

I need this function for rejecting invalid inputs in my program.

Answer 1

First of all, to answer your actual question, don’t use (0, 0) as the exceptional result. Your function’s type should be:

matrixDim :: [[a]] -> Maybe (Int, Int)

If the matrix is invalid, the result will be Nothing. Now first check what the length of the first sublist is. I’m assuming that a matrix must have at least one row and one column:

matrixDim [] = Nothing

Now you can use the fact that Maybe is a monad:

matrixDim (xs:xss) = do
    let w = length xs
    guard (w > 0)
    (w', h) <- matrixDim xss <|> return (w, 0)
    guard (w == w')
    return (w, h + 1)

And now let’s get back to the actual problem at hand: A list of lists is not what you want. A much better data type for this kind of application is an array as defined in one of the Data.Array.* modules. A more experimental alternative, where you get parallelization for free, is to use repa.