What does this Haskell Data.Maybe function do?

Question

I would like to ask if you know what this function does:

hasUnassigned :: [Int] -> Bool
hasUnassigned board = isJust $ elemIndex 0 board

I found this function online and I plan to use it to help draw Sudoku game solutions.

Please could you explain to me how it works?

Answer 1

elemIndex searches for the first argument in the second argument, which is a list. It returns a Maybe Int - Just the index if the element is found, Nothing otherwise.

isJust returns true if a Maybe value is a Just, false if it is a Nothing.

In other words, this is a very awkward way of testing whether board contains 0. A better way is

hasUnassigned board = 0 `elem` board

Answer 2

elemIndex has signature a -> [a] -> Maybe Int. When applied to x and a list list, it gives us Just i if x appears for the first time in list at position i. If x is not in list, we get Nothing instead.

The outer function, isJust, takes a Maybe a and tells us if it's of the form Just whatever or Nothing. hasUnassigned thus checks to see if there's a 0 in the board list.