Haskell: How to combine scalar and monadic values?

Question

I am currently messing around with a haskell issue regarding 2D ST array and recursion.

Given a two-dimensional position and an array of directions, I wrote a line that returns a list of all the resulting points inside the array:

let cellsAround = [resPt | dir <- directions,
                         let resPt = (fst dir + fst point, snd dir + snd point),
                         fst resPt >= 0 && fst resPt <= fst maxIdx &&
                         snd resPt >= 0 && snd resPt <= snd maxIdx]

Now the goal is to enrich the resulting list items with the contents of the array, and I tried this:

cellsAround <- sequence [readArray board resPt | dir <- directions,
                         let resPt = (fst dir + fst point, snd dir + snd point),
                         fst resPt >= 0 && fst resPt <= fst maxIdx &&
                         snd resPt >= 0 && snd resPt <= snd maxIdx]

This also works great. But the goal is to get a combination of both [(Point, Int)], because I have to filter the contents of the array.

Any ideas how to combine this, say

(resPt, readArray board resPt)

?

Answer 1

Minimum change:

sequence [(,) resPt <$> readArray board resPt
         | ...