Question

How do you map a function to operate on lists within a list?
The following is just what I’m trying to do as an example, but I was just asking as a general question. Thanks in advance!

Right now, I’m trying to map a function, change, onto each lists of one list (returned by itrCol xs).

evalChange xs = map change $ itrCol xs

where itrCol returns a list of lists, where each containing list is a column.

itrCol xs = [getCol x xs | x <- (take (width xs) (iterate (\x -> (x + 1)*1) 0))]

getCol lists column given list of column indices

getCol :: Int -> [t] -> [t]

and change is:

change []     = []
change [x]    = [x]
change [x,y]  = [x,y]
change (x:y:z:ws) | x == y && y == z = 0 : y*(-1) : 0 : change ws
change (x:xs) =  x : change xs

Answer 1

Could just use currying and another call to map.

map (map change) $ itrCol xs

To learn more about currying take a look at that chapter in Learn You A Haskell, a great beginner book in Haskell.