Question

So, to explain the mouthful that is the question…

I’m trying to use filter, and not getting anywhere. So here’s the behaviour I want. Given a list of lists (of Integers) ie

[[1,2,3],[23456,4,3,2],[1,3,4,5,6],[3,2,1],[4,2,1],[5,6,7],[1,2,5]]

I want to take another list ie [1,2] and take all the lists that contain both those elements. (It doesn’t matter if any of the others are preserved at this moment in time, though that might be needed later). So in this example my output would be something like

[[1,2],[2,1],[2,1],[1,2]]

or preferably

[[1,2,3],[3,2,1],[4,2,1],[1,2,5]]

Answer 1

So you want to keep a list candidate if all elements of the list criterion are also elements of candidate. Thus your filter condition is

condition candidate = all (`elem` candidate) criterion

which, using flip we can write as

condition candidate = flip all criterion (flip elem candidate)

thus giving

filter (flip all criterion . flip elem)