Question

I have tried to solve this problem before, and I’ve searched for a solution and could never find one.

I need a function that takes a list xs and an integer n and returns all list of length n with elements from xs. For example:

function [0,1] 3 = [[0,0,0],[0,0,1],[0,1,0],[0,1,1],[1,0,0],[1,0,1],[1,1,0],[1,1,1]]

I have tried this:

list _ 0 = []
list xs n = do
    y <- xs
    ps <- list xs (n-1)
    return y : ps

and this:

list _ 0 = []
list xs n = do
    y <- xs
    y : list xs (n-1)

None work as intended. I want to know two things:
Why doesn’t these work?
How should I modify them so that they work?

Answer 1

You’re very close! Your problem is your base case, list _ 0 = [].

What you’re saying there is that there are no lists of length 0 with elements from xs, when in fact there is one, the empty list.

Try

list _ 0 = [[]]
list xs n = do
  y <- xs
  ps <- list xs (n-1)
  return $ y : ps