Question

here is my code:

boolTrueList :: [Bool] -> Bool
boolTrueList xs
  | length (filterFalse xs) > 0 = False
  | otherwise = True
  where
    filterFalse = filter (==False)

This is perfectly working, however I would like to rewrite the same thing with foldr/foldl, but I am stuck.

My idea is to fold a list until I find a false value and then, stop.
Any hint?

Answer 1

My idea is to fold a list until I find a false value and then, stop. Any hint?

If you want to stop early, you have to use foldr. foldl always has to traverse the entire list (so doesn’t work at all on infinite lists).

So you want

foldr combine default list

The default is the result for an empty list, that would be True here. Now, how would we want to combine?

When a False is encountered, we want to return False immediately, so

combine False _ = False

and when the value is True, we must go on, so

combine True more = more

In other words, combine = (&&), so

boolTrueList = foldr (&&) True

or, even shorter

boolTrueList = and