Question

I am writing the following function the second time in unrelated projects (first was XML processing, now custom command line flag processing), and I have a feeling that it should exist in some library, I just cannot find it. It groups list elements, each group starting at an element where a predicate is true.

Any simpler ways to do this?

groupStartBy :: (a -> Bool) -> [a] -> [[a]]
groupStartBy pred xs = reverse $ map reverse $ foldl' step [] xs
  where
    step as x | pred x = [x]:as
    step (a:as) x = (x:a):as
    step [] x = [[x]]

Answer 1

You can use groupBy to do this:

import Data.List (groupBy)

groupStartBy :: (a -> Bool) -> [a] -> [[a]]
groupStartBy pred = groupBy (const (not . pred))
-- or in point free style: groupStartBy = groupBy . const . (not .)