I was mainly asking if it makes sense to include these functions in base/Data.List. Since the base package is maintained and ships along with ghc, and the issues are also raised at ghc trac I thought this is the right list. I am copying to libraries@haskell.org as well. -harendra On 5 February 2018 at 09:53, David Feuer wrote:

This is the wrong list. You probably meant to email haskell-cafe or perhaps libraries@haskell.org.

David Feuer Well-Typed, LLP

-------- Original message -------- From: Harendra Kumar Date: 2/4/18 10:50 PM (GMT-05:00) To: ghc-devs@haskell.org Subject: rolling span and groupBy for lists

Hi,

For a small problem, I was looking for a groupBy like function that groups based on a predicate on successive elements but I could not find one. I wrote these little functions for that purpose:

-- | Like span, but with a predicate that compares two successive elements. The -- span ends when the two successive elements do not satisfy the predicate. rollingSpan :: (a -> a -> Bool) -> [a] -> ([a], [a]) rollingSpan _ xs@[] = (xs, xs) rollingSpan _ xs@[_] = (xs, []) rollingSpan p (x1:xs@(x2:_)) | p x1 x2 = let (ys, zs) = rollingSpan p xs in (x1 : ys, zs) | otherwise = ([x1], xs)

-- | Like 'groupBy' but with a predicate that compares two successive elements. -- A group ends when two successive elements do not satisfy the predicate. rollingGroupBy :: (a -> a -> Bool) -> [a] -> [[a]] rollingGroupBy _ [] = [] rollingGroupBy cmp xs = let (ys, zs) = rollingSpan cmp xs in ys : rollingGroupBy cmp zs

Are there any existing functions that serve this purpose or is there any simpler way to achieve such functionality? If not, where is the right place for these, if any. Can they be included in Data.List in base?

Thanks, Harendra