Re: [Haskell-cafe] Fwd: rolling span and groupBy for lists

Hi Harendra -- I think you just need to register to libraries@haskell.org to post there. On Mon, Feb 5, 2018 at 6:02 AM, Harendra Kumar wrote:

Hi,

I am looking for the decision making authority or a place where questions about haskell libraries like in the email below can be asked or answered. ghc-devs told me that their list is not the right place for this and redirected me to haskell-cafe or libraries@haskell.org. I tried sending an email to libraries@haskell.org but that seems to be a closed list, my email got rejected, so that also does not seem like the right place. Is haskell cafe the right place? I was under the impression that haskell-cafe is for general discussions and not an owner of any of the libraries or any other haskell code. Are there any other mailing lists that I am missing? I can raise a ticket at ghc trac but I guess that cannot be the primary way to ask simple questions.

-harendra

---------- Forwarded message ---------- From: Date: 5 February 2018 at 09:34 Subject: Re: rolling span and groupBy for lists To: harendra.kumar@gmail.com

You are not allowed to post to this mailing list, and your message has been automatically rejected. If you think that your messages are being rejected in error, contact the mailing list owner at libraries-owner@haskell.org.

---------- Forwarded message ---------- From: Harendra Kumar To: David Feuer , libraries Cc: ghc-devs@haskell.org Bcc: Date: Mon, 5 Feb 2018 10:07:55 +0530 Subject: Re: rolling span and groupBy for lists 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

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-- Markus Läll