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

[Harendra Kumar]

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 at haskell.org. I tried sending an
email to libraries at 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: <libraries-owner at haskell.org>
Date: 5 February 2018 at 09:34
Subject: Re: rolling span and groupBy for lists
To: harendra.kumar at 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 at haskell.org.



---------- Forwarded message ----------
From: Harendra Kumar <harendra.kumar at gmail.com>
To: David Feuer <david at well-typed.com>, libraries <libraries at haskell.org>
Cc: ghc-devs at 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 at haskell.org as well.

-harendra

On 5 February 2018 at 09:53, David Feuer <david at well-typed.com> wrote:

> This is the wrong list. You probably meant to email haskell-cafe or
> perhaps libraries at haskell.org.
>
>
>
> David Feuer
> Well-Typed, LLP
>
> -------- Original message --------
> From: Harendra Kumar <harendra.kumar at gmail.com>
> Date: 2/4/18 10:50 PM (GMT-05:00)
> To: ghc-devs at 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20180205/cd20959a/attachment.html>