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

[Markus Läll]

Hi Harendra -- I think you just need to register to libraries at haskell.org
to post there.

On Mon, Feb 5, 2018 at 6:02 AM, Harendra Kumar <harendra.kumar at gmail.com>
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 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
>>
>
>
>
>
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-- 
Markus Läll
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