rolling span and groupBy for lists

[Brandon Allbery]

Why do I suddenly catch a whiff of https://xkcd.com/927/ ?

On Mon, Feb 5, 2018 at 1:13 PM, Harendra Kumar <harendra.kumar at gmail.com>
wrote:

> According to hayoo there seem to be 7 different implementations of this
> same function. Yours is 8th and mine is 9th and other people may have more
> not uploaded or maybe the ones that hayoo is not able to find. Does that
> make a case for including this in some standard place?
>
> -harendra
>
> On 5 February 2018 at 12:22, Evan Laforge <qdunkan at gmail.com> wrote:
>
>> I have my own list library with a bunch of things like this.  I think
>> it's what most people do, and some upload them to hackage, e.g.
>> utility-ht or the split package, or data-ordlist.
>>
>> Specifically, I think rollingGroupBy is what I call splitWith:
>>
>> -- | Split @xs@ before places where @f@ matches.
>> --
>> -- > split_with (==1) [1,2,1]
>> -- > --> [[], [1, 2], [1]]
>> split_with :: (a -> Bool) -> [a] -> NonNull [a]
>>     -- ^ output is non-null, and the contents are also, except the first
>> one
>>
>> You can probably find something like this in 'split', or if not, that
>> might be a good place to contribute it.
>>
>> I have a bunch of grouping functions too, which I use all the time, so
>> if there's some kind of general list grouping package then maybe I
>> could put them there.
>>
>> On the other hand, this sort of thing is pretty individual, so it
>> doesn't seem so bad for each person to have their own local library.
>> That way you know it fits your style.  Ultimately I think that's why
>> none of the split functions made it into Data.List, every person has a
>> slightly different idea of what it should be.
>>
>> On Sun, Feb 4, 2018 at 7:50 PM, Harendra Kumar <harendra.kumar at gmail.com>
>> wrote:
>> > 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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>> > ghc-devs at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> >
>>
>
>
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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