rolling span and groupBy for lists

[Harendra Kumar]

Yes, I did too :-) But there is a key difference in this case, all these
definitions are mathematically equivalent with identical semantics instead
of being some fuzzy subjective standards.

-harendra

On 5 February 2018 at 23:46, Brandon Allbery <allbery.b at gmail.com> wrote:

> 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
>>> >
>>> > _______________________________________________
>>> > ghc-devs mailing list
>>> > 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
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> unix, openafs, kerberos, infrastructure, xmonad
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