[Haskell-cafe] rolling span and groupBy for lists

Harendra Kumar harendra.kumar at gmail.com
Mon Feb 5 15:43:53 UTC 2018

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.

The irony is that theoretically you can find a Haskell package or
implementation of whatever you can imagine but quite often it takes more
time to discover it than writing your own. And then uploading what you
wrote to hackage compounds the problem. A hoogle search only shows the
groupBy in base, signature search also does not yield other results, it
seems hoogle does not cover those other packages. After writing my own and
spending quite a bit of time I could find two other similar implementations
of groupBy, one in "utility-ht" package and the other in
"data-list-sequences" but they don't turn up in hoogle search. It looks
like hoogle database should cover more packages, or maybe the search has
some issues. This state of affairs encourages people to write their own
rather than find and reuse stuff. My example in this email can be dismissed
as a small one but I think it is a larger problem.

> You can probably find something like this in 'split', or if not, that
> might be a good place to contribute it.

Yes, that is the fallback option I was considering, split seems to be the
most comprehensive of all such list related packages.

> 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.

It will be a great service to other Haskell explorers if we can consolidate
all such packages and make one standard package covering most use cases and
deprecate the other packages. Also it may be a good idea to have a see-also
or related packages kind of field in packages so that discovery is easy.

> 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.

I thought that rollingGroupBy would have been a better default option as it
can practically subsume the purpose of groupBy. groupBy in base is not well
documented, and intuitively many think it works the way rollingGroupBy
works i.e. compare two successive elements rather than comparing a fixed
element. See this stack overflow question
, I thought the same way. I guess if we compare two successive elements, by
transitive equality the existing groupBy implementation will practically
get covered by that, not strictly compatible but should serve all practical
purposes. That was the point why I was asking to consider having it in base
alongside groupBy. It seems more useful, general and intuitive than the
existing groupBy.


> 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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