[GHC] #971: Add intercalate and split to Data.List
Jón Fairbairn
jon.fairbairn at cl.cam.ac.uk
Mon Oct 30 13:25:43 EST 2006
"Josef Svenningsson" <josef.svenningsson at gmail.com> writes:
> On 10/30/06, Christian Maeder <maeder at tzi.de> wrote:
> > I also agree to add a split function. The problem was to agree which
> > variant should be called split (wrt. to treatment of multiple and final
> > separators as well as to "Eq" context or predicate argument).
> >
> > http://article.gmane.org/gmane.comp.lang.haskell.libraries/4862
> > http://article.gmane.org/gmane.comp.lang.haskell.libraries/4869
> >
> Right. I had totally forgotten about that discussion. Was there any
> consensus about the names?
I'm not sure there was yet consensus about the functions --
it seems I tried to have the bikeshed demolished, rebuilt
as a pagoda and painted red:
http://article.gmane.org/gmane.comp.lang.haskell.cafe/13807
while there's some merit in ending discussion and deciding
on /something/, I reckon it's worth trying to think about
laws and such for a while yet.
It strikes me that the discussion suggests that there are
several distinct design cases: does a separator at the
end/do two consecutive separators result in empty elements?
Do the separators form part of the output?
To sloganise for a moment, libraries should provide
components, not finished products. So what we should aim
for is a collection of functions that facilitate the
construction of all the design cases.
One of the components /might/ be:
> spanMb p [] = Nothing
> spanMb p l = Just $ span p l
As to nomenclature, since we already have span, I'd suggest
spans p l should return a list of all the contiguous
segments of l with members satisfying p (and discard the
rest).
Here's a definition:
> spans p = unfoldr (fmap (id >< dropWhile (not . p)) . spanMb p)
> (f >< g) (a,b) = (f a, g b)
We can write â€œgroupBy (\a b -> p a && p b)â€ for another of
the design cases, though I think some discussion of groupBy
belongs in here: why does it require an equivalence
relation? Shouldn't we at least have a version that works
for any binary predicate?
Here's an off the top of my head ugly version:
> runsBy = unfoldr . splitRun
> where splitRun p [] = Nothing
> splitRun p [x] = Just ([x],[])
> splitRun p (a:rest@(b:_))
> | p a b = fmap ((a:) >< id) (splitRun p rest)
> | otherwise = Just ([a], rest)
(maybe splitRun is generally useful?)
--
Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk
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