Re: [GHC] #971: Add intercalate and split to Data.List

"Josef Svenningsson" writes:

On 10/30/06, Christian Maeder 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 “groupBy (\a b -> p a && p b)” 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?) -- J�n Fairbairn Jon.Fairbairn@cl.cam.ac.uk