Proposal: Add split and splitWith (trac #2048)

[Henning Thielemann]

On Thu, 17 Jan 2008, Henning Thielemann wrote:

> On Thu, 17 Jan 2008, Twan van Laarhoven wrote:
>
> > Hello Haskellers,
> >
> > An often requested function is 'split', to split a list into parts delimited by
> > some separator. ByteString has the functions split and splitWith for this
> > purpose. I propose we add equivalents to Data.List:
> >
> >  > split :: Eq a => a -> [a] -> [[a]]
> >  > split x = splitWith (x==)
> >  >
> >  > splitWith :: (a -> Bool) -> [a] -> [[a]]
> >  > splitWith p xs = ys : case zs of
> >  >                         []   -> []
> >  >                         _:ws -> splitWith p ws
> >  >                   where (ys,zs) = break p xs
> >
> > trac: http://hackage.haskell.org/trac/ghc/ticket/2048
> > deadline: two weeks from now, January 30
>
> The type could be more specific. We know that the result list is always
> non-empty, and that the first element of each sub-list matches 'p', except
> the leading sub-list. Ideally we had a list type for elements of
> alternating type, like this one:
>   http://darcs.haskell.org/event-list/src/Data/AlternatingList/List/Uniform.hs
>  Then we could use the signature:
>     splitWith :: (a -> Bool) -> [a] -> AlternatingList.T [a] a
>
> We can simulate this type by:
>     splitWith :: (a -> Bool) -> [a] -> ([a], [(a, [a])])

Ah, I see that your function filters out the elements, that match 'p'.
This simplifies the resulting list type ...