[Haskell-cafe] Behavior of groupBy

[Hans van Thiel]

Thanks for the help! Strangely, I just now received your messages from
April 28, hence the late reply...

Hans van Thiel
On Sat, 2007-04-28 at 18:09 +0200, Henning Thielemann wrote:
> On Sat, 28 Apr 2007, Hans van Thiel wrote:
> 
> > Hello All,
> >
> > The standard function groupBy of List.hs doesn't work as I expect in
> > this case:
> >
> > groupBy (\x y -> (last x) == (last y)) ["abc", "bd","cac"]
> >
> > results in:
> >
> > [["abc"],["bd"],["cac"]]
> >
> > where I want:
> >
> > [["abc","cac"], ["bd"]]
> 
> I think you must roll your own one. How about repeated 'partition' to
> extract all elements that have the same trailing character like the head
> word of the list?
> 
> I have done this in a more complex setting and called it 'slice':
>   http://darcs.haskell.org/haskore/src/Haskore/Basic/TimeOrderedList.lhs
> 
> 
> > slice :: (Eq a, Num time) =>
> >    (body -> a) -> T time body -> [(a, T time body)]
> > slice f perf =
> >   let splitByHeadKey pf =
> >          fmap
> >             (\ev ->
> >                let i = f (eventBody ev)
> >                    (pf0, pf1) = partition ((i==) . f) 0 0 pf
> >                in  ((i,pf0), pf1))
> >             (listToMaybe pf)
> >   in  List.unfoldr splitByHeadKey perf
>