[Haskell-cafe] List operation question

[Robert Dockins]

On Sunday 04 February 2007 14:24, Nicolas Frisby wrote:
> I've always thought that when certain operations are of particular
> interest, it's time to use more appropriate data structures, right?
> Lists are great and simple and intuitive, but if you need such
> operations as shifts, something like a deque is the way to go.
>
> I'm not a data structure pro, but I'm sure someone on this list could
> post a neat example. Or you could look for work by Osaki - he seems to
> be the reference for functional data structures. "Finger trees" and
> "tries" also get a lot of attention around here.


Also, take a look at Edison.  It has a variety of sequence implementations 
with different properties.  Several of them have efficient access to both 
ends of the sequence.


http://www.eecs.tufts.edu/~rdocki01/edison.html


> Enjoy.
>
> On 2/4/07, Lennart Augustsson <lennart at augustsson.net> wrote:
> > Not much better.  You could define shiftl such that is does a single
> > traversal and
> > returns both the last element and all but the last.  That will save
> > you one traversal.
> >
> > On Feb 4, 2007, at 18:44 , Eric Olander wrote:
> > > Hi,
> > >    I'm still somewhat new to Haskell, so I'm wondering if there are
> > > better ways I could implement the following functions, especially
> > >
> > > shiftl:
> > > >> moves the first element to the end of the list
> > >
> > >     shiftr :: [a] -> [a]
> > >     shiftr [] = []
> > >     shiftr (x:y) = y ++ [x]
> > >
> > > >> moves the last element to the head of the list
> > >
> > >     shiftl :: [a] -> [a]
> > >     shiftl [] = []
> > >     shiftl x = [last x] ++ init x
> > >
> > > -Eric
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