[Rd] relist, an inverse operator to unlist

[Martin Maechler]

>>>>> "GaGr" == Gabor Grothendieck <ggrothendieck at gmail.com>
>>>>>     on Wed, 23 May 2007 08:56:50 -0400 writes:

    GaGr> On 5/23/07, Seth Falcon <sfalcon at fhcrc.org> wrote:
    >> Andrew Clausen <clausen at econ.upenn.edu> writes:
    >> 
    >> > Hi Seth,
    >> >
    >> > On Mon, May 21, 2007 at 05:15:10PM -0700, Seth Falcon wrote:
    >> >> I will also add that the notion of a default argument on a generic
    >> >> function seems a bit odd to me.  If an argument is available for
    >> >> dispatch, I just don't see what sense it makes to have a default.  In
    >> >> those cases, the default should be handled by the method that has a
    >> >> signature with said argument matching the "missing" class.
    >> >>
    >> >> What often does make sense is to define a generic function where some
    >> >> argument are not available for dispatch.  For example:
    >> >>
    >> >> setGeneric("foo", signature="flesh",
    >> >>            function(flesh, skeleton=attr(flesh, "skeleton")
    >> >>                standardGeneric("foo")))
    >> >
    >> > That's an excellent suggestion.  Thanks!  However, I had to set the signature
    >> > to c("numeric", "missing") rather than just "numeric".
    >> >
    >> > I have uploaded a new version here:
    >> >
    >> >       http://www.econ.upenn.edu/~clausen/computing/relist.R
    >> 
    >> I misunderstood.  You aren't using S4 classes/methods at all
    >> and so I don't actually see how my comments could have been helpful in
    >> any way.  relist seems like a really odd solution to me, but based on
    >> the discussion I guess it has its use cases.

    GaGr> You didn't misunderstand but there was an offline
    GaGr> discussion pointing out that one primary use is in the
    GaGr> inner loop of an optimization so it should be made as
    GaGr> efficient as possible and it was changed with that in
    GaGr> mind.

Thank you Gabor for that explanation.
(I had wondered, too, and was glad that Andrew had dropped S4
 generics seemingly "by himself"  ;-))

Re your proposal of mixing this into reshape():

I think it's a nice and didactly helpful idea to point out the
similarity in concepts between reshape() and relist().

However, I wouldn't like to make reshape() generic in this
sense:  As Andrew has mentioned as well, I think the two topics
of data.frame (/matrix) reshaping and list <-> vector transformation
seem too much different...

Martin