[R] pairlists (was: data manipulation function descriptions)

[Peter Dalgaard BSA]

"Warnes, Gregory R" <gregory_r_warnes at groton.pfizer.com> writes:

> > -----Original Message-----
> > From: Luke Tierney [mailto:luke at stat.uiowa.edu]
> 
> > R does not provide a pairlist data structure. This creates a dilemma
> > when translating some list-based xlispstat code, or, more
> > importantly, when implementing an algorithm for which parilists are
> > the natural data structure to use.
> > ...
> > Pairlists were and still are used internally for many things. 
> > ...
> 
> Wouldn't it, therefore, make sense to provide a 'pairlist' package which
> exposes the internal pairlist structure and provides appropriate functions
> (car, cdr, ...), instead of expecting people to keep re-implementing these
> features?

Some ancient consideration pops up here. We do actually expose
pairlists in a few places (try mode(.Options)). Some people consider
that this is a remnant and should be stamped out, but we might also
consider doing what you suggest. 

The big problem with old R was not so much the pairlists but that they
were used for representing objects of mode "list" so to get to X[[n]]
you had to count through the list from the beginning which killed
performance in some important cases. Then again, adding elements to a
generic vector requires copying the whole thing. Of course all the
legacy S code tended to do the former and not the latter, so generic
vectors ended up winning.

One or two reservations: With full lisp style access, could we end up
with (circular) data structures that confuse the garbage collector?
And might we -- supposing we allowed destructive list modifications --
end up with strange semantics a la the .Alias mess we had for a while?
Of course Luke would be the first to know about this.

Then of course there is the question of reverse compatibility. I don't
consider it much of a loss if R code doesn't run in Splus, but others
might.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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