On Tue, 1 Apr 2008, Dr. Ferdinand Jamitzky wrote: > Anyway something like "yield" would be very nice to have. > Would it be hard to implement?
Short answer: yes, it would be hard. Longer answer: R's interpreter is based on lots of recursive C calls. There are standard ways of implementing continuations in that sort of context, but they are not entirely portable (involves C stack copying), require assembly language programming on some platforms (register window flushing on sparc for example) and can be fairly inefficient. It might be possible to implement something more restricive that is sufficient for implementing generators, but I don't think it would be much easier given the current architecture. It is worth keeping this in mind if we make changes to the evaluator, but that is not likely to happen real soon. Some of us worked on something along these lines a while back in a branch called R-uthreads, but the cost of converting to that approach and the overhead of maintaining it seemed too high for the benefit, at least at the time. Best, luke > It could be in a similar way to scala where you can define for-comprehensions. > I really miss them in R. > best wishes > ferdinand > > On Tue, Apr 1, 2008 at 12:32 PM, Luke Tierney <[EMAIL PROTECTED]> wrote: >> No. First class continuations of the kind provided in scheme can be >> used as a means to implement generators, but downward-only >> continuations as currently provided in R are not sufficient for that. >> This version is intended only as a non-local exit mechanism. >> >> Best, >> >> luke >> >> On Mon, 31 Mar 2008, f.jamitzky wrote: >> >> > >> > callcc is similar to the "yield" keyword in python and c# >> > it lets you define e.g. a generator of lists of numbers. >> > >> > >> > >> > >> > Gabor Grothendieck wrote: >> >> >> >> Would anyone like to explain if callCC in R 2.7.0 gives >> >> anything that on.exit does not already provide? >> >> >> >> It seems that the exit condition once defined cannot >> >> be added to overridden whereas with on.exit multiple >> >> on.exit's add additional on.exits rather than being ignored. >> >> >> >> Is this important? >> >> >> >> ______________________________________________ >> >> R-devel@r-project.org mailing list >> >> https://stat.ethz.ch/mailman/listinfo/r-devel >> >> >> >> >> > >> > >> >> -- >> Luke Tierney >> Chair, Statistics and Actuarial Science >> Ralph E. Wareham Professor of Mathematical Sciences >> University of Iowa Phone: 319-335-3386 >> Department of Statistics and Fax: 319-335-3017 >> Actuarial Science >> 241 Schaeffer Hall email: [EMAIL PROTECTED] >> Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu >> > > > > -- Luke Tierney Chair, Statistics and Actuarial Science Ralph E. Wareham Professor of Mathematical Sciences University of Iowa Phone: 319-335-3386 Department of Statistics and Fax: 319-335-3017 Actuarial Science 241 Schaeffer Hall email: [EMAIL PROTECTED] Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
