On Mon, 11 Aug 2003, Saikat DebRoy wrote: > When an attribute is a delayed expression sometimes it is not forced > when it is extracted. > > > x <- list() > > attr(x, "p") <- delay(1) > > x > list() > attr(,"p") > <promise: 0x11e4bb8> > > val <- attr(x, "p") > > val > [1] 1 > > attr(x, "p") > <promise: 0x11e4bb8> > > I am not quite sure whether the above is a bug or not
Promises are not forced when retrieving them from a data structure. I don't think this is a bug (though I don't think the semantics of user level access to promises are exactly cast in stone). > but I think the > following is a bug - a promise is supposed to give its value once > evaluated! > > > eval(attr(x, "p")) > <promise: 0x11e4bb8> This should probably be considered a bug in do_eval (internal eval would force the promise). I'd be careful fixing it though as it might break other things. Promises are really intended to support lazy evaluation and work best if they are stored as values of variables in environments. I'm not sure I would consider other uses reliable in the long run. Best, luke -- Luke Tierney 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 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-devel
