Gabor Grothendieck
2013-Jan-12 16:02 UTC
[Rd] Tricking Promises into Sending Info Via Args into Caller
The is.pos function below results in the variable, out, being set to TRUE if the first argument to is.pos is positive and to FALSE otherwise. It does this without using the return value or using scoping tricks to reach into the caller. Instead it tricks the promise into communicating one bit of information upwardly from the function to its caller via the second argument. One would have thought this to be impossible. Is this intended behavior? is.pos <- function(i, x) { if (i > 0) x; NULL } # in this example actual arg1 of is.pos is positive out <- FALSE is.pos(1, out <- TRUE) out # TRUE # in this example actual arg1 of is.pos is negative out <- FALSE is.pos(-1, out <- TRUE) out # FALSE -- Statistics & Software Consulting GKX Group, GKX Associates Inc. tel: 1-877-GKX-GROUP email: ggrothendieck at gmail.com
peter dalgaard
2013-Jan-12 17:19 UTC
[Rd] Tricking Promises into Sending Info Via Args into Caller
On Jan 12, 2013, at 17:02 , Gabor Grothendieck wrote:> The is.pos function below results in the variable, out, being set to > TRUE if the first argument to is.pos is positive and to FALSE > otherwise. > > It does this without using the return value or using scoping tricks to > reach into the caller. Instead it tricks the promise into > communicating one bit of information upwardly from the function to its > caller via the second argument. > > One would have thought this to be impossible. Is this intended behavior?Yes, this is a generic consequence of lazy evaluation: delayed and unpredictable side effects. Whether it is desirable is an open issue; it is the sort of thing that creates serious headaches for compiler constructors, but it is pretty much unavoidable once you include the lazy eval feature.> > is.pos <- function(i, x) { if (i > 0) x; NULL } > > # in this example actual arg1 of is.pos is positive > out <- FALSE > is.pos(1, out <- TRUE) > out # TRUE > > # in this example actual arg1 of is.pos is negative > out <- FALSE > is.pos(-1, out <- TRUE) > out # FALSE > > -- > Statistics & Software Consulting > GKX Group, GKX Associates Inc. > tel: 1-877-GKX-GROUP > email: ggrothendieck at gmail.com > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel-- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Email: pd.mes at cbs.dk Priv: PDalgd at gmail.com