R devel - Jan 2013 - 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

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