[Rd] Tricking Promises into Sending Info Via Args into Caller

Gabor Grothendieck ggrothendieck at gmail.com
Sat Jan 12 17:02:04 CET 2013

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

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

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