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
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