[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
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email: ggrothendieck at gmail.com
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Re: [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@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@cbs.dk Priv: pda...@gmail.com
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Re: [Rd] Tricking Promises into Sending Info Via Args into Caller
In the spirit of this, but AFAIK not due to lazy evaluation, here's
another illustration why it's easy to mistakes when doing inline
assignments:
x - 0
TRUE (x - 1)
[1] TRUE
x
[1] 1
FALSE (x - 2)
[1] FALSE
x
[1] 1
(x - 3) FALSE
[1] FALSE
x
[1] 3
FALSE (x - 4)
[1] FALSE
x
[1] 4
/Henrik
On Sat, Jan 12, 2013 at 9:19 AM, peter dalgaard pda...@gmail.com wrote:
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@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@cbs.dk Priv: pda...@gmail.com
__
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
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