Thanks-- Luke Tierney's reply below is very helpful. 'makeActiveBinding' is
brilliant and I'm pretty sure I can make it do just what I want.
One question though: my first experiment with it was this
> makeActiveBinding( 'myAB', function( x) if( missing( x)) get( 'myABguts',
> env=.GlobalEnv) else assign( 'myABguts', x, .GlobalEnv), .GlobalEnv)
> exists( 'myAB')
which appeared to return absolutely nothing-- not even a missing or a null. The
problem, of course, is that 'myABguts' doesn't exist yet; what seems to happen,
though, is that the failure to 'get' causes a *messageless* error inside the
active binding function. Is this intended?
I did think that class(env$x) != evalq( class( x), env) was a bit weird, but
because it was useful to me, I wan't going to complain. Of course it's entirely
true that reliance on the undocumented is dangerous and entirely my liability--
but even ___documented___ features in R have been know to come & go a bit :) .
Nowadays I just expect to be buffetted by the winds of change every so often...
Mark
-----Original Message-----
From: Luke Tierney [mailto:[EMAIL PROTECTED]
Sent: Sat 12/03/2005 3:03 PM
To: Bravington, Mark (CMIS, Hobart)
Cc: [EMAIL PROTECTED]; r-devel@stat.math.ethz.ch
Subject: RE: [Rd] delay() has been deprecated for 2.1.0
On Sat, 12 Mar 2005 [EMAIL PROTECTED] wrote:
> Uh-oh... I've just written a bunch of code for 'mvbutils' using
'delay', and am worried by the statement that "there should be no way to see a
<promise:...> object in R". At present, it's possible to check whether 'x' is a
promise via e.g. 'class( .GlobalEnv$x)'. This will be different to 'class( x)'
if 'x' is a promise, regardless of whether the promise has or has not been
forced yet. This can be very useful; my recent code relies on it to check
whether certain objects have been changed since last being saved. [These
certain objects are originally assigned as promises to load from individual
files. Read-accessing the object keeps it as class 'promise', whereas
write-access creates a non-promise. Thus I can tell whether the individual
files need re-saving when the entire workspace is saved.]
Relying on undocumented features when designing a package is not a
good idea. In this case the feature of env$x returning a promise
contradicts the documentation and is therefore a bug (the
documentation says that env$x should behave like the corresponding
get() expression, and that forcec promises and returns their values).
>
> The has-it-changed test has been very valuable to me in allowing fast
handling of large collections of large objects (which is why I've been adding
this functionality to 'mvbutils'); and apart from is-it-still-a-promise, I
can't think of any other R-level way of testing whether an object has been
modified. [If there is another way, please let me know!]
>
> Is there any chance of retaining *some* R-level way of checking
whether an object is a promise, both pre-forcing and post-forcing? (Not
necessarily via the 'class( env$x)' method, if that's deemed objectionable.)
This would not be a good idea. The current behavior of leaving an
evaluated promise in place is also not a documented feature as far as
I can see. It is a convenient way of implementing lazy evaluation in
an interpreter but it has drawbacks. One is the cost of the extra
dereference. Another is the fact that these promises keep alive their
environments that might otherwise be inaccessible and hence available
for GC. These environments might in turn reference large data
structures, keeping them alive. At this point it seems too complicated
to deal with this in the interpreter, but a compiler might be able to
prove that a promise can be safely discarded. (In fact a compiler
might be able to prove that a promise is not needed in the first
place).
There are other possible approaches that you might use, such as active
bindings (see the help for makeActiveBinding). If that won't do we
can look into developing a suitable abstraction that we can implement
and document in a way that does not tie the hands of the internal
implementation.
Best,
luke
> Mark Bravington
> [EMAIL PROTECTED]
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] on behalf of Duncan Murdoch
> Sent: Sat 12/03/2005 3:05 AM
> To: r-devel@stat.math.ethz.ch
> Cc: Gregory Warnes; David Brahm; Torsten Hothorn; Nagiza F.
Samatova
> Subject: [Rd] delay() has been deprecated for 2.1.0
>
>
>
> After a bunch of discussion in the core group, we have decided
to
> deprecate the delay() function (which was introduced as
"experimental"
> in R 0.50). This is the function that duplicates in R code the
> delayed evaluation mechanism (the promise) that's used in
evaluating
> function arguments.
>
> The problem with delay() was that it was handled inconsistently
(e.g.
> sometimes you would see an object displayed as <promise:...>,
> sometimes it would be evaluated); it tended to be error-prone
in usage
> (e.g. this was the cause of the bug that makes the curve()
function
> create a "pu" object in the global environment); and it was
generally
> difficult to figure out exactly what the semantics of it should
be in
> order to be consistent.
>
> delay() has been replaced with delayedAssign(). This new
function
> creates a promise and assigns it into an environment. Once one
more
> set of changes is made and delay() is gone, there should be no
way to
> see a <promise: ...> object in R: as soon as the object is
accessed,
> it will be evaluated and you'll see the value.
>
> A few packages made use of delay(). I have replaced all of
those uses
> with delayedAssign(). The most common usage was something like
the QA
> code uses:
>
> assign("T", delay(stop("T used instead of TRUE")),
> pos = .CheckExEnv)
>
> This translates to
>
> delayedAssign("T", stop("T used instead of TRUE"), eval.env =
> .GlobalEnv, assign.env = .CheckExEnv)
>
> In most cases the "eval.env = .GlobalEnv" argument is not
necessary
> (and in fact it is often a bug, as it was in curve()). The
> environment where the promise is to be evaluated now defaults
to the
> environment where the call is being made, rather than the global
> environment, and this is usually what you want.
>
> Package writers who use delay() will now get a warning that it
has
> been deprecated. They should recode their package to use
> delayedAssign instead.
>
> Examples from CRAN of this (I am not sure if this list is
exhaustive):
>
> exactRankTests, genetics, g.data, maxstat, taskPR, coin
>
> I have cc'd the maintainers of those packages.
>
> If you want a single code base for your package that works in
both the
> upcoming R 2.1.0 and older versions, this presents a problem:
older
> versions don't have delayedAssign. Here is a workalike
function that
> could be used in older versions:
>
> delayedAssign <- function(x, value,
> eval.env = parent.frame(),
> assign.env = parent.frame()) {
> assign(x, .Internal(delay(substitute(value), eval.env)),
> envir = assign.env)
> }
>
> Because this function calls the internal delay() function
directly, it
> should work in R 2.1.0+ as well without a warning, but the
internal
> function will eventually go away too, so I don't recommend
using it in
> the long term.
>
> Sorry for any inconvenience that this causes.
>
> Duncan Murdoch
>
> ______________________________________________
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>
> ______________________________________________
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>
--
Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
241 Schaeffer Hall email: [EMAIL PROTECTED]
Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
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