RE: [Rd] delay() has been deprecated for 2.1.0

From: Luke Tierney <luke_at_stat.uiowa.edu>
Date: Sat 12 Mar 2005 - 16:14:08 GMT

On Sat, 12 Mar 2005 Mark.Bravington@csiro.au wrote:

> 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?

What I get is this:

     > makeActiveBinding( 'myAB', function( x)
                                      if( missing( x))
                                          get( 'myABguts', env=.GlobalEnv)
                                      else assign( 'myABguts', x, .GlobalEnv),
                          .GlobalEnv)
     NULL
     Warning message:
     saved workspaces with active bindings may not work properly when loaded ...
     > exists( 'myAB')
     Error in get(x, envir, mode, inherits) : variable "myABguts" was not found

[We might want to get rid of the warning message at this point.]

Currently exists() for active bindings calls the value function even if mode is "any". Given that fact and your implementation the message makes sense to me, and you get the same message if you try to access the variable:

     > myAB
     Error in get(x, envir, mode, inherits) : variable "myABguts" was not found

You probably should write your function a bit more defensively.

For ordinary bindings that contain promises from delayed evaluation exists() does not evaluate the promise for mode "any", but does for other modes of course. The behavior of exists() for active bindings is bug. A quick look at the code suggests that it will be a bit tricky to fix, so I'm not sure it will get done before 2.1.0.

Best,

luke

>
> I did think that class(env$x) != evalq( class( x), env) was a bit weird, but because it was useful to me, I wan't going to complain. Of course it's entirely true that reliance on the undocumented is dangerous and entirely my liability-- but even ___documented___ features in R have been know to come & go a bit :) . Nowadays I just expect to be buffetted by the winds of change every so often...
>
> Mark
>
> -----Original Message-----
> From: Luke Tierney [mailto:luke@stat.uiowa.edu]
> Sent: Sat 12/03/2005 3:03 PM
> To: Bravington, Mark (CMIS, Hobart)
> Cc: murdoch@stats.uwo.ca; r-devel@stat.math.ethz.ch
> Subject: RE: [Rd] delay() has been deprecated for 2.1.0
>
>
>
> On Sat, 12 Mar 2005 Mark.Bravington@csiro.au wrote:
>
> > Uh-oh... I've just written a bunch of code for 'mvbutils' using 'delay', and am worried by the statement that "there should be no way to see a <promise:...> object in R". At present, it's possible to check whether 'x' is a promise via e.g. 'class( .GlobalEnv$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
> > mark.bravington@csiro.au
> >
> >
> > -----Original Message-----
> > From: r-devel-bounces@stat.math.ethz.ch 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
> >
> > ______________________________________________
> > R-devel@stat.math.ethz.ch mailing list
> >
https://stat.ethz.ch/mailman/listinfo/r-devel
> >
> > ______________________________________________
> > R-devel@stat.math.ethz.ch mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
> --
> 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: luke@stat.uiowa.edu
> Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
>
> ______________________________________________
> R-devel@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
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:      luke@stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu

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Received on Mon Mar 14 10:33:56 2005

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