[Rd] delayedAssign

[Gabor Grothendieck]

Thanks for the explanation.

For lists either: (a) promises should be evaluated as they
enter the list or (b) promises evaluated as they exit the
list (i.e. as they are compared, inspected, etc.).  I gather
the intent was (a) but it does not happen that way due to
a bug in R.   Originally I thought (b) would then occur but
my surprise was that it does not occur either which is why
I feel its more serious than I had originally thought.

I think its ok if promises only exist in environments and not
lists.  Items that would be on my wishlist would be to be able
to do at R level the two mentioned previously

https://stat.ethz.ch/pipermail/r-devel/2007-September/046943.html

and thirdly an ability to get the evaluation environment, not just the
expression,
associated with a promise -- substitute only gets the expression.
Originally I thought I would need some or all of these wish items
and then thought not but am back to the original situation again as I use
them more and realize that they are at least important
for debugging (its very difficult to debug situations involving promises as
there is no way to inspect the evaluation environment so you are never sure
which environment a given promise is evaluating in) and possibly
for writing programs as well.

On 9/27/07, Luke Tierney <luke at stat.uiowa.edu> wrote:
> On Wed, 26 Sep 2007, Gabor Grothendieck wrote:
>
> > I thought that perhaps the behavior in the previous post,
> > while inconsistent with the documentation, was not all that
> > harmful but I think its related to the following which is a potentially
> > serious bug.
>
> The previous discussion already established that as.list of an
> environment should not return a list with promises in as promises
> should not be visible at the R level.  (Another loophole that needs
> closing is $ for environments). So behavior of results that should not
> exist is undefined and I cannot see how any such behavior is a further
> bug, serious or otherwise.
>
> > z is a list with a single numeric component,
> > as the dput output verifies,
>
> Except it isn't, as print or str verify, which might be a problem if z
> was an input these functions should expect, but it isn't.
>
> > yet we cannot compare its first element
> > to 7 without getting an error message.
> >
> > Later on we see that its because it thinks that z[[1]] is of type "promise"
>
> As z[[1]] is in fact of type promise that would seem a fairly
> reasonable thing to think at this point ...
>
> > and even force(z[[1]]) is of type "promise".
>
> which is consistent with what force is documented to do. The
> documentation is quite explicit that force does not do what you seem
> to be expecting.  That documentation is from a time when delay()
> existed to produce promises at the R level, which was a nightmare
> because of all the peculiarities it introduced, which is why it was
> removed.
>
> force is intended for one thing only -- replacing code like this:
>
>       # I know the following line look really stupid and you will be
>       # tempted to remove it for efficiency but DO NOT: it is needed
>       # to make sure that the formal argument y is evaluated at this
>       # point.
>       y <- y
>
> with this:
>
>      force(y)
>
> which seems much clearer -- at least it suggest you look at the help
> page for force to see what it does.
>
> At this point promises should only ever exist in bindings in
> environments. If we wanted lazy evaluation constructs more widely
> there are really only two sensible options:
>
>     The Scheme option where a special function delay creates a deferred
>     evaluation and another, called force in Scheme, forces the evaluation
>     but there is no implicit forcing
>
> or
>
>     The Haskell option where data structurs are created lazily so
>
>         z <- list(f(x))
>
>     would create a list with a deferred evaluation, but any attempt to
>     access the value of z would force the evaluation. So printing z,
>     for example, would force the evaluation but
>
>        y <- z[[1]]
>
>     would not.
>
> It is easy enough to create a Delay/Force pair that behaves like
> Scheme's with the tools available in R if that is what you want.
> Haskell, and other fully lazy functional languages, are very
> interesting but very different animals from R. For some reason you
> seem to be expecting some combination of Scheme and Haskell behavior.
>
> Best,
>
> luke
>
> >
> >> f <- function(x) environment()
> >> z <- as.list(f(7))
> >> dput(z)
> > structure(list(x = 7), .Names = "x")
> >> z[[1]] == 7
> > Error in z[[1]] == 7 :
> >  comparison (1) is possible only for atomic and list types
> >> force(z[[1]]) == 7
> > Error in force(z[[1]]) == 7 :
> >  comparison (1) is possible only for atomic and list types
> >>
> >> typeof(z)
> > [1] "list"
> >> typeof(z[[1]])
> > [1] "promise"
> >> typeof(force(z[[1]]))
> > [1] "promise"
> >> R.version.string # Vista
> > [1] "R version 2.6.0 beta (2007-09-23 r42958)"
> >
> >
> > On 9/19/07, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> >> The last two lines of example(delayedAssign) give this:
> >>
> >>> e <- (function(x, y = 1, z) environment())(1+2, "y", {cat(" HO! "); pi+2})
> >>> (le <- as.list(e)) # evaluates the promises
> >> $x
> >> <promise: 0x032b31f8>
> >> $y
> >> <promise: 0x032b3230>
> >> $z
> >> <promise: 0x032b3268>
> >>
> >> which contrary to the comment appears unevaluated.  Is the comment
> >> wrong or is it supposed to return an evaluated result but doesn't?
> >>
> >>> R.version.string # Vista
> >> [1] "R version 2.6.0 alpha (2007-09-06 r42791)"
> >>
> >
> > ______________________________________________
> > R-devel at r-project.org 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 at stat.uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu
>