From brahm@alum.mit.edu  Wed Aug  7 20:58:35 2002
From: brahm@alum.mit.edu (David Brahm)
Date: Wed, 7 Aug 2002 15:58:35 -0400
Subject: ESS assigns .Last.value to the wrong place in R
Message-ID: <15697.31723.719742.514063@gargle.gargle.HOWL>

I repeat my emails of 11/15/01 and 2/26/02, since it looks like this ESS bug is
still not fixed in ESS 5.1.23, and I think some resolution is needed.

When help() is invoked, ESS makes a copy of .Last.value in the .GlobalEnv,
which is *not* where R normally stores it (R stores it in package:base).
When this copy becomes stale it leads to wrong answers.  The bug is in
essd-r.el, lines 63-64:

    (ess-retr-lastvalue-command
     . ".Last.value <- get(\".ess.lvsave\",inherits=TRUE)\n") ; envir=1

which should be changed (as I have done on my machine) to:

    (ess-retr-lastvalue-command
     . "assign(\".Last.value\", .ess.lvsave, envir=NULL)\n") ; package:base

Here's an example of how things can go wrong:
R> find(".Last.value")
   [1] "package:base"
R> 0+1
   [1] 1
R> .Last.value
   [1] 1
R> ?seq       # At this point a stale copy of .Last.value is written to pos=1
R> 0+2
   [1] 2
R> .Last.value                                        # WHOOPS!  WRONG VALUE!
   [1] 1
R> find(".Last.value")          # The right value is masked by the stale copy
   [1] ".GlobalEnv"   "package:base"


Whenever I bring this up, a debate begins over who should change (ESS or R).
In particular, my solution will not work if package:base is "sealed".  Here's
the tail end of our last discussion:

Rich Heiberger <rmh@surfer.sbm.temple.edu> wrote:
> ... The effect of sealing package:base from the user means that R will have
> to store .Last.value in .GlobalEnv, which is the place ESS is using.  This
> in effect is your solution 2) Change R to store .Last.value in .GlobalEnv
>
> Net result, ESS does not change behavior, R has already promised to
> change behavior.

and Kurt Hornik <hornik@mithrandir.hornik.net> replied:
> Not at all.  Sealing base does not mean that .Last.value needs to be
> stored in the user's workspace.  The standard approach would be to use
> dynamic variables which are in fact functions with internal state
> (typically maintained in their environments).  E.g., .libPaths() in R
> 1.4 or better illustrates the idea.  There might be a .Last.value()
> along these lines.  The most prominent example is options() which is
> used for manipulating .Options (see also the R FAQ): this is currently
> visible to the user but it really should not.
> 
> Let me discuss our strategy re .Last.value with some r-core members.

Also, Rodney Sparapani <rsparapa@post.its.mcw.edu> suggested:
> I may be in way over my head here.  But, I think there is a 4th solution.
> Have ESS save .Last.value as some other variable in .GlobalEnv like
> .Last.value.saved.  That way .Last.value would not be hidden, and it could
> still be retreived if someone had done a help() or whatever.

Any more thoughts?
-- 
                              -- David Brahm (brahm@alum.mit.edu)
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ESS assigns .Last.value to the wrong place in R
David Brahm