[Rd] WISH: Optional mechanism preventing var <<- value from assigning non-existing variable

[Bill Dunlap]

Why should it make an exception for cases where the about-to-be-assigned-to
name is present in the global environment?  I think it should warn or give
an error if the altered variable is in any environment on the search list.

-Bill

On Sun, Mar 19, 2023 at 10:54 AM Duncan Murdoch <murdoch.duncan using gmail.com>
wrote:

> I think that should be the default behaviour. It's pretty late to get
> that into R 4.3.0, but I think your proposal (with check.superassignment
> = FALSE being the default) could make it in, and 4.4.0 could change the
> default to TRUE.
>
> Duncan
>
>
>
> On 19/03/2023 12:08 p.m., Henrik Bengtsson wrote:
> > I'd like to be able to prevent the <<- assignment operator ("super
> > assignment") from assigning to the global environment unless the
> > variable already exists and is not locked.  If it does not exist or is
> > locked, I'd like an error to be produced.  This would allow me to
> > evaluate expressions with this temporarily set to protect against
> > mistakes.
> >
> > For example, I'd like to do something like:
> >
> > $ R --vanilla
> >> exists("a")
> > [1] FALSE
> >
> >> options(check.superassignment = TRUE)
> >> local({ a <<- 1 })
> > Error: object 'a' not found
> >
> >> a <- 0
> >> local({ a <<- 1 })
> >> a
> > [1] 1
> >
> >> rm("a")
> >> options(check.superassignment = FALSE)
> >> local({ a <<- 1 })
> >> exists("a")
> > [1] TRUE
> >
> >
> > BACKGROUND:
> >
> >  From help("<<-") we have:
> >
> > "The operators <<- and ->> are normally only used in functions, and
> > cause a search to be made through parent environments for an existing
> > definition of the variable being assigned. If such a variable is found
> > (and its binding is not locked) then its value is redefined, otherwise
> > assignment takes place in the global environment."
> >
> > I argue that it's unfortunate that <<- fallbacks back to assigning to
> > the global environment if the variable does not already exist.
> > Unfortunately, it has become a "go to" solution for many to use it
> > that way.  Sometimes it is intended, sometimes it's a mistake.  We
> > find it also in R packages on CRAN, even if 'R CMD check' tries to
> > detect when it happens (but it's limited to do so from run-time
> > examples and tests).
> >
> > It's probably too widely used for us to change to a more strict
> > behavior permanent.  The proposed R option allows me, as a developer,
> > to evaluate an R expression with the strict behavior, especially if I
> > don't trust the code.
> >
> > With 'check.superassignment = TRUE' set, a developer would have to
> > first declare the variable in the global environment for <<- to assign
> > there.  This would remove the fallback "If such a variable is found
> > (and its binding is not locked) then its value is redefined, otherwise
> > assignment takes place in the global environment" in the current
> > design.  For those who truly intends to assign to the global, could
> > use assign(var, value, envir = globalenv()) or globalenv()[[var]] <-
> > value.
> >
> > 'R CMD check' could temporarily set 'check.superassignment = TRUE'
> > during checks.  If we let environment variable
> > 'R_CHECK_SUPERASSIGNMENT' set the default value of option
> > 'check.superassignment' on R startup, it would be possible to check
> > packages optionally this way, but also to run any "non-trusted" R
> > script in the "strict" mode.
> >
> >
> > TEASER:
> >
> > Here's an example why using <<- for assigning to the global
> > environment is a bad idea:
> >
> > This works:
> >
> > $ R --vanilla
> >> y <- lapply(1:3, function(x) { if (x > 2) keep <<- x; x^2 })
> >> keep
> >> [1] 3
> >
> >
> > This doesn't work:
> >
> > $ R --vanilla
> >> library(purrr)
> >> y <- lapply(1:3, function(x) { if (x > 2) keep <<- x; x^2 })
> > Error in keep <<- x : cannot change value of locked binding for 'keep'
> >
> >
> > But, if we "declare" the variable first, it works:
> >
> > $ R --vanilla
> >> library(purrr)
> >> keep <- 0
> >> y <- lapply(1:3, function(x) { if (x > 2) keep <<- x; x^2 })
> >> keep
> >> [1] 3
> >
> > /Henrik
> >
> > PS. Does the <<- operator have an official name? Hadley calls it
> > "super assignment" in 'Advanced R'
> > (https://adv-r.hadley.nz/environments.html), which is where I got it
> > from.
> >
> > ______________________________________________
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> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
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