[Rd] WISH: Optional mechanism preventing var <<- value from assigning non-existing variable
Duncan Murdoch
murdoch@dunc@n @end|ng |rom gm@||@com
Sun Mar 19 20:39:41 CET 2023
On 19/03/2023 2:43 p.m., Gabriel Becker wrote:
> I have to say <<- is a core debugging tool when assigning into the
> global environment. I suppose I could use assign but that would be
> somewhat annoying.
>
> That said I'm still for this change, the vast overwhelming number of
> times that <<- is in my package code - already rare but it does happen -
> it would absolutely be a bug (typo most likely) for it to get to the
> global environment and assign into it. Assigning into thr global
> environment from package code is a serious anti pattern anyway.
>
> To be honest from the developer perspective what id personally actually
> want is an assigner that was willing to go up exactly one frame from the
> current one to find its binding. That is how I essentially always am
> using <<- myself.
This sounds like a linter would be appropriate: any time you make an
assignment that goes more than one level up, it warns you about it.
Other linter rules could limit the destination in other ways, e.g.
assigning to globalenv() or things in the search list could be disallowed.
Another error I've made a few times is to use "<-" by mistake when "<<-"
was intended. A linter could detect this by seeing both `x <- value1`
and `x <<- value2` in the same context. That's legal, but (for me at
least) it usually indicates that one of them is a typo.
Duncan Murdoch
>
> ~G
>
> On Sun, Mar 19, 2023, 11:16 AM Bill Dunlap <williamwdunlap using gmail.com
> <mailto:williamwdunlap using gmail.com>> wrote:
>
> 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 <mailto: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
> <https://adv-r.hadley.nz/environments.html>), which is where I got it
> > > from.
> > >
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