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

6 messages · Henrik Bengtsson, Bill Dunlap, Gabriel Becker +1 more

Original
Henrik Bengtsson
# 
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
[1] FALSE
Error: object 'a' not found
[1] 1
[1] TRUE


BACKGROUND:
"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
This doesn't work:

$ R --vanilla
Error in keep <<- x : cannot change value of locked binding for 'keep'


But, if we "declare" the variable first, it works:

$ R --vanilla
/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.
Duncan Murdoch
# 
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:
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 at gmail.com>
wrote:

  
  


  


      

    

    

      
      

        


  

        

      Duncan Murdoch
    

    

      
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On 19/03/2023 2:15 p.m., Bill Dunlap wrote:

            

      
      
      
    

        
I'd say code like this should work:

   x <- NULL
   f <- function() x <<- 123
   f()

and then x should be changed to 123 unless the binding to x is locked. 
I don't see why it should matter if this code is in local() or in a 
function, or if it is run at the top level.

For most things on the search list, the binding would be locked, but we 
do allow people to attach environments, and then they'd be on the search 
list, so this code should work too:

   g <- function() {

     attach(environment())

     x <- NULL
     f <- function() x <<- 123
     f()

   }

What shouldn't work would be something like

   mean <<- 3

but it already doesn't work (contrary to the documentation), giving

   Error: cannot change value of locked binding for 'mean'

(which makes sense; what if I locked a binding in the global 
environment?  Then we'd go to the fallback, but the fallback can't work, 
because the binding is already there but locked...)

Duncan Murdoch

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Gabriel Becker
    

    

      
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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.

~G

        
On Sun, Mar 19, 2023, 11:16 AM Bill Dunlap <williamwdunlap at gmail.com> wrote:

        

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      Duncan Murdoch
    

    

      
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On 19/03/2023 2:43 p.m., Gabriel Becker wrote:

            

      
      
      
    

        
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