R devel - May 2020 - Surpising behavior when using an active binding as loop index in R 4.0.0

Hi,

I stumbled upon a surprising behavior when using an active binding as a
loop index variable in R 4.0.0. In contrast, the behavior observed in R
3.6.1 is in line with my expectations.

Consider the code below:

makeActiveBinding("i", 
	function(value) {
		if (missing(value)) {
			x
	} else {
		print("set")
		x <<- value
	}
	}, globalenv())

i <- 1     # output "set"
print(i)   # output [1] 1

# Surprising behavior starts here:
for(i in 2:3) print(i) # output [1] "set"
                       #        NULL
                       #        NULL

print(i)               # output NULL
print(x)               # output NULL

i <- 4                 # output "set"
print(i)               # ouput [1] 4
print(x)               # ouput [1] 4

Regards
Thomas

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Possibly just a symptom of the earlier behavior, but I'll amend my
example, below, with an even more disturbing observation:

Am Sat, 23 May 2020 13:19:24 +0200
schrieb Thomas Friedrichsmeier via R-devel <r-devel at r-project.org>:
[...]
> Consider the code below:
> 
> makeActiveBinding("i", 
> 	function(value) {
> 		if (missing(value)) {
> 			x
> 	} else {
> 		print("set")
> 		x <<- value
> 	}
> 	}, globalenv())
> 
> i <- 1     # output "set"
> print(i)   # output [1] 1
> 
> # Surprising behavior starts here:
> for(i in 2:3) print(i) # output [1] "set"
>                        #        NULL
>                        #        NULL
> 
> print(i)               # output NULL
> print(x)               # output NULL
> 
> i <- 4                 # output "set"
> print(i)               # ouput [1] 4
> print(x)               # ouput [1] 4
ls()
# Error in ls() : 
#  Value of SET_STRING_ELT() must be a 'CHARSXP' not a 'NULL'

Regards
Thomas

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A shorter reproducible example:

example(makeActiveBinding)
for (fred in 1:3) { 0 }
ls()

Both problems go away if you first do

compiler::enableJIT(2)

So looks like a bug in compiling the for loop.

-Deepayan

On Sat, May 23, 2020 at 5:45 PM Thomas Friedrichsmeier via R-devel
<r-devel at r-project.org> wrote:
>
> Possibly just a symptom of the earlier behavior, but I'll amend my
> example, below, with an even more disturbing observation:
>
> Am Sat, 23 May 2020 13:19:24 +0200
> schrieb Thomas Friedrichsmeier via R-devel <r-devel at
r-project.org>:
> [...]
> > Consider the code below:
> >
> > makeActiveBinding("i",
> >       function(value) {
> >               if (missing(value)) {
> >                       x
> >       } else {
> >               print("set")
> >               x <<- value
> >       }
> >       }, globalenv())
> >
> > i <- 1     # output "set"
> > print(i)   # output [1] 1
> >
> > # Surprising behavior starts here:
> > for(i in 2:3) print(i) # output [1] "set"
> >                        #        NULL
> >                        #        NULL
> >
> > print(i)               # output NULL
> > print(x)               # output NULL
> >
> > i <- 4                 # output "set"
> > print(i)               # ouput [1] 4
> > print(x)               # ouput [1] 4
>
> ls()
> # Error in ls() :
> #  Value of SET_STRING_ELT() must be a 'CHARSXP' not a
'NULL'
>
> Regards
> Thomas
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