[Rd] Calling a replacement function in a custom environment

[Duncan Murdoch]

I think there isn't a way to make this work other than calling `is.na<-` 
explicitly:

   x <- b$`is.na<-`(x, TRUE)

It seems like a reasonable suggestion to make

   b$is.na(x) <- TRUE

work as long as b is an environment.

If you wanted it to work when b was a list, it would be more problematic 
because of partial name matching.  E.g. suppose b was a list containing 
functions partial(), partial<-(), and part<-(), and I call

   b$part(x) <- 1

what would be called?

Duncan Murdoch

On 27/08/2023 10:59 a.m., Konrad Rudolph wrote:
> Hello all,
> 
> I am wondering whether it’s at all possible to call a replacement function
> in a custom environment. From my experiments this appears not to be the
> case, and I am wondering whether that restriction is intentional.
> 
> To wit, the following works:
> 
> x = 1
> base::is.na(x) = TRUE
> 
> However, the following fails:
> 
> x = 1
> b = baseenv()
> b$is.na(x) = TRUE
> 
> The error message is "invalid function in complex assignment". Grepping the
> R code for this error message reveals that this behaviour seems to be
> hard-coded in function `applydefine` in src/main/eval.c: the function
> explicitly checks for `::` and :::` and permits those assignments, but has
> no equivalent treatment for `$`.
> 
> Am I overlooking something to make this work? And if not — unless there’s a
> concrete reason against it, could it be considered to add support for this
> syntax, i.e. for calling a replacement function by `$`-subsetting the
> defining environment, as shown above?
> 
> Cheers,
> Konrad
>