Calling a replacement function in a custom environment

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

Konrad Rudolph // @klmr

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