[Rd] The default behaviour of a missing entry in an environment

[Trishank Karthik Kuppusamy]

Greetings everyone,

I have a question about the default behaviour of a missing entry in an environment.
Let us look at the following sequence of R statements:

> e <- new.env()
> e$a <- 1
> e$a
[1] 1
> e$b
NULL
> 

I think I understand the logic for returning NULL to a missing entry in an environment,
but I do not think that it is fully justified.
I am sure that the R developers must have seen this argument before,
but I wish to call for attention to this problem again,
because I think that it is important to the default safety of the R programming language.

I suppose that one could argue that a good R programmer must be careful
not to use NULL in any of his environment entries,
but I think it is better to remove altogether this burden from the programmer
and simply raise a good, old-fashioned exception when the "$" operator
encounters a missing entry in an environment.
The biggest advantage is that it will easily eliminate a whole class of programming error.
The biggest disadvantage is that it is not backwards-compatible with old R programs.

I suppose a personal solution would be to simply redefine the "$" operator in my programs.
However, I really do think that the default safety of an R environment matters very much.
At the very least, it would be nice to be able to configure the safety of a new environment,
perhaps through a parameter.

-Trishank