The default behaviour of a missing entry in an environment

Note that one should use inherits = FALSE argument on get and exists
to avoid returning objects from the parent, the parent of the parent,
etc.

On Fri, Nov 13, 2009 at 2:27 PM, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:

On 11/13/2009 2:03 PM, Trishank Karthik Kuppusamy wrote:

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.

You get the same behaviour when asking for a nonexistent element of a list,
or a nonexistent attribute.   If you want stricter checking, don't use $,
use get():

get("b", e)

Error in get("b", e) : object 'b' not found

or check first with exists():

exists("b", e)

[1] FALSE

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.

But then it would be inconsistent with what it does in other situations.

Duncan Murdoch

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