On 3/23/19 3:26 PM, Konrad Rudolph wrote:
I was surprised just now to find out that `topenv(emptyenv())` equals
… `.GlobalEnv`, not `emptyenv()`. From my understanding of the
description of `topenv`, it should walk up the chain of enclosing
environments (as if by calling `e = parent.env(e)` repeatedly; in
fact, that is almost exactly its implementation in envir.c) until it
hits a top level. However, `emptyenv()` has no enclosing environments
so it should be its own top-level environment (I thought).
Unfortunately the documentation on environments is relatively sparse,
and the R Internals document doesn’t mention top-level environments.
The intuition here is that topenv() should be used with a reasonable
execution environment and should return a reasonable execution
environment (e.g. things you get from parent.frame(), etc). globalenv()
is a reasonable default execution environment, emtyenv() is not). If you
are using topenv() and especially if you are using it for something that
is not an execution environment, there is a risk you are not doing the
right thing - and this list may be a good place to ask for other
solutions. topenv() as Martin said is not useful in normal user code. It
is used in object systems (S3 and S4), in the byte-code compiler,
sometimes in unit testing frameworks, etc.
As Martin already explained, reading the documentation as that
"topenv(emptyenv())" would be documented to return "emptyenv()" is
logically incorrect.
I agree, however, that it is not completely clear from the documentation
what topenv() would do when given emptyenv() as either argument and we
are working on addressing it. I've been looking into how much of the
CRAN+BIOC tests (all packages) run into the special cases of topenv(),
which btw required many hours of CPU time and some for analysis. The
cases when topenv() is used with emptyenv() are very rare, but they
exist (emptyenv() used as execution environment and S3 dispatch happens,
dummy environment for S4 methods, etc).
Best
Tomas
Concretely, I encountered this in the following code, which signals an
error if `env` is the empty environment:
while (! some_complex_condition(env) && ! identical(env, toplevel(env))) {
env = parent.env(env)
}
Of course there’s a trivial workaround (add an identity check for
`emptyenv()` in the while loop condition) but it got me wondering if
there’s a rationale for this result or if it’s “accidental”/arbitrary:
the C `topenv` implementation defaults to returning R_GlobalEnv for an
empty environment. Is this effect actually useful (and used anywhere)?
This is in R 3.4.4 but I can’t find an indication that this behaviour
was ever changed.
Cheers
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