setenv and let-binding process-environment

setenv and let-binding process-environment

[Michael Heerdegen]

Hello,

I've fallen into a trap.  I had expected that (something like)

(progn
  (setenv "A" "1")
  (let ((process-environment process-environment))
    (setenv "A" "2"))
  (getenv "A"))

would return "1", but it returns "2".  A similar expression and the
false assumption about it's semantics introduced a weird issue in my
code.

But I think others could come to the same conclusion that I came to
after reading the docs (e.g. setenv doc: "This function works by
modifying ‘process-environment’").

Should we document that case, or change the behavior, or do I miss
something?


BTW, after having located the cause of the trouble, I changed the code
to use

  (let ((process-environment (cons additional-binding process-environment)))
    { start the process })

following an advice in the manual.  That works without problems.


Thanks,

Michael.

Re: setenv and let-binding process-environment

[Wolfgang Jenkner]

On Sun, Mar 20 2016, Michael Heerdegen wrote:

>   (let ((process-environment process-environment))
>     (setenv "A" "2"))

This just establishes a new binding to the same object, so the code
works as expected.

The following would give the desired result (but, as you said, consing
the new value to the beginning of process-environment is better).

(let ((process-environment (copy-sequence process-environment)))
  (setenv "A" "2"))

Re: setenv and let-binding process-environment

[Michael Heerdegen]

Wolfgang Jenkner <wjenkner@inode.at> writes:

> On Sun, Mar 20 2016, Michael Heerdegen wrote:
>
> >   (let ((process-environment process-environment))
> >     (setenv "A" "2"))
>
> This just establishes a new binding to the same object, so the code
> works as expected.

Ah!  Shame on me - of course!  I though this is somehow related to how
`sentenv' internally works, but it's just all normal Lisp.


Thanks,

Michael.