Odd behavior of `eval-when-compile'

Odd behavior of `eval-when-compile'

[Jay Belanger]

I came across the following surprising (to me) behavior for
`eval-when-compile'.  Suppose I have two files:
a.el, containing
   (defvar vara 2)
   (provide 'a)
and b.el, containing
   (require 'a)
   (defvar varb (eval-when-compile (+ 1 vara)))
If I byte compile b.el, it works fine.
However, if I have a file b.el containing
   (defvar vara 2)
   (defvar varb (eval-when-compile (+ 1 vara)))
and then byte compile it (in a new Emacs session), I get an error
    Symbol's value as variable is void: vara
Is there a reason for this behavior (I didn't find it in the elisp
manual), or is this a bug?

Jay

Re: Odd behavior of `eval-when-compile'

[Stefan Monnier]

> However, if I have a file b.el containing
>    (defvar vara 2)
>    (defvar varb (eval-when-compile (+ 1 vara)))
> and then byte compile it (in a new Emacs session), I get an error
>     Symbol's value as variable is void: vara

Of course: there is no `vara' in the running Emacs: it's only present in the
b.el file which is being compiled but hasn't been loaded yet.

`require' is handled specially by the byte-compiler in that it doesn't just
place a call to `require' in the .elc file but it also loads the file during
byte-compilation.  `defvar' isn't special in this way.  Maybe we could make
`defconst' special in this way as well, but it hasn't seemed particularly
important until now.  You can just do

    (eval-and-compile (defvar vara 2))
    (defvar varb (eval-when-compile (+ 1 vara)))


-- Stefan

Re: Odd behavior of `eval-when-compile'

[Jay Belanger]

Stefan Monnier <monnier@iro.umontreal.ca> writes:
...
> Of course: there is no `vara' in the running Emacs: it's only present in the
> b.el file which is being compiled but hasn't been loaded yet.
...
>     (eval-and-compile (defvar vara 2))
>     (defvar varb (eval-when-compile (+ 1 vara)))

Thanks for the explanation; it makes sense now.

Jay