Trouble with eval-when-compile.

Trouble with eval-when-compile.

[Alan Mackenzie]

Hi, everybody.

Any chance of a tip?

I have two files like this (grossly simplified, of course):

when-compile-sub.el:
    (defconst str "Foo")
    (provide 'when-compile-sub)

when-compile.el:
    (eval-when-compile
      (require 'when-compile-sub))

    (defconst strng `,str)
    (message "strng is %s" strng)

I think it's clear what I want to do - that is, to (require
when-compile-sub.elc) only at compile time, and use the value of a
constant at runtime in the main file.

Both files compile OK.  But when I try to load when-compile.elc, with

    emacs -Q -batch -l when-compile.elc

, I get the error message:

    Symbol's value as variable is void: str

What is going on here?  str is merely a compile time constant.  Why is
anything about str still hanging around at runtime?

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Trouble with eval-when-compile.

[Helmut Eller]

* Alan Mackenzie [2011-08-31 13:55] writes:

> Hi, everybody.
>
> Any chance of a tip?
>
> I have two files like this (grossly simplified, of course):
>
> when-compile-sub.el:
>     (defconst str "Foo")
>     (provide 'when-compile-sub)
>
> when-compile.el:
>     (eval-when-compile
>       (require 'when-compile-sub))
>
>     (defconst strng `,str)
>     (message "strng is %s" strng)
>
> I think it's clear what I want to do - that is, to (require
> when-compile-sub.elc) only at compile time, and use the value of a
> constant at runtime in the main file.

(defconst strng (eval-when-compile str))

would work better.

>
> Both files compile OK.  But when I try to load when-compile.elc, with
>
>     emacs -Q -batch -l when-compile.elc
>
> , I get the error message:
>
>     Symbol's value as variable is void: str
>
> What is going on here?

`,str is rewritten to str

> str is merely a compile time constant.  Why is
> anything about str still hanging around at runtime?

The value of a named constant is stored in the value slot of its symbol.
I guess that's less problematic with regard to eq-ness than dumping the
value to the .elc file and reconstructing it on load time.

Helmut