Using the content of a dynamic variable in a macro

Using the content of a dynamic variable in a macro

[Damien Cassou]

Hi,

I'm wondering why the code below works but won't compile.

foo.el:
  (defvar foo-var '((message "hello world")))
  
  (defmacro foo-macro ()
    `(progn
       ,@foo-var))
  
  (defun foo-fun ()
    (foo-macro))


$ emacs --batch -l foo.el --eval '(foo-fun)'
hello world

$ emacs --batch --eval '(find-file "foo.el")' --eval '(emacs-lisp-byte-compile)'
In toplevel form:
foo.el:32:1: Error: Symbol’s value as variable is void: foo-var

Why isn't the compiler aware of the foo-var variable?

-- 
Damien Cassou

"Success is the ability to go from one failure to another without
losing enthusiasm." --Winston Churchill

Re: Using the content of a dynamic variable in a macro

[Michael Heerdegen]

Damien Cassou <damien@cassou.me> writes:

> I'm wondering why the code below works but won't compile.
>
> foo.el:
>   (defvar foo-var '((message "hello world")))
>   
>   (defmacro foo-macro ()
>     `(progn
>        ,@foo-var))
>   
>   (defun foo-fun ()
>     (foo-macro))
> [...]
> foo.el:32:1: Error: Symbol’s value as variable is void: foo-var
>
> Why isn't the compiler aware of the foo-var variable?

The compiler is aware of it, but it doesn't eval the definition.  It is
a feature that the compiler normally does not load any part of the
program when compiling, including variable definitions.  Note that in
the general case, evaluating a defvar form might require to load more
parts of a file, or even the complete file.

If you do need to eval something when compiling, use `eval-when-compile'
or `eval-and-compile' respectively.


Michael.