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

[Lynn Winebarger <owinebar@gmail.com>]

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On Mon, Feb 27, 2023, 2:05 PM Basil Contovounesios <contovob@tcd.ie> wrote:

> Richard Stallman [2023-02-26 22:24 -0500] wrote:
>
> >   > Lexical binding isn't the issue - the variable is global in scope.
> >   > The issue is that when you explicitly run the byte-compiler in batch
> mode,
> >   > the "defvar" expression is only compiled, not evaluated, while the
> defmacro
> >   > is evaluated, and the application of the macro function is evaluated
> during
> >   > compilation.
> >
> > Should we change this to evaluate defvar in batch mode?
>
> The byte-compiler avoids evaluating the defvar also in non-batch mode.
>
> So the question is how much of the code that it is compiling should the
> byte-compiler evaluate.
>
> We may need to reuse or possibly extend the notions of Lisp code safety
> we have before the byte-compiler can start evaluating arbitrary
> expressions.  I'm not sure it's worth the complexity.
>

I didn't word my response very precisely.  I mentioned batch mode because
the user was byte compiling in a separate batch process, so it seemed
clearer to say that the compile time and evaluation time environments must
be distinct in that case.
The difference I described above happens any time a file is compiled as a
unit and not evaluated.
One reason to not make defvar (and defun) take effect in the compile-time
environment is that newcomers to across may mistakenly believe that
changing the value of a variable should be able to dynamically change the
code produced by instances already loaded.  In other words, believing that
macro expansion can be dynamic, when in fact the result of the expansion is
a constant embedded in the compiled file (modulo optimization by the byte
compiler).

Lynn

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