On Fri, Jan 26, 2024 at 08:16:46PM +0000, Alan Mackenzie wrote:
> Hello, Emacs.
> 
> I have a problem in a macro, where I have the expression ,expr.  I want
> to evaluate it, but get the error message void-variable (expr).
> 
> In the good old days, I could have tested this with
> 
>     (if (boundp 'expr) (eval ,expr))
> 
> , but now, with lexical variables, I can't find the equivalent to boundp.
> Is there such a facility for lexical variables, and if so what is it?

To offer something which might be half-right, but less whimsical
than the typical Andreas's answers: lexical variables are only
"known" at compile time (something to bear in mind when building
your macro; I take you know that). But functions, like "boundp"
take a symbol at run time and query it. The whole compile-time
"world" is now somewhere under the Atlantic Ocean and only
legends talk about it :-)

One work-around would be "ignore-errors" (I take this is roughly
what Common Lispers do). Assume lexical bindings, assume foo hasn't
been defined:

  (ignore-errors foo)
  => nil

  (let ((foo 12))
    (ignore-errors foo))
  => 12

(Ignore-errors is a macro, so it gets expanded when useful to you)

Perhaps there's a solution along these lines.

Cheers, hth
-- 
tomás