Re: Macro expansion: Why doesn't the invoked macro see (let (variables))from the invoking one?

[Alan Mackenzie]

On Wed, Feb 08, 2012 at 02:52:45PM -0500, Stefan Monnier wrote:
> > (defmacro run-hooks-here ()
> >    (setq hooks-called t)        <================= flag variable
> >   `(run-hooks ',hook (if ,mode ',hook-on ',hook-off)))

> > (defmacro define-minor-mode (....)
> > ....
> >   (let (... hooks-run)
> > ....
> >     ,@body                  <================= expand invoker's forms

> This comment is wrong: ",@body" just plugs in the `body' without
> macro-expanding it.

> >                             <====== There may be (run-hooks-here) here.
> >     ,@(unless hooks-run `((run-hooks-here))) <========= test flag

> You can do that, but you then need to make sure the `body' gets
> macro-expanded while the `let' is live, i.e. during the expansion of the
> call to `define-minor-mode'.

Surely it should be done that way anyway?  I think I'm beginning to get a
clue, but why would expansion of macros be delayed?  That seems to
preclude normal healthy use of side effects.

> You can do it with something like

>    (defmacro define-minor-mode (....)
>    ....
>      (let (... hooks-run)
>    ....
>        ,@(macroexpand-all body) <================= expand invoker's forms
>                                <====== There may be (run-hooks-here) here.
>        ,@(unless hooks-run `((run-hooks-here))) <========= test flag


> But note that this counts as ugly.

I wouldn't disagree with that.

> We use such tricks in cl-macs.el to figure out whether `body' uses
> `return-from' within a `block' (in order to optimize away the `catch'
> that's otherwise needed), but it's ugly, inefficient, and brittle.  An
> :after-hook (or :late-code or some other name you prefer) is much
> better in this regard.

I think you're right.  Ah well.  I learnt this afternoon that quoting a
macro invocation

    '(foo-macro)

doesn't stop it being expanded.  I don't think that's in the elisp
manual.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).