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Re: Macro expansion: Why doesn't the invoked macro see (let (variables))
From: |
Alan Mackenzie |
Subject: |
Re: Macro expansion: Why doesn't the invoked macro see (let (variables))from the invoking one? |
Date: |
Wed, 8 Feb 2012 19:28:00 +0000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Hello, Drew!
On Wed, Feb 08, 2012 at 09:46:45AM -0800, Drew Adams wrote:
> > (defmacro BAR ()
> > (message (if (boundp 'asdf) "asdf" "no asdf"))
> > '(message "bar"))
> > (defmacro FOO () (let (asdf) `(BAR)))
> > One macro FOO binds a let variable, then invokes another
> > macro BAR. BAR doesn't see this let variable. Why not?
> That `let' binding is evaluated only when FOO is expanded.
This is what I want.
> It is not part of the resulting expansion (which is then evaluated).
> > Is there anything I can do about this?
> 1. Don't use side effects in the macro definition.
I need side effects during macro expansion (see below).
> 2. You might be looking for something like this (dunno):
> (defmacro FOO () `(let (asdf) ,(BAR)))
> When `(FOO)' is expanded, the expansion includes a `let' binding.
> This is the result of `(macroexpand '(FOO))':
> (let (asdf) "bar")
> Not sure what you're really trying to do, though.
OK, here it is in grisly detail. I want to amend define-minor-mode so
that the position of calling the mode hooks can be specified by d-m-m's
invoker. To do this, the invoker should insert the macro
(run-hooks-here)
at the appropriate place. During its expansion, run-hooks-here needs to
set a flag for define-minor-mode meaning "hook expansion already done".
Should this flag not get set, d-m-m inserts the hook calls in the default
place.
What I was trying to do looks like this:
(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
<====== There may be (run-hooks-here) here.
,@(unless hooks-run `((run-hooks-here))) <========= test flag
Should run-hooks-here appear in ,@body, it should inhibit the later
expansion of r-h-h.
########################################################################
So, we've got a dynamically scoped language. run-hooks-here is invoked
from define-minor-mode. A variable let-bound in the latter should be
dynamically available in the former. It isn't.
What am I missing here?
--
Alan Mackenzie (Nuremberg, Germany).
Re: Macro expansion: Why doesn't the invoked macro see (let (variables)) from the invoking one?, Tassilo Horn, 2012/02/08