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Re: macro temp variables


From: Pascal J. Bourguignon
Subject: Re: macro temp variables
Date: Sun, 21 Sep 2014 22:59:47 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Eric Abrahamsen <address@hidden> writes:

> "Pascal J. Bourguignon" <address@hidden> writes:
>
>> Eric Abrahamsen <address@hidden> writes:
>>
>>> I've never actually needed to write a macro that provided temporary
>>> local variables, and consequently am not very good at it. Despite having
>>> read the docs and basically followed the examples there, my attempt is
>>> producing errors.
>
>> The problem is that you want your body to access those variables.  So
>> the body must know their names.  But you are computing new names that
>> are uninterned, and therefore unaccessible.  Therefore there's no way to
>> access those temporary variables, from the body.  Only code generated by
>> your macro could access those variables (since the macro has their name
>> stored in its head .. tags variables.
>
> Wow, thanks so much to both of you, this is an excellent lesson in using
> macros. Richard, the walk-through was much appreciated -- things like
> that always start out seeming obvious, but then by step three or so I'm
> perplexed. pp-macroexpand-* will be helpful in the future.
>
> So I think I've got it. The behavior that everyone's trying to avoid by
> using make-symbol is in fact the precise behavior I want: leaking
> symbols from the macro into the body code. I'm going to hold my brain
> perfectly still until that sinks in.
>
> I had suspected that I'd have to do something along the lines of
> Pascal's solution -- explicitly providing the args to be bound on each
> pass -- but I'm inclined to go with Richard's version, since the whole
> point of this function is to be a *scratch*-buffer way of doing one-off
> things with Org headings, and I'd like it to be as easy to write as
> possible.


Then you would just bind those normal, interned, symbols, and document
that your macro does this binding:

(defmacro org-iterating-headings (&rest body)
  "Binds the following lexical variables: head item todo tags body-ars
to the corresponding org headings, while iterating headings."
  (destructuring-bind (head item todo tags body-pars) vars
    `(call-org-iterating-headings
      (lambda (head item todo tags body-pars) ,@body))))

Notice the advantage of factoring yout the call-org-iterating-heading
function: you can easily change the user interface of your macro after
the fact, and even provide both.


       
-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk


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