NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/comp/lang/lisp/3185

Text File | 1993-01-06 | 2.6 KB | 65 lines

Path: sparky!uunet!mcsun!julienas!corton!ilog!davis
From: [e-mail] (Harley Davis)
Newsgroups: comp.lang.lisp
Subject: Re: macro question
Message-ID: <[e-mail]>
Date: 6 Jan 93 09:27:02 GMT
References: <[e-mail]>
    <[e-mail]>
Sender: [e-mail]
Organization: ILOG S.A., Gentilly, France
Lines: 51
In-reply-to: [e-mail]'s message of 6 Jan 93 04:52:05 GMT


In article <[e-mail]> [e-mail] (timoshenko) writes:

   >I would like to define a macro which in turn defines several functions.
   >For example:

   >(defmacro a (x y)
   >  `(defun ,x () (+ 2 2))
   >  `(defun ,y () (* 2 3)))

   >As it stands, the macro only defines the second function, since that is the
   >returned value.  How can I get both functions defined?

   I suppose you could do something like

   (defmacro a (x y)
      `(progn (defun ,x () (+ 2 2))
          (defun ,y () (* 2 3))))

   This is a macro that expands to an expression that, when evaluated,
   will cause both of the defun forms to be evaluated.  You're right
   that the last form in the defmacro is returned as the expansion, and
   so the first defun in your version above has no effect.

   Why do you want to do this, though?  It's likely to be confusing,
   both because it's unusual (though legal) to have defun forms that
   aren't top-level, and because this will cause functions to be defined
   without a corresponding explicit defun form in the code.  I suspect
   that there is some better way to do whatever you're trying to do;
   in particular, if you are defining functions that are used only once,
   it would be better to use a lambda-expression than to automatically
   define a named function before calling it.

Your solution is correct, but I don't really understand why you don't
like it.  If all functions "should" be defined by DEFUN, then you are
severely limiting Lisp's potential for higher level syntactic
abstractions based on macros.  For example, you would also have to be
against DEFCLASS forms which define accessor functions since these
function definitions aren't visible directly at the top level.

On the other hand, I would recommend simply as a stylistic guideline
that a macro which defines functions be only used at the top level and
have its name prefixed with DEF.

-- Harley Davis
--

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