From: Glenn Meader
Subject: Macros for the elementary lithp programmer
Date:
Message-ID: <4455@mit-eddie.MIT.EDU>
Could someone who is not offended by the triviality of this question please
tell me how to test things inside a defmacro definition. By the way I use
Common Lisp.
Example: (defmacro foo(&rest body))
I want this form to translate to (/ body) if (oddp (car body))
and to (* body) if (evenp (car body)). This example is only
to demonstrate a principle. My LISP, when given (foo 2 3 4), complains
that '2 is a bad function. Also is there a more appropriate group for
this type of elementary question? Thanks.
From: Barry Margolin
Subject: Re: Macros for the elementary lithp programmer
Date:
Message-ID: <4483@mit-eddie.MIT.EDU>
In article <····@mit-eddie.MIT.EDU> ·····@mit-eddie.MIT.EDU (Glenn Meader) writes:
>Example: (defmacro foo(&rest body))
>
>I want this form to translate to (/ body) if (oddp (car body))
>and to (* body) if (evenp (car body)). This example is only
>to demonstrate a principle. My LISP, when given (foo 2 3 4), complains
>that '2 is a bad function. Also is there a more appropriate group for
>this type of elementary question? Thanks.
It would have helped if you had included the rest of your macro in your
example, but I think I can guess what it looked like:
(defmacro foo (&rest body)
(if (oddp (car body))
`(/ ,body)
`(* ,body)))
The problem is not in the way you do the test, it is in the expansions.
Since the value of BODY is a list, the expansion of (foo 2 3 4) is
(* (2 3 4))
Now it should be obvious why it complained that 2 is a bad function.
The fix is to use ". ,body" in the macro instead of ",body".
One way to learn these things by yourself is to make use of the
MACROEXPAND function to see what the expansion is. If you're lucky,
your implementation will have an interactive macroexpander, like the
Lisp Machine's MEXP.
--
Barry Margolin
ARPA: ······@MIT-Multics
UUCP: ..!genrad!mit-eddie!barmar
From: Pascal Kuczynski
Subject: Re: Macros for the elementary lithp programmer
Date:
Message-ID: <1714@crcge1.UUCP>
In article <····@mit-eddie.MIT.EDU> ·····@mit-eddie.UUCP writes:
>Example: (defmacro foo(&rest body))
>
>I want this form to translate to (/ body) if (oddp (car body))
>and to (* body) if (evenp (car body)). This example is only
>to demonstrate a principle. My LISP, when given (foo 2 3 4), complains
>that '2 is a bad function.
(defmacro foo (&rest body)
(cons (if (oddp (car body))
'/
'*)
body))
or more legible:
(defmacro foo (&rest body)
(if (oddp (car body))
`(/ ,@body)
`(* ,@body)))
--
Pascal Kuczynski.
Laboratoires de Marcoussis. Route de Nozay. Marcoussis 91460. FRANCE
...mcvax!inria!crcge1!kuczynsk