Double unquote in a macro

[Adam Warner]

Hi all,

This doesn't work because defparameter is a macro that doesn't evaluate
the second term of the input list to obtain the symbol name:

(defmacro grab-defun-name (list)
`(defparameter (second ,list) ,list))

I want to obtain the second value of the variable list which requires an
unquote on the list and an unquote on (second ,list)--which leads to too
many commas for one backquote.

I just want to understand whether achieving this is possible without
resorting to reimplementing defparameter or using EVAL. I believe I have
struck this situation in the FAQ:

http://www.faqs.org/faqs/lisp-faq/part3/

On the other hand, EVAL can sometimes be necessary when the only
portable interface to an operation is a macro.

I need to capture each function and store its list representation
(conveniently with the same name) along with evaluating the function
because I am simultaneous using the function and parsing its elements to
produce presentation markup.

Thanks,
Adam

[Timothy Moore]

"Adam Warner" <use...@consulting.net.nz> writes:

> Hi all,
>
> This doesn't work because defparameter is a macro that doesn't evaluate
> the second term of the input list to obtain the symbol name:
>
> (defmacro grab-defun-name (list)
> `(defparameter (second ,list) ,list))
>
> I want to obtain the second value of the variable list which requires an
> unquote on the list and an unquote on (second ,list)--which leads to too
> many commas for one backquote.

Don't you just want
(defmacro grab-defun-name (list)
`(defparameter ,(second list) ,list))

>
> I just want to understand whether achieving this is possible without
> resorting to reimplementing defparameter or using EVAL. I believe I have
> struck this situation in the FAQ:
>
> http://www.faqs.org/faqs/lisp-faq/part3/
>
> On the other hand, EVAL can sometimes be necessary when the only
> portable interface to an operation is a macro.
>

I don't think this is one of those times.

Tim

[Kenny Tilton]

Adam Warner wrote in message ...

>Hi all,
>
>This doesn't work because defparameter is a macro that doesn't evaluate
>the second term of the input list to obtain the symbol name:
>
> (defmacro grab-defun-name (list)
> `(defparameter (second ,list) ,list))
>
>I want to obtain the second value of the variable list which requires an
>unquote on the list and an unquote on (second ,list)--which leads to too
>many commas for one backquote.
>

howseabout `(defparameter ,(second list) ,list)? Then your problem may be
... well, I'll shut up. But I am surprised the defparam value ",list" is not
quoted: "',list". But I am only guessing at your intent. OK, I'll shut up.

kenny
clinisys

[Adam Warner]

Hi Timothy Moore,

> Don't you just want
> (defmacro grab-defun-name (list)
> `(defparameter ,(second list) ,list))

Almost! Before the example I posted I was continually doing the same thing:

[1]> (defmacro grab-defun-name (list)
`(defparameter ,(second list) ,list))
grab-defun-name
[2]> (grab-defun-name '(defun test () 123))

*** - defparameter: non-symbol (defun test nil 123) cannot be a variable

Now I realise I was actually finding the second list value of
(quote (defun test nil 123))!

But now that I am no longer quoting the input value I need to add a quote
to the last list item:

(defmacro grab-defun-name (list)
`(defparameter ,(second list) ',list))

Now without quoting the input:

(grab-defun-name (defun test () 123)

The function is simultaneous captured and evaluted!

[7]> test
(defun test nil 123)
[8]> (test)
123

Regards,
Adam

[Adam Warner]

Hi Kenny Tilton,

You wouldn't be surprised if you tried to get it to work using this input:

(grab-defun-name '(defun test () 123))

I had originally intended to create the global variable and then evaluate
the list. Impressively it all works by the macro capturing the defun list
before it evaluates.

Thanks,
Adam

[Martin Simmons]

"Adam Warner" <use...@consulting.net.nz> wrote in message
news:pan.2002.10.15....@consulting.net.nz...

The defun won't be evaluated if you quote the list. Depending on how it will be
used, I would probably do it differently:

(defmacro defun-grab (name lambda-list &body body)
(let ((defun-form `(defun ,name ,lambda-list ,@body)))
`(progn
(defparameter ,name ',defun-form)
,defun-form)))

(defun-grab test2 () 123)

--
Martin Simmons, Xanalys Software Tools
zne...@xanalys.com
rot13 to reply

[Adam Warner]

Hi Martin Simmons,

> The defun won't be evaluated if you quote the list. Depending on how
> it will be used, I would probably do it differently:
>
> (defmacro defun-grab (name lambda-list &body body)
> (let ((defun-form `(defun ,name ,lambda-list ,@body)))
> `(progn
> (defparameter ,name ',defun-form)
> ,defun-form)))
>
> (defun-grab test2 () 123)

This is much preferred thanks Martin. And it's not just because it will be
evaluated even if I quote the list: it's also a hassle closing the bracket
on the separate defun-grab.

Regards,
Adam