Comments on: Constants In Lisp Macro Calls

By: Head Geek

Head Geek — Wed, 10 Oct 2007 21:52:18 +0000

I’ve discovered that the proper answer to this is to use SYMBOL-VALUE instead of EVAL. As described on this page:

Novices almost always misuse EVAL. When experts use EVAL, they often would be better off using APPLY, FUNCALL, or SYMBOL-VALUE. Use of EVAL when defining a macro should set off a warning bell — macro definitions are already evaluated during expansion. See also the answer to question 3-12. The general rule of thumb about EVAL is: if you think you need to use EVAL, you’re probably wrong.

I’ve confirmed that SYMBOL-VALUE will do the trick for this too.

By: Head Geek

Head Geek — Sun, 30 Sep 2007 04:35:40 +0000

Nifty, yes, but everything I’ve read suggests that you avoid using it. Although this would be a special case where it’s okay to do so, because Lisp macros are processed at compile-time anyway.

By: Joshua Lee

Joshua Lee — Sun, 30 Sep 2007 00:54:54 +0000

(newbie lisper hat on)

(eval) is pretty nifty in general, an interpreter within an interpreter, if I remember correctly.

By: Head Geek

Head Geek — Sat, 29 Sep 2007 01:13:08 +0000

Nice — it does the trick. Thanks, that’ll make things a little easier.

By: C-Squared

C-Squared — Fri, 28 Sep 2007 15:25:56 +0000

You’re right, the macro isn’t translating the constant to its value before using it. As an example of this:

(defconstant +shift-array-size+ 10)

(defmacro constant-macro (var)
(format t “VAR is ~a~%” var))

(defun constant-macro-test ()
(constant-macro +shift-array-size+))

When compiled, the following is printed:

VAR is +SHIFT-ARRAY-SIZE+

So, to get it to use the constant, you have to force it to evaluate the constant string inside the macro by using (eval). For example:

(defconstant +shift-array-size+ 10)

(defmacro constant-macro (var)
(format t “VAR is ~a~%” (eval var)))

(defun constant-macro-test ()
(constant-macro +shift-array-size+))

When compiled, the following is printed:

VAR is 10

So, to have your shift-array-down function work like you want it you, you just need to have it evaluate the arr-length parameter:

(defmacro shift-array-down (arr arr-length newitem)
`(shiftf ,@(loop for i from (1- (eval arr-length)) downto 0 collecting `(svref ,arr ,i)) ,newitem))

That will work both when the arr-length is a number and when it’s a reference to a constant.

By: Head Geek

Head Geek — Fri, 28 Sep 2007 04:12:14 +0000

It seems that I have no problem if I use the constant within the macro itself, but I’m not permitted to use it as a parameter to the macro. Which, again, doesn’t make sense — in this case, it means that I’d have to make a different macro for each different size of array I want to use this with. I’ll keep looking for an answer.