Lisp: Pass &rest parameters to a macro

Question

I'm trying to build a function to access records in a database as such:

(select :title "milk" :rating 7)

However, it's just returning all of the records in the database. I believe this is because I'm passing &rest arguments to a macro and it's interpreting the parameter name "fields" literally. I've tried removing &rest from my macro, but then I get an error about not passing a list. I've tried calling (list fields) in various places to no avail (in the macro, in the function that calls the macro, in the function that the macro calls).

The following code works as expected:

(select-custom (where :title "milk" :rating 7))

And returns only the records which match the arguments.

source code:

(defun select (&rest fields)
  (select-custom (where fields)))

(defun select-custom (selector-function)
  (remove-if-not selector-function *db*))

(defmacro where (&rest fields)
  `#'(lambda (cd) (and ,@(make-comparison-list fields))))    

(defun make-comparison-list (fields)
  (loop while fields
        collecting (make-comparison-exp (pop fields) (pop fields))))

(defun make-comparison-exp (field value)
  `(equal (getf cd ,field) ,value))

Macros are expanded at compile time, they can't make use of run-time variables. — Barmar
– Barmar, Commented Dec 18, 2015 at 22:57

6502 · Accepted Answer · 2015-12-19 10:35:54Z

7

You could just make select a macro instead

(defmacro select (&rest fields)
  `(select-custom (where ,@fields)))

You can check that

(macroexpand-1 '(select :title "milk" :rating 7))

returns

(SELECT-CUSTOM (WHERE :TITLE "milk" :RATING 7))

answered Dec 19, 2015 at 10:35

6502

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Comments

Barmar · Accepted Answer · 2015-12-18 23:02:02Z

2

The best solution would be to create a function version of where. But as a simple kludge, you can use eval

(defun select (&rest fields)
    (select-custom (eval `(where ,@fields))))

answered Dec 18, 2015 at 23:02

Barmar

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