Using loop on a macro to generate classes slots in Common Lisp

Question

While making classes on CLOS I've met the same pattern multiple times:

(defclass class-name () 
  ((field-1
      :initarg field-1
      :initform some-value
      :accessor field-1)
   (field-2
      :initarg field-2
      :initform another-value
      :accessor field-2)
   (...)
   (field-n
      :initarg field-n
      :initform n-value
      :accessor field-n)))

(whether this is good design is something I'll learn with time)

I've tried to tackle this with a macro so I could call, say:

(defclass-with-accessors 'class-name
   (('field-1 some-value)
    ('field-2 another-value)
    (...)
    ('field-n n-value)))

My first tackle (ignoring hygiene for now) was to split into two macros: one to make each field, other to make the class itself.

The macro to make the accessor fields seems to be correct:

(defmacro make-accessor-field (name form)
  `(,name
   :initarg ,(make-keyword name)
   :initform ,form
   :accessor ,name))

But I'm not getting the main macro right. My first attempt was:

(defmacro defclass-with-accessors (name body)
  `(defclass ,name () \(
     ,(loop for my-slot in body collect
       (make-accessor-field (car my-slot) (cadr my-slot)))))

But this isn't valid, SBCL giving me the following error at the defmacro evaluation:

; in: DEFMACRO DEFCLASS-WITH-ACCESSORS
;     (MAKE-ACCESSOR-FIELD (CAR MY-SLOT) (CADR MY-SLOT))
; 
; caught ERROR:
;   during macroexpansion of (MAKE-ACCESSOR-FIELD (CAR MY-SLOT) (CADR MY-SLOT)).
;   Use *BREAK-ON-SIGNALS* to intercept.
;   
;    The value (CAR MY-SLOT)
;    is not of type
;      (OR (VECTOR CHARACTER) (VECTOR NIL) BASE-STRING SYMBOL CHARACTER).
; 
; compilation unit finished
;   caught 1 ERROR condition
STYLE-WARNING:
   redefining COMMON-LISP-USER::DEFCLASS-WITH-ACCESSORS in DEFMACRO

What's happening exactly? How can the compiler tell the type of (car slot) when slot isn't even defined? How can I proceed to correctly define this macro?

Consider sidetracking the issue with [yasnippet](https://github.com/AndreaCrotti/yasnippet-snippets/blob/master/lisp-mode/class). — PuercoPop, Aug 29 '15 at 02:36
I guess that every lisper has done something like this somewhere in his carreer. The question is whether it is worthwhile to introduce such a thin wrapper around an already quite concise standard form. — Svante, Aug 30 '15 at 21:58

Rainer Joswig · Accepted Answer · 2015-08-29T10:08:17.210

The basic mistakes

This macro is wrong, because it should not be a macro:

(defmacro make-accessor-field (name form)
  `(,name
   :initarg ,(make-keyword name)
   :initform ,form
   :accessor ,name))

Macro forms should expand into code. This macro expands a form into a list used for a slot description. A slot description is not code, but a part of the defclass slots list. Thus you can't use a macro like this, because the value returned should be code, not a slot description list.

Also one usually would not use MAKE- in a macro name. That's more a convention. MAKE-SOMETHING should be a function. Whenever you make something, there is something being created at runtime and thus it should be a function. Sometimes one also wants to apply make to a list of things and then again, a function is preferred.

This also is wrong, because there is a symbol with a parenthesis as its name:

(defmacro defclass-with-accessors (name body)
  `(defclass ,name () \(    ;  <-  what is this?
     ,(loop for my-slot in body collect
       (make-accessor-field (car my-slot) (cadr my-slot)))))

This code is also not a good idea, because the quotes are not useful:

(defclass-with-accessors 'class-name
   (('field-1 some-value)
    ('field-2 another-value)
    (...)
    ('field-n n-value)))

If you look at defclass, it does not need quoted names. Thus in your defclass variant, there should be no quotes either.

Let's try to improve it

Example form from some imaginary code base:

(defclass-with-accessors foo
   ((bar 10)
    (baz (sin pi)))

MAKE-ACCESSOR-FIELD is now a function:

(defun make-accessor-field (name form)
  `(,name
    :initarg  ,(intern (symbol-name name) "KEYWORD")
    :initform ,form
    :accessor ,name))

The new DEFCLASS-WITH-ACCESSORS:

(defmacro defclass-with-accessors (name slot-descriptions)
  `(defclass ,name ()
     ,(loop for (slot-name form) in slot-descriptions
            collect (make-accessor-field slot-name form))))

Let's check the expansion:

macroexpand-1 expands a form once at the toplevel and pprint prints the s-expression in some automagically formatted way:

CL-USER 12 > (pprint (macroexpand-1 '(defclass-with-accessors foo
                                         ((bar 10)
                                          (baz (sin pi))))))

(DEFCLASS FOO
          NIL
          ((BAR :INITARG :BAR :INITFORM 10 :ACCESSOR BAR)
           (BAZ :INITARG :BAZ :INITFORM (SIN PI) :ACCESSOR BAZ)))

Looks okay.

Using loop on a macro to generate classes slots in Common Lisp

1 Answers1