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From: jeff@aiai.ed.ac.uk (Jeff Dalton)
Newsgroups: comp.lang.lisp
Subject: Re: structures
Keywords: structures
Message-ID: <4535@skye.ed.ac.uk>
Date: 22 Apr 91 18:00:14 GMT
References: <1991Apr09.091234.18122@kub.nl> <1991Apr9.202146.18803@Think.COM>
Reply-To: jeff@aiai.UUCP (Jeff Dalton)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 78

In article <1991Apr9.202146.18803@Think.COM> barmar@think.com (Barry Margolin) writes:
>In article <1991Apr09.091234.18122@kub.nl> pberck@kub.nl writes:
>>How can I 'choose slots' in a structure I have defined? Say I have the
>>structure (user) with the slots (name) and (adres), and I want
>>a function which accesses one of the two, supplied as an argument
>>to the function. (I hope I am making myself clear.)
>>For example:
>>
>>(defun prt (someone slot)
>>    (print (user-slot someone)))
>>
>>I would like to supply 'name' or 'adres' as argument (slot) to the function.
>>How do I do this? I could probably set things up differently, but I
>>would like to do it this way :)

>There's currently no portable way to do this automatically.  Eventually it
>will be doable using the CLOS metaobject protocol, though.
>
>Right now, the only thing you can do is write a function that manually
>selects between the slots, e.g.:
>
>(defun prt (someone slot)
>  (print (ecase slot
>    	   (name (user-name someone))
>    	   (adres (user-adres someone))))
>
>You can also abstract this out so that you don't have to repeat it in
>different functions:
>
>(defun user-accessor (slot)
>  (ecase slot
>    (name #'user-name)
>    (adres #'user-adres)))

Etc.

An variation of this approach I've sometimes used is to define my own
structure-defining macro that expands into a DEFSTRUCT plus some code
that records the name of the access function that goes with each slot.
A simple version is defined below.  It doesn't support any of the 
DEFSTRUCT options :INCLUDE, :CONC-NAME, etc.  The mapping from names
of slots to names of accessor functions is stored in the SLOT-MAP
property of the structure name.

(defmacro define-structure (struct-name &rest slots)
  (let* ((slot-names
           (mapcar #'(lambda (s) (if (symbolp s) s (car s)))
                   slots))
         (accessors
           (mapcar #'(lambda (s) (concat-symbol struct-name "-" s))
                   slot-names)))
    `(progn
       (defstruct ,struct-name ,@slots)
       (setf (get ',struct-name 'slot-map)
             ',(mapcar #'list slot-names accessors))
       ',struct-name)))

(defun concat-symbol (&rest parts)
  (intern (apply #'concatenate 'string (mapcar #'string parts))))

Now we can define (GET-SLOT struct slot-name) -> value to return
the value of a slot given its name.  GET-SLOT relies on TYPE-OF
to return the type of a structure.  Although one is discouraged
from using TYPE-OF in programs, it is defined to work as required
for this case.

(defun get-slot (struct slot-name)
  (let ((entry (assoc slot-name (get (type-of struct) 'slot-map))))
    (if entry
        (funcall (second entry) struct)
      (error "Unknown slot ~S for struct ~S." slot-name struct))))

Your PRT function would then be

(defun prt (someone slot)
  (print (get-slot someone slot)))

-- Jeff