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From: davel@cbnewsl.att.com (David Loewenstern)
Newsgroups: comp.lang.lisp
Subject: Re: Accessing a field of a Lisp structure
Message-ID: <1991Apr10.215214.2827@cbnewsl.att.com>
Date: 10 Apr 91 21:52:14 GMT
References: <DPX_J3+@ee.eng.ohio-state.edu>
Distribution: usa
Organization: AT&T Bell Labs, Whippany, NJ
Lines: 37

In article <DPX_J3+@ee.eng.ohio-state.edu> mzaik@ee.eng.ohio-state.edu (Tahsin Mzaik) writes:
>Hello Lisp experts:
>
>I have a common lisp question:
>
>I would like to write a macro that can access a particular field 
>in a structure. 
>
>Example: Let the structure be:
>      (defstruct test a b c)
>
>      (make-test instance)
>
>I would like to wite a macro 'access-field' that I can invoke
>something like  
>
>      (access-field instance 'a)
>
>in order to access field a of the structure. I have tried the
>following, but it does not work:
>
>      (defmacro access-field (instance field)
>        '(setq (test-,field ,instance) any-value)
>      )

Try:
(defmacro access-field (instance field)
  `(,(intern (format nil "~A-~A" 'test field)) ,instance))

Note that this will allow 
(access-field instance a)
to work but not, for example
(access-field instance (get-slot-name-from-some-wierd-place))

>
>
>Any ideas? Thanks for any reply.