Newsgroups: comp.lang.lisp
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From: mzaik@ee.eng.ohio-state.edu (Tahsin Mzaik)
Subject: Accessing a field of a Lisp structure
Message-ID: <DPX_J3+@ee.eng.ohio-state.edu>
Date: Wed, 10 Apr 91 19:34:34 GMT
Organization: The Ohio State University Dept of Electrical Engineering
Distribution: usa
Originator: mzaik@fujiyama.eng.ohio-state.edu

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)
      )


Any ideas? Thanks for any reply.