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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: <1991Apr11.215248.9075@cbnewsl.att.com>
Date: 11 Apr 91 21:52:48 GMT
References: <DPX_J3+@ee.eng.ohio-state.edu> <1991Apr10.215214.2827@cbnewsl.att.com> <28047A1F.44CE@ibma0.cs.uiuc.edu>
Distribution: usa
Organization: AT&T Bell Labs, Whippany, NJ
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In article <28047A1F.44CE@ibma0.cs.uiuc.edu> epstein@sunc4.cs.uiuc.edu (Milt Epstein) writes:
>In <1991Apr10.215214.2827@cbnewsl.att.com> davel@cbnewsl.att.com (David Loewenstern) writes:
>
>>In article <DPX_J3+@ee.eng.ohio-state.edu> mzaik@ee.eng.ohio-state.edu (Tahsin Mzaik) writes:
>>>
>>>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)
>>
>>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))
>
>Someone else also posted this question just a couple of days ago, to
>which Barry Margolin responded suggesting a function like:
>
>(defun access-field (instance field)
>  (case field
>    (a (test-a instance))
>    (b (test-b instance))
>    (c (test-c instance))))
>
>Note that if you use a function, it doesn't matter where you get the
>slot-name from.

Oh, I agree.  It is, however, slower.
>
>I sent a message to Barry suggesting the INTERN-FORMAT version, and he
>pointed out a couple of things:
>
>1. It's better to use FIND-SYMBOL than INTERN, since the symbol (the
>structure accessor function) should already exist.

Maybe.  The cost of using find-symbol is that the macro writer
must add some error handling code, or else (access-field instance oops)
will generate some unreadable nonsense about test-NIL being undefined.

Besides, while it is good coding style, and helps the compiler generate
better code, to define before using a structure, 
it is legal to compile (access-field instance a) or
even (test-a instance) before loading the definition of test.

>
>2. The CASE version appeared to be much faster (he did some timing
>test on a Symbolics).

This, however, I do not believe.  After all, the macro 

(access-field instance a)

expands during compile time directly into
(test-a instance)

The function access-field *calls* test-a and so cannot possibly
be faster than test-a alone.

Worse, the macro (access-field instance c) expands into (test-c instance),
and during run-time is no slower than (access-field instance a).  The
function access-field is O(n), where n is the number of slots in the
test structure.

The only way the function could be faster than the macro is if you
are including the macroexpansion time with the execution time.

Comments, Barry?

>
>-- 
>Milt Epstein
>Department of Computer Science
>University of Illinois
>epstein@cs.uiuc.edu

David Loewenstern
AT&T Bell Laboratories
14B-253 
Whippany, NJ 07981
email: davel@whutt.att.com || whutt!davel
at&t: 201-386-6516