Path: utzoo!attcan!uunet!dalsqnt!pollux!ti-csl!pf@csc.ti.com
From: pf@csc.ti.com (Paul Fuqua)
Newsgroups: comp.lang.lisp
Subject: Re: Common Lisp Problems with Hash-table in Macros
Message-ID: <65699@ti-csl.CSNET>
Date: 14 Dec 88 21:30:09 GMT
References: <701@crin.crin.fr>
Sender: news@ti-csl.CSNET
Organization: TI Computer Science Center, Dallas
Lines: 34


    Date: Tuesday, December 13, 1988  1:15pm (CST)
    From: stephan at crin.crin.fr (Stephan BRUNESSAUX)
    Subject: Re: Common Lisp Problems with Hash-table in Macros
    Newsgroups: comp.lang.lisp

    On Explorer, the hash table seems to be dumped but no longer works since you
    cannot add or remove any entries.

The problem is that the quoted-constant hash-table is being compiled
into a read-only area, and MAPHASH errors when it tries to write to the
hash-table-lock.  Our compiler guy entered a bug-report on the problem,
since MAPHASH is not a destructive operation, so hash-tables ought to be
special-cased.  His further comments:

"This is interesting because the ANSI Common Lisp compiler subcommittee
is currently discussing rules for how constants are to be dumped and
loaded and one of the stickiest problems has been deciding which
attributes of a hash table need to be preserved.  I had thought the
issue was rather academic since I hadn't ever seen anyone try to use a
hash table as a constant before."
    
    My second question is : Am I right to write that kind of macro ? 

I would not write such a macro because any quoted constant is likely to
be write-protected -- you shouldn't be modifying a quoted constant in
your program, it's "constant."  I have a distaste for dumping complex
structures as constants in any case, preferring to write macros that
expand into construction forms that are evaluated at load time.

Paul Fuqua
Texas Instruments Computer Science Center, Dallas, Texas
CSNet:  pf@csc.ti.com (ARPA too, sometimes)
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