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Issue: CONSTANT-SIDE-EFFECTS (no proposal)
- To: masinter.pa@Xerox.COM
- Subject: Issue: CONSTANT-SIDE-EFFECTS (no proposal)
- From: Jon L White <email@example.com>
- Date: Thu, 27 Oct 88 20:10:08 PDT
- Cc: KMP@STONY-BROOK.SCRC.Symbolics.COM, CL-Cleanup@SAIL.Stanford.EDU
- In-reply-to: masinter.pa@Xerox.COM's message of 26 Oct 88 11:42 PDT <881026-114227-13467@Xerox>
re: While it is certainly possible to address the issue by making "constant" a
first-class run-time accessable property of data, I don't think we are
going to make a lot of progress if we choose that direction.
Well, I mentioned this avenue -- elevating to first class the hidden concept
of "read-only", or whatever it is that is different about "constants" --
because it was one of the more sensible criticisms that came out of six
months of haggling. "Haggling" first on the common-lisp@sail mailing list,
and verbally at the Scheme community meetings at Snowbird in mid July.
I would very much like to know what implementations do something akin to
the PDP10 MacLisp treatment of "constants", or maybe an extension thereto:
(1) except possibly for symbols, they are stored in a separate storage
area, which will be made write-protected at the next "disksave";
(2) they ("constants") are identified by the compiler as the result of
any evaluable program like (QUOTE <anything>). [Note that certain
datatypes other than lists and symbols may be "implicitly quoted".]
I think that Symbolics and Lucid do this, and suspect TI of doing it too;
but what of the many others?
Lucid also treats many "compiled-functions" as constants (i.e. "read-only"),
namely those loaded in from a file produced by compile-file; in effect,
(FUNCTION (LAMBDA ...)) is recognized as a "constant" too [note that this
isn't related to the discussion about a constant-function declaration, which
was primarily about what kinds of "functions" would ever be assigned to the
declared name]. For purposes of this discussion, (DEFUN FOO ...) is handled
as if it were just (SETF (SYMBOL-FUNCTION 'FOO) (FUNCTION (LAMBDA ...))).
Additionally, before investing any time in such a proposal, I wonder how
controversial it would be to insist (or, maybe just "suggest") that the
interpreter evaluate (QUOTE <anything>) not merely into <anything>, but
rather into a possibly-cached copy of <anything> that is known to be
CONSTANT-STORAGE-P. That is, the interpreter must do something very similar
to what the compiler does for constants. Without this clause -- without
rectifying the Lisp 1.5 mistake that defines QUOTE like CAR -- there is
far too much potential for interpreted/compiled mismatch. I really
wouldn't want to address "constants" if it applied _only_ to the output
of the compiler.
Garbage-collection of "constants" is a separate issue that I think
needn't be addressed by our emerging (non) proposal.
-- JonL --