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Re: Issue: DECLARE-TYPE-FREE (Version 9)
- To: masinter.pa@Xerox.COM
- Subject: Re: Issue: DECLARE-TYPE-FREE (Version 9)
- From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
- Date: Tue, 3 Jan 89 01:31 EST
- Cc: Moon@STONY-BROOK.SCRC.Symbolics.COM, CL-Cleanup@SAIL.STANFORD.EDU
- In-reply-to: <890102-221315-1967@Xerox>
Date: 2 Jan 89 22:12 PST
If I say (DECLARE (TYPE (SIGNED-BYTE 12) X)), I'd think I'd like it to mean
that X never even momentarily holds a value that isn't of the declared
The problems I have with this are the following:
- It isn't enforceable.
- In exactly those cases where it is enforceable, it's useful to enforce.
In those case where it is not enforceable (the odd middle-ground cases
in the Examples), it doesn't help you any to enforce the restriction, and
it might get in your way.
Put another way: If you grant that a language could be internally
consistent, well-formed, and all that sort of thing with the LEXICAL
proposal, then why not attach a useful meaning to the middle ground case?
There are potentially useful things (mostly "patches") that you could
write that way, and even if you permit it, you don't hinder the compiler's
ability to make useful inferences.
- My general rule of thumb is that if someone says they need a feature
and someone else says they don't, that the person claiming they do need
the feature has an a priori more interesting case. As such, LEXICAL is
more interesting partly because the fact that some people don't need all
its features is not a compelling thing to me. If you could make a case for
why LEXICAL caused some actual -problem-, that would make the other case
more interesting. After all, nothing about LEXICAL prevents you personally
from meaning that "X never momentarily holds..." since that's compatible
with the meaning proposed in LEXICAL -- just as nothing about the way
false and the empty-list are treated in Lisp keeps you from meaning
that () always means the empty list (even if Lisp doesn't care).
Similarly here, LEXICAL admits both views of the universe while ALLOW
admits only one. To me, this seems unfair, since ALLOW restricts things
to lock out a view of the world, without providing any hint of a reason
for why that view of the world is an unreasonable one.