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- To: labrea!hilfingr%berkeley.edu@labrea.Stanford.EDU
- Subject: LEAST-POSITIVE-<mumble>-FLOAT
- From: Jon L White <edsel!jonl@labrea.Stanford.EDU>
- Date: Mon, 1 Feb 88 07:23:13 PST
- Cc: labrea!common-lisp%sail@labrea.Stanford.EDU
- In-reply-to: Paul Hilfinger's message of Sun, 31 Jan 88 14:42:15 PST <8801312242.AA29857@tully.Berkeley.EDU>
re: [in the Ada Standard ] . . . Indeed, as far as
denormalized numbers are concerned, the interpretation of some of the
constants is still under discussion. ... [denormalized numbers] are
tolerated, but nothing specifically is said about them.
Thanks for your warnings about the continuing ferment in the ADA community on
related matters. The issue, here, is the "unexpected dividends" obtained when
trapping is turned on -- that LEAST-POSITIVE-<mumble>-FLOAT may be subject to
different interpretations depending on whether producing it is a normal or an
exceptional condition. Does ADA specify these kinds of constants? No issue
of confusion arises in Common Lisp, as far as I know, with the other elective
traps (such as the inexact trap).
re: . . . I get a bit nervous seeing arguments
based on what a particular piece of hardware does. IEEE arithmetic is
a user interface standard, and says nothing of the division of labor
between chip and interface software.
I don't think the arguments arise from any peculiarites of one chip or another;
it was just that some Common Lisp implementations defaultly had trapping turned
off, and thus no one had observed the dilemma. The IEEE Standard provides for
reasonable behaviour when a particular trap is not enabled.
What really my quest hoped to get at was "Just what value are these constants
to Common Lisp users anyway?". Who, if anyone, would be affected if their
values were changed from one state to the other? Could a user profit from
the existence of two defined constants, one normalized and the other possibly
denormalized (as Symbolics has recently done)? As long as this kind of value
is specified by the language standard (CLtL, in this case), test suites will
have a "need" for them; but beyond that, I just don't know.
-- JonL --