Re: CLOS vs. subtypes of FLOAT

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

    Date: 21 May 87 12:07 PDT
    From: Masinter.pa@Xerox.COM

    You omitted the possibility where all the classes exist in all
    implementations, but that some of the classes might be empty (have no
    instances) in some implementations.

Unfortunately this elegant-sounding choice can't be taken without
changing Common Lisp incompatibly.  The problem is that Common Lisp
specifies (CLtL p.19) that in an implementation without short-floats,
(typep x 'short-float) does the same thing as (typep x 'single-float),
rather than always returning nil.  We want to keep typep consistent
with class-instance relationships, to avoid giving Common Lisp too
subtly-incompatible type systems.