[MLton] ML/Lisp/Scheme, =, eq?, eqv?, equal?

[Bradley Lucier]

This is an ML semantics question, so not exactly about implementing 
MLton, but perhaps you can help me out here.

Over in the Scheme world there's a new administrative structure to 
extend Scheme and possibly come up with R6RS.  Marc Feeley, the 
editor-in-chief of this effort, asked me to contribute some changes to 
the Scheme standard that might make it more compatible with IEEE [78]54 
arithmetic.

No problem, I thought, just divorce = from eq?, eqv?, and equal? (with 
numerical = meaning what it means in IEEE arithmetic) and define eqv? 
on IEEE numbers by

> 6.1 Equivalence predicates
>
> <Replace the parts about numbers with what I've written here.  BTW,
> since each of these subclauses are supposed to be complete sentences
> (notice the periods ending them), they should each begin with capital 
> letters.>
>
> The eqv? procedure returns #t if:
>
> *  obj1 and obj2 are both IEEE 754/854 format numbers, and obj1 and 
> obj2 have
> the same base, sign, number of exponent digits, exponent bias, biased 
> exponent,
> number of significand digits, and significand.
>
> *  obj1 and obj2 are both exact numbers and are numerically equal (see 
> =, section 6.2).
>
> The eqv? procedure returns #f if:
>
> *  one of obj1 and obj2 is an exact number but the other is an inexact 
> number.
>
> *  obj1 and obj2 are both IEEE 754/854 format numbers, and the base, 
> sign,
> number of exponent digits, exponent bias, biased exponent, number of 
> significand
> digits, or significand of obj1 and obj2 differ.
>
> *  obj1 and obj2 are exact numbers for which the = procedure returns 
> #f.
>
> Note: This section does not state under which conditions eqv? returns 
> #t or #f
> for inexact numbers that are not in IEEE 754/854 format.  We recommend 
> that
> numbers not in IEEE 754/854 format for which a base, sign, number of 
> exponent
> digits, exponent bias, biased exponent, number of significand digits, 
> and
> significand can be defined follow the same rules as above.

I'm trying not to be a cowboy language designer, so I looked up ML's 
version of Real, and found at

http://www.standardml.org/Basis/real.html#Real:STR:SPEC

that basically ML equality (whatever that means, is there a precise 
Lisp equivalent?) does not apply to Reals; at least that's how I 
understand the phrase "real is not an equality type".

I think the ML folks are bright guys and gals, so I'm disturbed. Why 
not?  What's wrong with the definition I gave above?

Brad