[Scheme-reports] clarification of proposal for eqv?/eq? on procedures

It was pointed out that my counter to Will's proposal may

not have been clear.

I propose we restore the semantics of eqv? and eq? on

procedures to IEEE/R5RS as the WG had intended.

From this we make only one change, which is what the

WG voted on in ballot item #467, which is that eqv? may

make a broader distinction than eq?. However, this is

in the context of retaining the R5RS semantics for eq?

on procedures, so effectively we will duplicate the

same semantics for eqv? and eq? on procedures.

The change is largely ideological, and would make it

easier should a future standard wish to make eq?

unspecified on procedures. It would only break R5RS

programs in very pathological cases, and only in

implementations which as yet do not exist.

Unfortunately, not using eqv? to define eq? on

procedures becomes clumsy. A potential specification

is as follows, though I'm open to suggestions for

improvement:

----

The eqv? procedure returns #t if:

* obj1 and obj2 are procedures whose location tags are

equal (section 4.1.4).

The eqv? procedure returns #f if

* obj1 and obj2 are procedures that would behave

differently (return different values or have different side

effects) for some arguments.

...

On symbols, booleans, the empty list, pairs, records, and

non-empty strings, vectors, and bytevectors, eq? and eqv? are

guaranteed to have the same behavior. On procedures eq? returns

true if the arguments' location tags are equal, and returns false

if the arguments would behave differently for some arguments. On

numbers and characters, eq?'s behavior is

implementation-dependent, but it will always return either true

or false, and will return true only when eqv? would also return

true. On empty strings, empty vectors, and empty bytevectors, eq?

may also behave differently from eqv?.

----

Finally, in addition to this, WG2 will include a declare

syntax to specify various standard optimizations.

This is of great practical use, and is likely the best way

to provide R6RS-style safety guarantees. Among the

declarations provided will be one which allows for eq?

to be unspecified on procedures.

Alex