John Cowan quoting me quoting him: > > > Different definitions for different contexts. SML equality has ad-hoc > > > polymorphism; Scheme `eqv?` has universal polymorphism. > > > > That's news to me. Which of the Scheme reports/standards do you wish > > to cite as your source? > > Any of them will do: given two values x, y, `(eqv? x y)` answers either > #t or #f. This is not the case in SML, where values cannot be compared > unless they have the same type, and only certain types can be compared > for equality at all.
I misinterpreted your "universal polymorphism" to mean parametric polymorphism, with universal quantification over type parameters. It looks as though you were really using that phrase to mean ad hoc polymorphism with a domain that includes all objects, as with Java's equals(Object), hashCode(), and toString() methods. Among other things, it's worth noting that Java does not require the equals(Object) method to agree with == (which is Java's closest equivalent to Scheme's eq?) on function objects such as comparators. As I explained earlier [1], the R5RS and R6RS requirement for eq? and eqv? to agree on all procedure values impedes implementation of the more useful behavior described by Alaric Snell-Pym. > I suppose you mean "have never been mutable". Yes. Thank you for that correction. Will [1] http://lists.r6rs.org/pipermail/r6rs-discuss/2012-July/006405.html _______________________________________________ r6rs-discuss mailing list r6rs-discuss@lists.r6rs.org http://lists.r6rs.org/cgi-bin/mailman/listinfo/r6rs-discuss
