one simplifying view of the phrasing is that the order matters as soon as they
"play the same role". (except the "*" role, that is.)
seriously, this is the only way i have been able to make that passage make
sense. the key distinction being whether "the effective method is the same" as
far as the standard method->role distribution algorithm is concerned. except for
the "*" role, that mechanism is sufficient to put each method in a specific
(read "implementation independent") position in the two-dimensional (role x
specialization) space. the "*" role being the exception, in that the combination
is specified to be responsible for the structure of that space.
> -----Original Message-----
> From: cmucl-imp-bounce@... [mailto:cmucl-imp-bounce@...
> Behalf Of Christophe Rhodes
> Sent: Wednesday, November 17, 2004 17.14
> To: Martin Simmons
> Cc: sbcl-devel@...; cmucl-imp@...
> Subject: Re: [Sbcl-devel] Re: DEFINE-METHOD-COMBINATION bug (?)
> Martin Simmons <martin@...> writes:
> > Christophe Rhodes wrote:
> >> I believe this is a bug in the specification -- or rather in the
> >> example, which is not part of the specification.
> >> The spec for DEFINE-METHOD-COMBINATION says
> >> Note that two methods with identical specializers, but with
> >> different qualifiers, are not ordered by the algorithm described in
> >> Step 2 of the method selection and combination process described in
> >> Section 7.6.6 (Method Selection and Combination). Normally the two
> >> methods play different roles in the effective method because they
> >> have different qualifiers, and no matter how they are ordered in
> >> the result of Step 2, the effective method is the same. If the two
> >> methods play the same role and their order matters, an error is
> >> signaled. This happens as part of the qualifier pattern matching in
> >> define-method-combination.
> > I don't see how d-m-c can know that "their order matters".
> Well, no, nor do I in general, short of solving the halting problem.
> As I said, it would be nice to understand what was going through the
> standardization committee's (or the CLOS/MOP authors') minds when they
> wrote that paragraph.