[Sbcl-devel] patch to defcombin.lisp

[Wendall M. <wam...@sp...>]

I'll try to keep this short and sweet.

One portion of the 7.7 description of define-method-combination states:

"It is also possible to bypass the method group specifier mechanism and 
do everything in the body forms. This is accomplished by writing a 
single method group with * as its only qualifier-pattern; the variable 
is then bound to a list of all of the applicable methods, in 
most-specific-first order."

Attempting to do this by rolling your own version of the standard method 
combination fails in sbcl (as well as cmucl and acl) with an error like:

debugger invoked on a SB-PCL::LONG-METHOD-COMBINATION-ERROR in thread 18371:
  More than one method of type ALL-METHODS with the same specializers.
See also:
  The ANSI Standard, Macro DEFINE-METHOD-COMBINATION

This error is thrown before the body has a chance to resolve anything. 
Lispworks
and anecdotally MCL work fine.

Another section of the same 7.7 define-method-combination states:

"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."

So, I split the difference. define-method-combination with exactly one 
method group whose qualifier-pattern is * allows you to do whatever 
you'd like in the body, as the first portion states - allowing full 
control. Everything else throws this error, as the second portion would 
seem to indicate. Patch is attached, and I'm willing to change it if 
it's not the right fix. (I have code that wants to do whatever I told it 
to do, rather than dying before it has the chance ;))

Thanks,
Wendall