Multimethods 101 (was Re: Multiple dispatch (Re: Q: Python 2.0 preliminary features?))

[Bruce Hoult]

In article <381A5FE7.1EBECBE2 at bigfoot.com>, Louis Madon
<madonl at bigfoot.com> wrote:

> > I note that the ambiguity can arise *only* if some module that (directly
> > or indirectly) calls foo() also (directly or indirectly) depends on both
> > AA and BB.
> > 
> > For example, you suggested that maybe something in module X calls
> > something in module Y, passing it a BB.  Module Y creates an AA (which X
> > doesn't know about) and calls foo(AA, BB).  oops.  The only place that
> > this problem can be detected at compile-time is in module X, and then only
> > if the compiler knows that module Y imports both AA and foo EVEN IF IT
> > DOESN'T RE-EXPORT THEM.  This seems like a big ask.
> 
> I agree, but isn't that what runtime exceptions are for?

That's right, but some people seem to prefer to get told about potential
problems at compile time -- there is always a danger that the particular
case that causes the exception doesn't get exercised in testing but gets
hit in the field.


> >  If not then perhaps foo() is trying to do too much and
> > should be split up?  In particular, perhaps there should instead be a
> > bar(A) and baz(B) and just a single, generic, version of foo(A, B) that
> > calls bar(A) and baz(B) as part of its operation.  In which case the
> > implementator of AA should make a bar(AA) and the implementatior of BB
> > should make a baz(BB). And then all is sweetness and light :-)
> 
> You're not suggesting explicit type tests inside foo(A, B) are you? 

Aww, *maate*.  Strewth, I'm not suggesting any such thing as a type test.


> Thats precisely the kind of thing we need to get away from.  I could
> come along and define a new class AB that inherits from both A and B and
> all of a sudden your typecase is broken.

The only typecase is in the (compiler-generated) code for the generic
function itself.

When you define your class AB, you'll just need to define appropriate
bar(AB) and baz(AB) methods.

-- Bruce