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From: Joachim Durchholz on 11 Nov 2007 18:03
Marshall schrieb:
> Very interesting. Can you expand on the difficulties that multiple
> dispatch brings to the question of composable modules?

Assume you have a multi-dispatching function

f (A, B)

Now the author of class A creates a subclass A', and the associated function

f (A', B)

Likewise for the author of B:

f (A, B')

Problems arise if the two authors don't know about each other (or can't
or don't want to collaborate). Then, combining classes A' and B' in one
system will lack a definition for

f (A', B')

There are several ways to deal with the situation:

1) Give priority to one of the parameters (usually the first one). This
means f (A', B') defaults to f (A', B). This can cause problems if the
implementations of A' and B' are so different from their respective base
classes that f (A', B') really would have to be written.
2) Force the integrator to write f (A', B'). Extra bonus if he doesn't
have the sources of A' and B'.

The "nonmodular" bit comes from the observation that A' and B' aren't
modules anymore: they cannot be added independently, there's an
interaction between them.

Regards,
Jo
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