How is Ruby module inclusion not really 'multiple inheritance' and how does the Ruby style avoid the problems associated with multiple inheritance?

Question

Matz supposedly said "mixins could do almost everything multiple inheritance do, without the associated drawbacks" (Matz words)."

First of all, why is Ruby module inclusion not 'multiple inheritance' ? It seems to me there is very little difference between modules and classes. The fact you cannot instantiate a module is irrelevant when it is used as a superclass.

I also know that successive module inclusion forms a single inheritance chain (not tree) extending upwards from the class. But this, to me, is not sufficient to distinguish it from 'multiple inheritance' as the Python multiple inheritance system also "linearizes" the superclass chain (using the C3 algorithm) it's just that Ruby 'linearizing' process is significantly simpler.

So what exactly distinguishes Ruby module mixins from multiple inheritance in say a language like Python? And why don't the arguments behind the Python adoption of the c3 MRO algorithm apply to Ruby? And if they do apply - why did Ruby decide not to adopt this algorithm ?

thanks

Answer 1

With MI, lots of the issues that arise can be trimmed down to implementation details; you can't just talk about "Multiple inheritance" in general, without talking specifics. So I'll be assuming that you mean "C++ multiple inheritance" when you say "multiple inheritance".

The most common problem with Multiple inheritance is the Diamond Problem. If several superclasses on the same level define the same method, how do you know which method is invoked on the subclass?

With modules, this problem is easily answered - the last included module always "wins". You can't include several modules "simultaneously", as you can do with classes in C++. So this problem never arises.

The fact you cannot instantiate a module is irrelevant when it is used as a superclass

I respectfully disagree.

First, modules are never "used as superclasses" in ruby; only superclasses are.

Second, with multiple inheritance, knowing exactly in what order the constructors (& destructors!) are called is not a trivial matter. The fact that ruby modules don't allow instantiation suppresses that problem altogether.