Ayende @ Rahien

Hi!
My name is Oren Eini
Founder of Hibernating Rhinos LTD and RavenDB.
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What does Mixin mean?


Bill Wagner has a proposal about the usage of mixins. He is talking about having a marker interface with minimal methods (or no methods), and then extending that using extension methods. To a point, he is correct, this will give you some sense of what a mixin is. But it is not enough.  

It is not enough because of the following reasons:

  • It is not really a cohesive solution. There is no really good way to specify something like SnapshotMixin. You need interface and static class and inherit from the marker interface, etc. Those are... problematic. I want to just be able to say: "also give me the functionality of this class"
  • A more important issue is one of state. The examples in Bill's proposal are all stateless methods, but I want to have a stateful mixin. I can think of several hacks around that, but they are hacks, not a proper way to work.

Comments

Francois Tanguay

Cohesion isn't an all-or-nothing philosophy. It is something we should tend for when it applies.

I think Mixin (Tag) interfaces + Extension Methods greatly overcome any drawbacks due to less cohesion (Discoverability being one.)

State "infrastructure" required by stateful mixins should be provided by the Mixin Interface itself.

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