[Haskell-cafe] Subclass or no subclass?

[Dan Burton]

>
> I am not trying to say "every building is a shelter", rather "anything
> that is a building must provide sheltering services".


Well if it walks like a shelter and quacks like a shelter... /shrug

The "is a" relationship is not a good way to think about type classes, in
my opinion. The "interface" or "services" viewpoint is, though. Class
instances "register" a service for a certain type, and constraints are
simply a requirement that certain other services also be registered.
Usually, this is because services of the class in question invoke some of
the services listed in that class's constraints.
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