On 24 August 2015 at 09:18, Clinton Mead
A second approach is an orphan instance. The recommendation here is to put the orphan instances in their own module, so the user can choose to import them.
This may works ok if your user is writing an executable. But what if your user is writing a library themselves. But once, you, or your user, directly uses one of the instances, they need to import it, and they pollute the global instance namespace for anyone that uses their package.
For this reason, I think the recommended course of action is to make a canonical place for the instance, so that everyone can use it. For example, if you have a library 'foo' providing T, and a library 'bar' providing C, put the instance in a new package 'foo-bar' (or 'bar-foo'). Then everyone can use that one instance, since Haskell is built on the assumption that every type has one unique instance per class.
I want to suggest a third option:
(3) Copying the class.
This would make a new distinct class, which means you can't call any methods which have the original class as the context (f :: C a => a -> a) since that class won't exist for type T (you are trying to avoid defining that orphan instance). So I don't think this is usable in most cases, unless I'm missing something. Erik