[Haskell-cafe] Alternative to newtypes or orphan instances

[Erik Hesselink]

On 24 August 2015 at 09:18, Clinton Mead <clintonmead at gmail.com> wrote:
> 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