On Wed, Sep 5, 2012 at 5:22 PM, Evan Laforge
Assuming no one uses orphan instances, adding instances is always safe because you can only add instances for types from packages you depend on, which means that they can't be depending on your package in turn and have defined a non-orphan instance for a type or class defined in your package.
The thing is if a library is missing some standard instance like Monoid it's very tempting to put in your own. So the instances the library author is most likely to eventually add in are also the ones people are most likely to have worked around locally by adding their own orphans.
E.g. Applicative for Parsec, Monoid for Text.PrettyPrint.Doc.
In this case I would add a newtype wrapper. I've been doing that lately and it doesn't add much noise. -- Johan