On Wed, Sep 24, 2008 at 05:30:27PM +0100, Claus Reinke wrote:
Orphan instances are usually wrong unless the orphans are also exported via the standard API for either the class or the type. That is, orphans are ok in the implementation of a package, but not in the exposed API, because that makes it possible for a client to import both the class and type without getting the instance, which is what we have to avoid.
Sadly, that has all been discussed to death already, and again, it is a matter of being precise. "Orphan" instances are not wrong per se - they encode and name the extent of type relations via modules, but one needs to think carefully about their intended use and whether that use is really supported by the language or just an illusion. Of the top of my head, I can think of two uses:
(a) having two instances of the same class for the same types in the same program only works by "virtue" of #2356, so should be avoided unless and until the positive aspects of #2356 are moved from accident to design decision
(b) giving clients control over which instances they want to use (eg, use set A or set B, or neither) should work, and mostly does, but may run into ghc #2182 and haddock #54. Also, it is advisable only for client applications, not for client libraries, as long as their users might run into unresolved aspect of (a).
In the interest of providing a concrete example, see the darcs bug: http://bugs.darcs.net/issue387 It's a wishlist bug that we can't implement because the Show instance for Control.Exception.Exception is *not* an orphan (and thus we cannot define a duplicate instance). If it were an orphan, we could define a second show instance and then we'd get an error message if anyone ever calls show on an exception (which is always a bug). Alas, we cannot do this, and so we're stuck perpetually auditing our code to ensure that noone calls this pernicious function. Admittedly, we only want this orphan instance as a workaround for a poor API. Nevertheless, I think it illustrates your point that orphan instances could be useful, *particularly* because we have very little control over instance propogation. David