Re: ``Orphan instances'' should be avoided anyway.

On Tue, 2008-08-12 at 17:38 +0000, kahl@cas.mcmaster.ca wrote:

Henning Thielemann wrote:

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On Tue, 12 Aug 2008 kahl@cas.mcmaster.ca wrote:

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I just would like to point out that there is nothing inherently bad about what GHC calls ``orphan instances''.

From a code structuring point of view, I frequently ``consider orphan'' instances useful for separation of concerns.

The problem is, that if you have a main instance of a class for a type and this one is not bundled with either the type or the class, then you are able to import the type and the class without the main instance (that is, you can accidentally miss that instance)

Or on purpose --- this is in fact another use of ``orphan instances'' I forgot to mention.

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, and thus you are able to define another instance.

Indeed --- this is the only way to have different instances for the same class, as long as we do not have something like the ``named instances'' of our Haskell-2001 paper (shameless plug ;-).

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This will likely cause clash with the main instance sooner or later, if other modules import your custom instance and the main one.

If there are several instances, there is very likely no ``main instance''.

If there is no main instance, there should very likely be no instance at all. We already have named instances: data ShowDict alpha = ShowDict alpha { namedShows :: alpha -> String -> String } show :: ?namedShow :: ShowDict alpha => alpha -> String show x = namedShows ?namedShow x "" Confusing this with type classes seems mostly redundant to me. jcc