``Orphan instances'' should be avoided anyway.

kahl at cas.mcmaster.ca kahl at cas.mcmaster.ca
Tue Aug 12 13:38:28 EDT 2008

Henning Thielemann <lemming at henning-thielemann.de> wrote:
 > On Tue, 12 Aug 2008 kahl at cas.mcmaster.ca wrote:
 > > 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.

 > , 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 ;-).

 > 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''.


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