[Haskell-cafe] Orphan instances
lemming at henning-thielemann.de
Wed Mar 5 01:53:19 EST 2008
Consider the following modules
class A t
data T a
class A t => B t
instance B t => A (T t)
GHC emits warning about orphan instance A (T t), since neither A nor T are
defined in B. However I can't move the instance to A or T since it depends
on B. Would it be fine to lift the restriction, such that it is accepted
to declare the instance in a module where the type or the class or a
superclass (this is new) is defined?
In my example class B provides conversion toInteger (that is Integral),
class A provides conversion toRational (that is Real) and T is Ratio.
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