I've got (maybe a bit naive) question, whether it is possible to have instance of class (already derived from another class) if it uses more concrete type than instances of predecesing classes. Let us assume the following example:
class A a where... class (A a) => B a where... class (B a) => C a where...
data MyData a = MD Int a
instance A (MyData a) where... instance B (MyData a) where... instance C (MyData Int) where...
Hugs/GHCi produce errors, of course. Nevertheless, is there any way to obtain such a feature? (In this example, classes A and B represent "abstract" classes providing functions with general algorithms, while class C cannot have a general implementation and requires a concrete data type for MyData.)
Thank you for any hints,
Dusan
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