Thanks for the extended response to my question about overlapping instances. Before my original posting, I had read a posting that included the example with Show that you included in your response.
I believed (and still do) that my specific case is a bit different.
| To determine (SubType y Value) which is just:
| (SubType y (Either Double BaseType))
| | it seems to me that GHC should (has to?) use
| instance (SubType a b) => SubType a (Either x b)
| | I do not see how the other alternative is applicable:
| instance SubType a (Either a x)
Well, the "target" (Subtype y (Either Double BaseType)) can match against *both* of these instances.
It can match instance (SubType a b) => SubType a (Either x b)
by taking a=y, x=Double, b=BaseType
*If y were instantiated to Double* it could match the second instance declaration too. Now, you say, y is an existential type variable, so it can't be instantiated to Double --- but that's tricky to pin down.
Within the GHC compiler > can't be instantiated to Double --- but that's tricky to pin down.
this may be tricky to pin down. But, there is specific information in my example to exclude Double: I had carefully constructed the type definitions to avoid ambiguity.
> | type BaseType = Either Integer ( Either Bool () ) > | > | type Value = (Either Double BaseType) > | > | data Foo = forall x. (SubType x BaseType) => MkFoo x > | > | test :: Foo -> Value > | test (MkFoo x) = inj x
'x' is the variable I am concerned about. Since it is an argument to MkFoo, we know that (SubType x BaseType) and we also know that:
NOT (SubType Double BaseType), so 'x' cannot be instantiated as Double.
Am I correct when I say that: this specific case is not ambiguous, but the compiler does not recognize the constraint on 'x' which dis-ambiguates this case. OR is this _really_ ambiguous.
I am not suggesting a change in the compiler to recognize such specific cases (though I would not object :-); but I want to be sure I understand what I have written.
thanks Ed Komp
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