Thomas Schilling wrote:
data T class Foo ns a b c | ns -> a, ns -> b, ns -> c where mkFoo :: ns defaultA :: a defaultB :: c -> IO b defaultC :: [T] -> c f :: c -> b -> a -> (b, Int)
data DefaultA instance Foo ns a b c => Apply DefaultA ns a where apply _ _ = defaultA
but I get:
Could not deduce (Foo ns1 a b1 c1) from the context (Apply MakeAs ns a, Foo ns a b c) arising from use of `defaultA'
Indeed. Let us examine the inferred type of defaultA: *Main> :t defaultA defaultA :: (Foo ns a b c) => a We see it is a value polymorphic over four type variables: ns, a, b, and c. The type variable 'a' is also the type of the value, so we have a way to instantiate it. There is no direct way to instantiate the remaining three. If there were a functional dependency a -> ns, a->b, a->c, we could have instantiated the remaining variables. But there are no such dependencies. So, there is really no way we can ever instantiate the type variables ns, b and c -- and so the typechecker will complain. So, we need either a functional dependency a -> ns in the definition of Foo, or defaultA should have a signature defaultA :: ns -> a (and ditto for other defaults). As I understand, the function 'defaultA' can be present in different components, identified by ns. When we write 'defaultA' however, how can we say that we mean defaultA of component X rather than of component Y? There isn't any way to name the desired component... Incidentally, if we represent components by records data XRec = XRec { defaultA :: XA } then the type of defaultA is Xref -> XA. It is the function from the type of the `namespace'. This seems to suggest the signature of defaultA should be ns -> a ... BTW, there are other ways to add the name of the namespace to the signature of defaultA. For example: newtype TaggedT ns a = TaggedT a class Foo ns a b c | ... defaultA :: TaggedT ns a or class Foo ns a b c | ... defaultA :: ns a etc.