问题
I have the following typeclass which models a SQL-like query optimization:
class OptimizableQuery q where
type Optimized q :: *
optimize :: q -> Optimized q
instance Query q => OptimizableQuery q where
type Optimized q = q
optimize q = q
instance (Query q, OptimizableQuery q) => OptimizableQuery (Select (Select q p) p) where
type Optimized (Select (Select q p) p) = Select (Optimized q) p
optimize (Select (Select q _) p) = Select (optimize q) p
the problem is that I get the error "Conflicting family instance declarations" on the Optimized type function. Why is that and how can I solve it? It would really be nice to have a "fallback instance" instead of having to exhaust all cases (which might be quite many)...
回答1:
It's illegal to have overlapping instances with type families. See the GHC manual, "Overlap of type synonym instances" for details.
The reason is that having two different possible results for a type function application depending on the available instances can lead to unsoundness.
来源：https://stackoverflow.com/questions/4463154/haskell-overlapping-instances-and-type-functions