Simple Haskell Instance Question

I'm trying different data structures for implementing Prim's algorithm. So I made a class to abstract what I want to do:

class VertexContainer a where
  contains :: a -> Vertex -> Bool
  insert :: a -> WeightedEdge -> a
  numVertices :: a -> Int

Now I want to use a heap (from Data.Heap) as my vertex container. But I can't for the life of me figure out the syntax. As you can see from the insert declaration, the container can only hold WeightedEdges, which are a data type. So I tried:

instance VertexContainer (Heap MinPolic开发者_运维知识库y WeightedEdge) where
  contains _ _ = True

It tells me it's an illegal type synonym. I've tried various other permutations, and none of them seem to work. Can anyone help me?

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If you read the entire error message you'll find that it tells you how to be able to use a type synonym in an instance declaration, namely by using the language extension TypeSynonymInstances. E.g., you can pass -XTypeSynonymInstances on the command line.

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I got it working by wrapping this into a newtype. Considered ugly. I guess you have to wait for one of the Haskell gurus to answer this.