Simple Haskell Instance Question

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 MinPolicy 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?

augustss · Accepted Answer · 2011-03-19 16:57:07Z

6

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.

answered Mar 19, 2011 at 16:57

augustss

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2 Comments

Xodarap Over a year ago

Sorry, should've explained the error more. I actually did pass this in, and the full error message that I got is "Illegal type synonym family application in instance:"

stephen tetley Over a year ago

Can you declare the instance on HeapT rather than the synonym Heap? Scanning the source the associated type Prio is possibly doing something "unusual".

fuz · Accepted Answer · 2011-03-19 17:20:44Z

1

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.

answered Mar 19, 2011 at 17:20

fuz

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