[Haskell-cafe] how to avoid overlapping instance error?

[S. Alexander Jacobson]

Currently, the HAppS.DBMS lib requires the user to provide a Show 
instance for their table types.  An example might be:

    instance (Show item) => Show (MyTable item) where
        showsPrec d table = showsPrec d $ Set.toList $ myTableSet table

But the Table class itself defines a toSet function so I think I 
should be able to do this once for all Table instances:

    instance (Show item,Ord item, Table table item p) => Show (table item) where
        showsPrec d table = showsPrec d $ Set.toList $ toSet table

But I get an error telling me I am overlapping with Show [a].

Since [] is not an instance of Table, I don't see why there should be 
an overlap.

-Alex-

______________________________________________________________
S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com