[Haskell-cafe] wierd type errros with MPTCs

[S. Alexander Jacobson]

This code gives me a Kind error because IVal isn't 
applied to enough type arguments.

      import qualified Set
      class Table table indexVal | indexVal->table where
          --insertIndex::item->indexVal item -> table item ->table item
          union::table item -> table item -> table item
          --union t1 t2 = t1
      data DBTable item = DBTable
      data IVal item = Name item
      instance Table DBTable (IVal ) where
Weirdly, when I uncomment the insertIndex 
function, things work.  But, if I then uncomment 
the default implementation of union, I get:

      No instance for (Table DBTable indexVal)
        arising from use of `Main.$dmunion' at example.hs:13
      In the definition of `union': union = Main.$dmunion
      In the definition for method `union'
      In the instance declaration for `Table DBTable IVal'
I don't know what this error even means.  But it 
goes away if I put the union implementation in the 
instance rather than in the class.

Bot these error messages seem unreasonable.  Can 
someone clarify?

Note: I am using GHC 6.2.2
-Alex-
______________________________________________________________
S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com
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