Just thought I ought to point out that all this is only necessary if the datasources may return different types... If you want them to return the same type you only need: instance (Datasource l k v,Datasource r k v) => Datasource (JoinedDS l r) k v ... As both datasources have the same key and value types, you then choose which 'v' to return at the value level. I am not sure whether you intended Datasources to contain heterogeneous key or value types, and whether the loolup is supposed to be value or type driven. My original answer assumed a single Datasource contains values of different types, selected by the type of the key... Keean. Robert van Herk wrote:
Yes, but this is not what I want. I want to be able to give a key that either the left or the right data source would take, and then return the appropriate value. Thus: if I pass it a key that would normally go into l, I want the value l returns me to be returned, and if I pass it the key that would normally go into r, I want to return the value r returns me.
The datasource class has a function dsread :: ds -> k -> (ds, v) -- read may have a side effect Thus I want want to do something like: instance (Datasource l k v) => Datasource (JoinedDS l r) k v where dsread (JoinedDS l r) k = let (l, v) = dsread l k in (JoinedDS l r, v) instance (Datasource r k v) => Datasource (JoinedDS l r) k v where dsread (JoinedDS l r) k = let (r, v) = dsread r k in (JoinedDS l r, v)
It would be perfectly okay to me when the compiler would complain if the key and value that go into l and r are the same, but for any useful purpose I can think of (e.g. glueing two database couplings together, since I also made a Datasource instance for database access), this will not happen and the duplicate instances should not really occur, since the context of the instances makes sure only 1 will be possible.
However, GHC only looks at the RHS (thus: Datasource (JoinedDS l r) k v) and then decides that both instances are the same.
So, my question was: how to overcome this.
Thanks, Robert