Keean Schupke wrote:
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.
Nono, the datasources I have implemented are a type safe means to extract (key,value) pairs from a data store. The idea is that this way, in a type safe fashion, e.g. database access can be abstract. I use HaskellDB as the database access layer, and then define a datasource instance for any given database, so that the user does not need to think about the details of the actual database access: he can just read and write from the datasource, and the datasource will make sure the actual queries will be executed. My idea now was that if I have 2 databases, and I construct datasources for them, it would be really cool if I was able to unite them, so that the programmer in the end could talk two 1 datasource, that allowed for accessing the 2 databases at one entry point. This was what I was making the JoinedDS for. So, suppose I have 2 datasources for two different databases. One may have keys: data KeysOfDS1 = KDB1_Table1 Int | KDB1_Table2 Int and values data ValuesOfDS1 = VDB1_Table1 (Int,Int,String) | VDB2_Table2 (Int,Int,String) and the other one: data KeysOfDS2 = KDB2_Table1 String | KDB2_Table2 String data ValuesOfDS2 = VDB2_Table1 (String, Float) | VDB2_Table2 (String, Float, Int) Now, these datastructures correspond to the actual tables in the database. My toolset will generate datasources for these types, thus we have instances: instance Datasource Database1 KeysOfDS1 ValuesOfDS1 instance Datasource Database2 KeysOfDS2 ValuesOfDS2 and the cool thing would be, to combine these two datasources at a higher level in my datasources graph, so that I would have 1 datasource that found out by itself which actual datasource to use, thus: x::JoinedDS x = JoinedDS db1 db2 -- where dbx is a datasource Databasex KeysOfDSx ValuesOfDSx Now, I would want the user to be able to read both KeysOfDS1 (which would yield a ValuesOfDS1) as well as KeysOfDS2 (which would yield a ValuesOfDS2) from x. Herefore, I need the instances mentioned before: 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) But this, thus, yields duplicate instance errors, which I don't like :-). Robert P.S. Sorry for any typos, I am enjoying a rather nice bottle of wine :-).
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