[Haskell-cafe] Data polymophism

Phlex Phlex at telenet.be
Sun Jun 10 06:34:45 EDT 2007

Just to keep you updated,

Ok so the design was bad ... a table of elements 'a' will have indexes 
that hold elements of the same type 'a' so the list of indexes can be 
[dbIndex a]..

There's was still one polymorphism issue :

data DbIndex a =
        DbIndex {dbiInsert :: Row a -> Maybe (DbIndex a),
                 dbiDelete :: Row a -> DbIndex a,
                 dbiFindWithKey :: Dynamic -> Maybe [Row a]}

indexDelete DbIndex{dbiDelete=delete} row = delete row
indexInsert DbIndex{dbiInsert=insert} row = insert row

findWithKey :: forall a k. (Typeable k) => (DbIndex a) -> k -> Maybe [Row a]
findWithKey DbIndex{dbiFindWithKey=find} key = find $ toDyn key

As far as i understand it, there is absolutely no way for the key in 
findWithKey to be fully typed while still having the indexes in the same 
list, so i took the given advice and used dynamics.

I guess this is poor style but it does the job !

Thanks again

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