Thanks to both Christian and Tomasz. This whole thing seems a bit too hard for the intended purpose. After much tries and errors, I think i'll go for closures returning DbIndex items. Something like this : uniqueIndex bkf datas = DbIndex{dbiInsertIndex=insertIndex, {- more methods -}} where index = UniqueIndex bkf datas insertIndex id item = do newIndex <- uniqueIndexInsert item index return $ uniqueIndex bkf $ uiItems newIndex -- more functions multiIndex bkf datas = DbIndex{dbiInsertIndex=insertIndex, {- more methods -}} where index = MultiIndex bkf datas insertIndex id item = do newIndex <- multiIndexInsert id item index return $ multiIndex bkf $ miItems newIndex -- more functions This keeps the definitions related to each index type close together, and allows for extention by adding new index types later on. Actually I don't even need the MultiIndex and UniqueIndex types anymore, making the code even shorter. Thanks for your time, Sacha Tomasz Zielonka wrote:
On Fri, Jun 08, 2007 at 07:49:20PM +0200, Tomasz Zielonka wrote:
On Fri, Jun 08, 2007 at 05:23:23PM +0200, Phlex wrote:
But i don't seem to find a way to get out of this DbIndex type to actually work on the enclosed index.
for instance, this doesn't work: liftDbIndex (DbIndex index) fun = DbIndex (fun index)
The compiler probably can't infer higher-ranker types, so you have to write you type signature explicitly. Try:
liftDbIndex :: Index_ i2 a2 k2 => (forall a1 k1 i1. Index_ i1 a1 k1 => i1 -> i2) -> DbIndex -> DbIndex
Now I think that this type signature will be too restrictive. Ideally, i2, a2 and k2 would be existentially quantified, like
liftDbIndex :: (forall a1 k1 i1. Index_ i1 a1 k1 => i1 -> (exists. Index_ i2 a2 k2 => i2)) -> DbIndex -> DbIndex
AFAIK such type isn't supported by any Haskell compiler, so we have to use the existential quantification from DbIndex:
liftDbIndex :: (forall a1 k1 i1. Index_ i1 a1 k1 => i1 -> DbIndex) -> DbIndex -> DbIndex liftDbIndex f (DbIndex i) = f i
Best regards Tomek