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