[Haskell-cafe] Data polymophism

[Tomasz Zielonka]

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

Best regards
Tomek