On Fri, May 10, 2013 at 5:49 PM, Alexander Solla
I'm not sure if it would work for your case, but have you considered using DataKinds instead of phantom types? At least, it seems like it would be cheap to try out.
http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/kind-polymorphism-and...
I do like DataKinds a lot, and I did think about them a little bit with respect to this problem, but a solution isn't obvious to me, and perhaps more importantly I'd like to be able to support older versions of GHC, probably back to 7.0 at least. The issue is that every call to init needs to return a slightly different type, and whether this is achieved via phantom types or datakinds, it seems to me some form of existential typing is required. As both Andres and MigMit pointed out, you can sort of achieve this by using a continuation-like construction and higher-ranked types (is there a name for this transform? I've seen it a number of times and it is pretty well known...), but this enforces a dynamic extent on the descriptor whereas the original interface I proposed allows an indefinite extent. Best, Leon