Hi all, Max asked earlier[1] how to create a new instance of a class in Persistent using a monad transformer. Without getting into the specific details of persistent, I wanted to pose a question based on a much more general question: how can we lift the inner monad of an enumerator? We can easily do so for an Iteratee[2], but there is nothing to allow it for an Enumerator.
At first glance, this problem looks very similar to the shortcomings of MonadIO when dealing with callbacks. In that case, you cannot use liftIO on a function that takes an `IO a` as a parameter. A solution to this issue is monad-control[3], which can be used to allow exception catching, memory allocation, etc. So I'm wondering: can we come up with a similar solution to this issue with enumerators? I have a working solution for the specific case of the ErrorT monad[4], but it would be great to be able to generalize it. Bonus points if we could express this in terms of the typeclasses already provided by monad-control. Michael [1] http://groups.google.com/group/yesodweb/browse_thread/thread/be2a77217a7f3343 [2] http://hackage.haskell.org/packages/archive/enumerator/0.4.14/doc/html/Data-Enumerator.html#v:liftTrans [3] http://hackage.haskell.org/package/monad-control [4] https://gist.github.com/1168128 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
