On Fri, May 4, 2012 at 9:40 AM, Michael Snoyman
I think the underlying point of distinction between conduit and pipes-core here is that, is conduit, a Pipe of type `Pipe i o m r` is *required* to provide an `r` value ultimately. If I understand correctly, this is not the case in pipes-core. I believe this also explains your question about the `MonadTrans` instance: `Finalize` is not simply "clean up resources," it's "clean up resources __and__ return the required `r` value." That's why we duplicate the base monad action: it's the only way to get a value of type `r` to return.
So, if I understand correctly, that means that if you use `lift` to specify a monadic action, there's no way to specify a different action for cleanup. That probably implies that there is no sensible way to convert a pipes-core pipe to a conduit pipe, at least not if you want the finalization semantics to be preserved. Thanks a lot for you reply! BR, Paolo