On Thu, Jan 1, 2009 at 5:17 AM, Ryan Ingram <ryani.spam@gmail.com> wrote:
It might be possible to build a "lazy-ify" monad transformer which would make this work. In particular, I think Oleg's LogicT transformer[1] does something of the sort, only applying side effects where they are required in order to get "the next result" in a nondeterministic computation.
LogicT is continuation-based, so it's strict in the first argument to (>>=). observeT $ undefined >>= \_ -> return () will throw an exception. On the other hand, LogicT is non-strict in the second argument to mplus if the transformed monad is non-strict in (>>=). take 4 . runIdentity . observeAllT . msum $ map return [0..] should return [0,1,2,3]. (My implementation does. I assume the LogicT on Hackage is the same.) -- Dave Menendez <dave@zednenem.com> <http://www.eyrie.org/~zednenem/>