On Fri, May 14, 2010 at 4:25 PM, Derek Elkins
You did it wrong. All you did was Church encode the Either type. Your bind is still doing a case-analysis. All you have to do is use ContT r (Either e). The bind implementation for ContT is completely independent of the underlying monad. It doesn't even require the m in ContT r m to be a functor, let alone a monad. Therefore the ContT bind doesn't do any case-analysis because it doesn't know anything about the underlying monad. One way to look at what is happening is to compare it to Andrzej Filiniski's work in "Representing Monads" and "Representing Layered Monads".
Would you then use callCC to get the required short-circuit-on-error behavior? A church encoding of Either coded as a monad transformer still wouldn't hit the inner monad on bind, even if it is weaving the left and right continuations together. Antoine