I personally would be happy with either Maybe or MonadPlus. I hope we don't miss out on getting rid of fail by arguing about Maybe vs. MonadPlus. Dan Doel wrote:
I don't see how specializing to Maybe and requiring people to explicitly inject into an arbitrary MonadPlus gains anything over restricting to MonadPlus in the first place.
Maybe is not arbitrary. It is the "unit" instance of MonadPlus. It can be lifted trivially into any other instance. Krasimir Angelov wrote:
I think the proposal to generalize to MonadPlus is even worse. In this case the function can indicate failure with either fail or mzero which makes things even worse.
The same is true for Maybe. But once we are in Maybe or MonadPlus, fail becomes a red herring. Any program that uses fail can be marked as unsafe, just like any program that uses error. -Yitz