Isaac Dupree schrieb:
Either's Monad instance (whatever we decide the canonical instance ought to be*) should be in Control.Monad.Instances just like Either's Functor instance is.
* how many people use the current "(Error e) => Monad (Either e)" instance who would break if we change it? If we don't like the Error class, then what do we replace it with? (Monad Maybe ignores the message -- but Monad (Either e) has to generate an "e", unless we let fail=error which may be an acceptable decision after all. If we have a class to generate an "e", we could use that to make a MonadPlus instance too (mzero)... but even then, the Monad instance perhaps shouldn't depend on that class...)
Assuming we do want to keep Either having a Monad instance at all, we do as Ross says have to pick the "leftmost error" rather than "all errors" Applicative instance. (Monad cannot produce errors beyond the leftmost, due to its later-actions-depend-on-results-from-earlier-ones nature.)
I can live without the Monad Either instance.