Invalid law for MonadPlus: v >> mzero = mzero
The documentation of
v >> mzero = mzero. This law is broken by
MaybeT, and a few other monads with some notion of "irreversible effects". I propose to just remove that law.
Another alternative is to keep it but remove unlawful instances, so
MonadPlus instances can signal that those laws hold, on top of the
Alternative instances. But that breaks backwards compatibility, because
MaybeT is already used in many places.
This thread previously brought up the issue but hasn't been followed up: https://mail.haskell.org/pipermail/libraries/2014-January/021993.html, continued on https://mail.haskell.org/pipermail/libraries/2014-February/022004.html with some interesting discussion about what those laws should be.
This even older thread also discusses
MonadPlus laws, hence it also seems relevant to improve the documentation: https://mail.haskell.org/pipermail/haskell-cafe/2005-January/008751.html.