Invalid law for MonadPlus: v >> mzero = mzero
The documentation of MonadPlus states: v >> mzero = mzero. This law is broken by IO, MaybeT, and a few other monads with some notion of "irreversible effects". I propose to just remove that law.
https://hackage.haskell.org/package/base-4.11.0.0/docs/Control-Monad.html\#t:MonadPlus
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 MonadPlus for IO and 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.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.2.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | libraries/base |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | lyxia |
| Operating system | |
| Architecture |