[Haskell-cafe] Fwd: incorrect MonadPlus law "v >> mzero = mzero"?

Petr Pudlák petr.mvd at gmail.com
Tue Feb 4 08:21:25 UTC 2014

My intention wasn't to add these laws, but to replace the existing
MonadPlus ones. (Adding new laws wouldn't help with the original ones being
invalid.) But you're right, the laws I proposed follow from Monad laws and

    mzero >>= f  =  mzero

This single law is enough to ensure that any chain of operations containing
`mzero` "ends" at this point.

So the best solution seems to just remove the problematic

    v >> mzero   =  mzero

2014-02-04 Dan Burton <danburton.email at gmail.com>:

> I'm not sure what should be the proper solution. Perhaps to change the
>> laws to:
>>   return x >> mzero  =  mzero
>>   (v >> mzero) >>= f  =  (v >> mzero)`
>> That is, if an expression ends with `mzero`, it behaves like `mzero`.
> These laws are redundant with existing laws. The first:
>     return x >> z = z
> Is true forall x and z, and can be proven by just the monad laws. The
> second:
>     (v >> mzero) >>= f  =  (v >> mzero)
> Can be proven by the associativity of monadic operations:
>     (v >> mzero) >>= f  =  v >> (mzero >>= f)
> And the other MonadPlus law already states that (mzero >>= f) = mzero. So
> I don't think any new laws are needed. I just think the (v >> mzero =
> mzero) law should be removed, or else a *lot* of instances of MonadPlus
> need to come with a disclaimer that they are not law-abiding.
> -- Dan Burton
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140204/f899b281/attachment-0001.html>

More information about the Haskell-Cafe mailing list