Re: [Haskell-cafe] Re: What are the MonadPlus laws?

Ashley Yakeley writes:

In article , Jules Bean wrote:

...

So, anyone? What are the laws that MonadPlus is supposed to satisfy?

These are what I think they should be:

mplus mzero a = a mplus a mzero = a mplus (mplus a b) c = mplus a (mplus b c) mzero >>= a = mzero (mplus a b) >>= c = mplus (a >>= c) (b >>= c)

These are what is found in Martin and Gibbons paper, even as they're wrong that Maybe can follow them.

<http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/ tactics

..pdf>

Philip Wadler listed those as the laws he "would usually insist on" in a 1997 message[1]. [1] http://www.dcs.gla.ac.uk/mail-www/haskell/msg00057.html He also mentions two other possible, but problematic, laws: m >>= \x -> mzero == mzero m >>= \x -> k x `mplus` h x == m >>= k `mplus` m >>= h The first doesn't hold when m is bottom. The second doesn't hold for lists. -- David Menendez http://www.eyrie.org/~zednenem/