Ashley Yakeley writes:
> In article <[EMAIL PROTECTED]>,
> Jules Bean <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> <http://www.eyrie.org/~zednenem/>
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