On 21.01.2011, at 11:52, Daryoush Mehrtash wrote:
Do I have to have MonadPlus m or would any other Monad class work
the same way?
Not all monad instances satisfy
undefined >>= return . Just = undefined
if that's what you are asking for. For example, consider the identity
monad.
instance Monad Identity where
return = Identity
m >>= k = k (runIdentity m)
Then we have
undefined >>= return . Just
= undefined >>= Identity . Just
= Identity (Just undefined)
/= undefined
If >>= performs pattern matching on its first argument like most
instances do then you get undefined >>= return . Just = undefined.
I think that the monadplus laws
mplus m n >>= f = mplus (m >>= f) (n >>= f)
called (Ldistr) in the paper and
mzero >>= f = mzero
called (Lzero) in the paper imply
undefined >>= return . Just = undefined
At least if you have mzero /= mplus m n which is quite desirable. I
don't think that this holds for continuation based monads. But
Sebastian will most certainly know better as he is one of the authors
of the paper.
Cheers, Jan
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