Re: [Haskell-cafe] Re: what is inverse of mzero and return?

Am Samstag, 22. Januar 2005 21:20 schrieb Keean Schupke:

Ashley Yakeley wrote:

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In article <41F211FB.3030706@imperial.ac.uk>,

Keean Schupke wrote:

...

This fits the above description, but I don't see how the following can be true:

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

Try it (and my test code) with [], which is an instance of MonadPlus. mplus is defined as (++) for [].

but what if (a >>= c) causes c to fail, and (b >>= c) lets c succeed. In this case the LHS will fail, whereas the RHS will succeed I think?

Keean.

That's probably a misunderstanding due to the notation, in the [] monad, it's just concat (map c (a ++ b)) = concat (map c a) ++ concat (Map c b), which is easily seen to be true (if applying c to an element of a causes an error, neither side will go past that). Daniel