On Mon, 2008-10-13 at 18:28 +0100, Andrew Coppin wrote:
Reid Barton wrote:
It's not difficult: the operation is called
mplus :: MyMonad a -> MyMonad a -> MyMonad a
and already exists (assuming the author of ListT has not forgotten to write a MonadPlus instance).
I see... I was under the impression that "mplus" is just any arbitrary binary operation over a given monad. How do you know what it does for a specific monad?
Process of elimination. Sometimes, this doesn't narrow things down to a single operation, but it gives you a good idea of what you're supposed to expect. Firstly, mplus and mzero form a (natural) monoid, put together. That rules out a number of binary operations right there. Secondly, mzero has a null law with (>>=): mzero >>= f = mzero So, if you have a `mplus` b and a calls mzero at some point (not inside another call to mplus --- nice and informal, that description :), then you know b will be executed instead. (Maybe b will be executed *anyway*. I didn't say anything about that). So mplus and mzero are basically suitable for three kinds of things: * Exception handling * Back-tracking * Parallelism Usually, when you see a MonadPlus instance, you expect one or more of these. That's in the general case. ListT is a special case; the (somewhat idealized) specification of ListT (what people want to happen when they use ListT) is that ListT m in some sense `adds back-tracking' to m. Where back-tracking choice is implemented by mplus. jcc