On Thu, Dec 15, 2011 at 2:20 AM, Gregory Crosswhite
On Dec 15, 2011, at 6:19 PM, Gregory Crosswhite wrote:
After all, we already have the Monad typeclass which gives them essentially the same functionality.
Make that the *Monoid* typeclass. :-)
And this is an interesting discussion all of its own! Should the monoid instance of a Functor do what List does - which is analogious to its append or choice operation (where applicable), or should it do what Maybe does, which is lift the operation into its contained type? (That is, (Just x) `mappend` (Just y) ==> Just (x `mappend` y)). Since the Monoid instance for Maybe doesn't offer choice between Nothing and Some, it would be nice to have a standard choice operation that we could use for Maybe. Which is sort of what Alternative is - offering choice over a functor which supports it. Except that the notion of what choice means is richer in a parser than in Maybe (parsers may backtrack (like List) and parsing has some sort of stateful effect, which affects the success of future parses). It is an interesting dilemma. I am also fond of using Alternative (disguised as MonadPlus) in the Happstack sense, for building a web-site routing table. In the truest sense I am composing alternative responses to an input request, but using 'many', 'some', or 'sepEndBy` in this context would be odd. Antoine