On 7 November 2014 12:36, David Feuer
Currently, Applicative is a superclass of Alternative. Unfortunately, the *only* laws for Alternative are the monoid laws. Sensible sets of laws have been proposed in the past, but unfortunately *none* of them cover all the current important instances. Thus we have a rather awkward situation where Alternative is a subclass of Applicative, but there's no real way to take advantage of that fact. There are essentially no useful functions that would end up with signatures that look like
p :: (Applicative f, Alternative f) => ...
I'm wondering, therefore, what people think of the idea of making Alternative entirely independent—just a version of Monoid with a different kind.
class Alternative f where empty :: f a (<|>) :: f a -> f a -> f a
A second option would be to go with a Functor superclass for Alternative; that might save some typing over the independent Alternative, and it comes with the free theorem
fmap f empty = empty
Control.Applicative.optional requires (Functor f, Alternative f), though that's the only function using Alternative that isn't a method of the typeclass that's in there. With AMP, what happens with MonadPlus? Isn't it equivalent to Monad+Alternative? -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com