On Nov 15, 2010, at 9:43 AM, Ling Yang wrote:
Specifically: There are some DSLs that can be largely expressed as monads, that inherently play nicely with expressions on non-monadic values.
This, to me, is a big hint that applicative functors could be useful. Every monad is an applicative functor. Given a monad instance for F, you can do: instance Applicative F where pure = return (<*>) = ap <$> is an alias of fmap. <*> can be interpreted as a kind of "lifting" product operator (Examine the types as you learn it. The notation will become transparent once you "get it"). So you write expressions like: data F a = F a -- We'll assume F is instantiated as a monad data Foo = Foo Int Int Int foo :: F Foo foo = Foo <$> monad_action_that_returns_an_int_for_your_first_argument <*> monad_action_that_returns_an_int_for_your_second_argument <*> monad_action_that_etc Your test test = liftM2 (+) (coin 0.5) (coin 0.5) translates to: test = (+) <$> (coin 0.5) <*> (coin 0.5) You can't really express a test in 5 arguments (I think there's no liftM5...) but it's easy with <$> and <*>: test = Five <$> one <*> two <*> three <*> four <*> five