[Haskell-beginners] WWAAA... I hate monads
daniel.carrera at theingots.org
Wed Apr 22 07:52:14 EDT 2009
Andrew Wagner wrote:
> Here's a clearer description:
> class Monad m where
> return :: a -> m a
> (>>=) :: m a -> (a -> m b) -> m b
> Instances of Monad must satisfy the following laws:
> return a >>= k == k a
> m >>= return == m
> m >>= (k >>= h) == (m >>= k) >>= h
> Much more clear and concise, don't you think?
Actually, that /is/ clearer for me. Though I still think I'll need the
tutorial Colin gave me to gain better intuition and understand the
motivations behind monads. That said, I do have questions:
1) Am I right in thinking that any type with two elements ("m a") is
always a Monad? (IO String, Maybe Int, etc).
2) When I say "m a >>= f" the >>= just pulls the "a" out of "m a" and
passes it to "f". Right?
3) The last two properties are not comparing values but functions.
Right? For example, reading the second-last property:
(m >>= return) :: a -> m a
Where (m >>= return) is defined by (\x -> m x >>= return). Right? And
this is why you can write the last property:
(k >>= h) :: a -> h a defined by (\x -> k a >>= h) This is what allows
us to make (k >>= h) the second argument of ">>=" as in the last property:
m >>= (k >>= h) == (m >>= k) >>= h
Am I right?
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