Why use such a peculiar function type in monads?

问题

New to Haskell, and am trying to figure out this Monad thing. The monadic bind operator -- >>= -- has a very peculiar type signature:

(>>=) :: Monad m => m a -> (a -> m b) -> m b

To simplify, let's substitute Maybe for m:

(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

However, note that the definition could have been written in three different ways:

(>>=) :: Maybe a -> (Maybe a -> Maybe b) -> Maybe b
(>>=) :: Maybe a -> (      a -> Maybe b) -> Maybe b
(>>=) :: Maybe a -> (      a ->       b) -> Maybe b

Of the three the one in the centre is the most asymmetric. However, I understand that the first one is kinda meaningless if we want to avoid (what LYAH calls boilerplate code). However, of the next two, I would prefer the last one. For Maybe, this would look like:

When this is defined as:

(>>=) :: Maybe a -> (a -> b) -> Maybe b

instance Monad Maybe where
   Nothing  >>= f = Nothing
   (Just x) >>= f = return $ f x

Here, a -> b is an ordinary function. Also, I don't immediately see anything unsafe, because Nothing catches the exception before the function application, so the a -> b function will not be called unless a Just a is obtained.

So maybe there is something that isn't apparent to me which has caused the (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b definition to be preferred over the much simpler (>>=) :: Maybe a -> (a -> b) -> Maybe b definition? Is there some inherent problem associated with the (what I think is a) simpler definition?

回答1:

It's much more symmetric if you think in terms the following derived function (from Control.Monad):

(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c)
(f >=> g) x = f x >>= g

The reason this function is significant is that it obeys three useful equations:

-- Associativity
(f >=> g) >=> h = f >=> (g >=> h)

-- Left identity
return >=> f = f

-- Right identity
f >=> return = f

These are category laws and if you translate them to use (>>=) instead of (>=>), you get the three monad laws:

(m >>= g) >>= h = m >>= \x -> (g x >>= h)

return x >>= f = f x

m >>= return = m

So it's really not (>>=) that is the elegant operator but rather (>=>) is the symmetric operator you are looking for. However, the reason we usually think in terms of (>>=) is because that is what do notation desugars to.

回答2:

Let us consider one of the common uses of the Maybe monad: handling errors. Say I wanted to divide two numbers safely. I could write this function:

safeDiv :: Int -> Int -> Maybe Int
safeDiv _ 0 = Nothing
safeDiv n d = n `div` d

Then with the standard Maybe monad, I could do something like this:

foo :: Int -> Int -> Maybe Int
foo a b = do
  c <- safeDiv 1000 b
  d <- safeDiv a c  -- These last two lines could be combined.
  return d          -- I am not doing so for clarity.

Note that at each step, safeDiv can fail, but at both steps, safeDiv takes Ints, not Maybe Ints. If >>= had this signature:

(>>=) :: Maybe a -> (a -> b) -> Maybe b

You could compose functions together, then give it either a Nothing or a Just, and either it would unwrap the Just, go through the whole pipeline, and re-wrap it in Just, or it would just pass the Nothing through essentially untouched. That might be useful, but it's not a monad. For it to be of any use, we have to be able to fail in the middle, and that's what this signature gives us:

(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

By the way, something with the signature you devised does exist:

flip fmap :: Maybe a -> (a -> b) -> Maybe b

回答3:

The more complicated function with a -> Maybe b is the more generic and more useful one and can be used to implement the simple one. That doesn't work the other way around.

You can build a a -> Maybe b function from a function f :: a -> b:

f' :: a -> Maybe b
f' x = Just (f x)

Or, in terms of return (which is Just for Maybe):

f' = return . f

The other way around is not necessarily possible. If you have a function g :: a -> Maybe b and want to use it with the "simple" bind, you would have to convert it into a function a -> b first. But this doesn't usually work, because g might return Nothing where the a -> b function needs to return a b value.

So generally the "simple" bind can be implemented in terms of the "complicated" one, but not the other way around. Additionally, the complicated bind is often useful and not having it would make many things impossible. So by using the more generic bind monads are applicable to more situations.

回答4:

The problem with the alternative type signature for (>>=) is that it only accidently works for the Maybe monad, if you try it out with another monad (i.e. List monad) you'll see it breaks down at the type of b for the general case. The signature you provided doesn't describe a monadic bind and the monad laws can't don't hold with that definition.

import Prelude hiding (Monad, return)

-- assume monad was defined like this
class Monad m where
  (>>=)  :: m a -> (a -> b) -> m b
  return :: a -> m a

instance Monad Maybe where
  Nothing  >>= f = Nothing
  (Just x) >>= f = return $ f x

instance Monad [] where
  m >>= f   =  concat (map f m)
  return x  =  [x]

Fails with the type error:

    Couldn't match type `b' with `[b]'
      `b' is a rigid type variable bound by
          the type signature for >>= :: [a] -> (a -> b) -> [b]
          at monadfail.hs:12:3
    Expected type: a -> [b]
      Actual type: a -> b
    In the first argument of `map', namely `f'
    In the first argument of `concat', namely `(map f m)'
    In the expression: concat (map f m)

回答5:

The thing that makes a monad a monad is how 'join' works. Recall that join has the type:

join :: m (m a) -> m a

What 'join' does is "interpret" a monad action that returns a monad action in terms of a monad action. So, you can think of it peeling away a layer of the monad (or better yet, pulling the stuff in the inner layer out into the outer layer). This means that the 'm''s form a "stack", in the sense of a "call stack". Each 'm' represents a context, and 'join' lets us join contexts together, in order.

So, what does this have to do with bind? Recall: