Why use such a peculiar function type in monads?

Question

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

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Answer 1

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)