Monad type signature example

The type signature of (+) is:

    (+) :: Num a => a -> a -> a

I can see this in:

    + 4 5

Resulting in 9. + takes 4 and returns a function of roughly:

    (4 + a) -> a

...which then takes 5 and evaluates to 9. Basically, two things in, one thing out. I don't see how this works using the bind operator's type signature. To me, in practice, it always looks like one in, one out.

Could someone please walk me through a simple example using a Maybe monad the way I've done above for (+) ? Hopefully, this will be generally useful to Haskell newbie's like me!

Solution

Here is the type signature of bind:

(>>=)
    :: Monad m
    => m a         -- Left argument
    -> (a -> m b)  -- Right argument
    -> m b

Here is an example of a bind with two arguments:

Just 1 >>= (\n -> Just (n + 1))
^^^^^^     ^^^^^^^^^^^^^^^^^^^^
Arg #1     Arg #2

.. and it evaluates to Just 2