Question

I’m trying to learn Haskell but I’m having problem in monad usage.

I imported the module Data.Maybe.

But I don’t know how to use the >>= operator.

Given (>>=) :: Monad m => m a -> (a -> m b) -> m b I cannot understand how to define a function (a -> m b).

Can someone provide some pedagogical example?

Answer 1

For Maybe monad bind function (>>=) looks like that:

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

So, let’s define some Maybe a value:

> let a = Just 1
a :: Maybe Integer

And :: a -> Maybe b function:

> let f = \x -> Just (x+1)
f :: Integer -> Maybe Integer

Now we can use bind like infix operator:

> a >>= f
Just 2
it :: Maybe Integer

---

Another example of really a -> Maybe b function could be:

let h :: Integer -> Maybe String; h = return . show . (+1)
h :: Integer -> Maybe String

So h increment integer number, convert it to string and make a Maybe value
with return function.

> a >>= h
Just "2"