Monads

Let’s say that there is a function called getHalf.

-- it halfs when even number is given 
getHalf x = 
  if even x
  then Just (x `div` 2)
  else Nothing

This function does not work if we feed wrapped values, for example, Just 3. To solve it, we should use bind(>>=) operator.

> Just 5 >>= half
Nothing
> Just 10 >>= half
Just 5

Monad is a typeclass.

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

And Maybe is a Monad.

instance Monad Maybe where
  Nothing >>= func = Nothing
  Just val >>= func = func val

Maybe is a Functor and also a Monad!

December 17, 2018