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In the earlier example, we used the transformation function to return another value, but what if we wanted to use it to return a new Result object?

func map<U>(f: T -> U) -> Result<U>

What is the practical reason for which it returns Result<U> instead of U? We wrap U inside Result then flatten it, to get U again?

In the Monads section of the post, there is a code snippet where you define flatten, and inside there is a reference to innerResult - is this a typo? Or where is this coming from? Maybe this is some magic swift thing I am not familiar with? Very informative post, but this is tripping me up a bit. (I'm just starting to learn Swift, so maybe this is a language feature I haven't quite grasped yet.)

Nice article Javi. I just came across a scenario where I would like to have a flatMap on optional, and I realized there isn't one.

Therefore, in the swift standard library Optional is current a Functor but not a Monad right?

I'm thinking this might be a great use-case to point out for a practical benefit having a monad.

You already encourage people to come up with their own flatMap for Optional in your article, which is great, but maybe you could also mention that there actually is no flatMap for optional, and you could use an extension to turn Optional into a Monad using your own implementation of flatMap.

Here is a trivial example of when you may want a flatMap in optional. The nesting comes when you are using an Optional return value within your map function.

func squareRoot(number: Int) -> Int? {
    let root = Int(sqrt(Float(number)))
    return root * root == number ? root : nil
}

let optionalInput: Int? = 4

let optionalOutput = optionalInput.map { squareRoot($0) }

println (optionalOutput)
//=> Optional(Optional(2))

if let unwrappedOutput = optionalOutput {
    println(unwrappedOutput)
    //=> Optional(2)
}