Second Monad Law:Unit

Question

I am reading James Iry's blog post on Monads in scala. I am in part three and I am confused about his description of the second law of monads regarding Unit. Specifically this claim: ``` unit(x) flatMap f = f(x) ``` When I apply my mental examples which are jame's previous examples of monadic types this never seems to work out ``` val x = 1 val f = (_:Int) * 2 f(x) == 2 //true List(x) flatMap f == 2 //fail Some(x) flatMap f == 2 //fail ``` As a matter of fact they don't even compile due to type issues. To clarify I understand why these are failing. I understand the how to fix them so they compile. My confusion is that these seem to be in conflict with the theory presented in the article. Is there a step I am missing? Are these types not really monads? Is the section entitled "Second Law of Monads:Unit" incorrect?

Accepted Answer

Scala's `flatMap` needs a function that returns a collection, not a function that returns a single element, like your function `f`.

Either use `map`:

```
List(x) map f
```

or make your function return a collection:

```
val f = (x: Int) => List(x * 2)

List(x) flatMap f
```

Note that it will also return a collection, not a single integer (you'll get `List(2)`, not just `2`).

Second Monad Law:Unit

Solution

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