A modern programming language that makes developers happier.

kotlinlang

in the case of `?` is just syntax to deal with null and absence but it's not a proper data type which you can abstract over or reason about. Kotlin is still on it's infancy when it comes to FP but there is already some great tutorials and articles around.
<https://medium.com/hacker-daily/kotlin-functors-applicatives-and-monads-in-pictures-part-1-3-c47a1b1ce251>
<https://medium.com/hacker-daily/kotlin-functors-applicatives-and-monads-in-pictures-part-2-3-f99a09efd1ec>
<https://medium.com/hacker-daily/kotlin-functors-applicatives-and-monads-in-pictures-part-3-3-832d58d92445>
Not kotlin specific but familiar enough : <https://www.scala-exercises.org/>