Vicky Chijwani, young coder from India, created an application for Android called Kotlin Koans. It is designed for stugying Kotlin.
— Vicky Chijwani (@vickychijwani) June 18, 2017
Learning process consists of 41 challenges, that last for few minutes. In each challenge, you will be able to read some documentation and write a code to pass the test. Application provides feedback with detailed desciption on your errors. Tips are also available.
Kotlin Koans learning process
The application is written in Kotlin and is open-source, so you are able to "hack" it.