Its always challenge to build a good abstraction during work on library. Library codebase must be flexible and modular when user code must be clean and easy understandable. One of variants to build API between library and customer its Free evaluation approach. In that case you as developer is free to interpret customer actions as you want and customer is free to define actions as he want. In my talk I will show how to build API based on recursive abstract data types, describe why its cool and demonstrate some tricks on type system to make customer experience with library better.