As domain driven design practitioners, we have to design datastructures a lot. Often we have to encode our knowledge into a not-so-expressive type system. That's when the trouble starts: our types don't represent exactly what we have. Algebraic Data Types (or ADTs) are a very powerful tool and help a lot when it comes to design our data types. Moreover, they share interesting properties with addition and multiplication (hence their name). In this short talk, I'll show you how to use them in scala and javascript, and how their mathematical properties can give you intuition and easy (re)factoring. Clément Delafargue is the CTO of Clever Cloud. A graduate from École Centrale de Nantes, he has been developing web projects since 2002. He enjoys working both on frontend and backend, as well as learning new languages and tech stacks. He's particulary interested in functional programming. [QBN-7622]