The Type Class Pattern

A type class is a pattern for adding behavior to types without modifying them. It is a form of ad-hoc polymorphism: the same operation works for many unrelated types, chosen by the compiler based on type.

It comes from Haskell and is widely used in Scala libraries like Cats.

Inheritance forces behavior into a type at definition time. But you often cannot edit a type (it is in a library), or you want different behaviors in different contexts. Type classes let you attach behavior externally.