CZ: Multiple Inheritance Without Diamonds

Multiple inheritance has long been plagued with the diamond inheritance problem, leading to solutions that restrict expressiveness, such as mixins and traits. Instead, we address the diamond problem directly, considering two important difficulties it causes ensuring a correct semantics for object initializers, and typechecking multiple dispatch in a modular fashion the latter problem arising even with multiple interface inheritance. We show that previous solutions to these problems are either unsatisfactory or cumbersome, and suggest a novel approach supporting multiple inheritance but forbidding diamond inheritance. Expressiveness is retained through two features a requires construct that provides a form of subtyping without inheritance inspired by Scala 29, and a dynamically-dispatched super call similar to that found in traits. Through examples, we illustrate that inheritance diamonds can be eliminated via a combination of requires and ordinary inheritance. We provide a sound formal model for our language and demonstrate its modularity and expressiveness.