Erlang is a functional programming language with dynamic typing. The language offers great flexibility for destructing values through pattern matching and dynamic type tests. Erlang also comes with a type language supporting parametric polymorphism, equi-recursive types, as well as union and a limited form of intersection types. However, type signatures only serve as documentation; there is no check that a function body conforms to its signature.

Set-theoretic types and semantic subtyping fit Erlang’s feature set very well. They allow expressing nearly all constructs of its type language and provide means for statically checking type signatures.

This talk explains how set-theoretic types work for Erlang and demonstrates how existing Erlang code can be statically type checked without or with only minor modifications to the code. We are working on an implementation of a type checker and there is an article that provides more details.