TypEr: a type annotator of Erlang code

Abstract

We describe and document the techniques used in TOOL, a fully automatic type annotator for Erlang programs based on constraint-based type inference of success typings (a notion closely related to principal typings). The inferred typings are fine-grained and the type system currently includes subtyping and subtype polymorphism but not parametric polymorphism. In particular, we describe and illustrate through examples a type inference algorithm tailored to Erlang's characteristics which is modular, reasonably fast, and appears to scale well in practice.