Trust, but Verify: Two-Phase Typing for Dynamic Languages

TL;DR

This paper introduces a two-phase typing framework for dynamic languages, combining flow-insensitive type checking with flow-sensitive refinement typing to handle value-based overloading and verify correctness.

Contribution

It proposes a novel two-phased approach that first assigns basic types and then refines them with logical predicates to ensure correctness in dynamic languages.

Findings

Effective handling of value-based overloading

Successful verification of casts using refinement types

Enhanced type safety in dynamic languages

Abstract

A key challenge when statically typing so-called dynamic languages is the ubiquity of value-based overloading, where a given function can dynamically reflect upon and behave according to the types of its arguments. Thus, to establish basic types, the analysis must reason precisely about values, but in the presence of higher-order functions and polymorphism, this reasoning itself can require basic types. In this paper we address this chicken-and-egg problem by introducing the framework of two-phased typing. The first "trust" phase performs classical, i.e. flow-, path- and value-insensitive type checking to assign basic types to various program expressions. When the check inevitably runs into "errors" due to value-insensitivity, it wraps problematic expressions with DEAD-casts, which explicate the trust obligations that must be discharged by the second phase. The second phase uses refinement typing, a flow- and path-sensitive analysis, that decorates the first phase's types with logical predicates to track value relationships and thereby verify the casts and establish other correctness properties for dynamically typed languages.