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

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

Static Types with Less Syntax: Locus Types

Optionally and gradually-typed languages allow types to be introduced to dynamic code as needed. While this approach allows some gradual movement from dynamically to statically-typed code, it requires rewriting object-constructing code to use conventional static types. We introduce a flexible notion of type, deemed "locus types", that aims to minimize syntactic burden and the need for refactoring when introducing types to dynamic code. Locus types are gained by objects that pass through an annotated code site, following the creed of "code is types". Their structure is inferred from local type information computed through flow-based type refinement. The design of LocusTypeScript, a language extending TypeScript with locus types, is detailed. Tooling support, building on that of TypeScript, for programming with locus types is described. As well, the general properties and applicability of locus types are explored. LocusTypeScript's simple algorithm for computing stable flow-based refinement types is presented. The implications and performance impact of making locus types sound are discussed

Building an IDE for the Calculational Derivation of Imperative Programs

In this paper, we describe an IDE called CAPS (Calculational Assistant for Programming from Specifications) for the interactive, calculational derivation of imperative programs. In building CAPS, our aim has been to make the IDE accessible to non-experts while retaining the overall flavor of the pen-and-paper calculational style. We discuss the overall architecture of the CAPS system, the main features of the IDE, the GUI design, and the trade-offs involved.Comment: In Proceedings F-IDE 2015, arXiv:1508.0338