Design and evaluation of contracts for gradual typing

Gradual typing aims to improve the correctness of dynamically typed programs by incrementally adding type information. Sound gradual typing performs static type checking and inserts run-time checks when a type cannot be guaranteed statically. This form of gradual typing offers many features, but also requires that the programmer uses a language with a specialised gradual type system. A lightweight form of gradual typing uses contracts to enforce types at run-time, assigning blame when a type assertion fails. Contracts can be implemented as a library, without requiring a specialised gradual type system. Contracts provide a lower barrier of entry into sound gradual typing. This thesis investigates the design and evaluation of contracts for gradual typing, focusing on bridging the gap between JavaScript (dynamic) and TypeScript (static). There are two key outcomes regarding theory and practice. Contracts for higher-order intersection and union types can be designed in a uniform way, using blame to derive the semantics of contracts satisfaction. Contracts and gradual typing can be evaluated using the DefinitelyTyped repository, where JavaScript libraries are annotated with TypeScript definition files. Contract composition is the fundamental method for building complex type assertions. Intersection and union types are well suited for describing patterns common to dynamically typed programs. Our first contribution is to present a calculus of contracts for intersection and union types with blame assignment, giving a uniform treatment to both operators. A correct model of contracts must include a definition of contract satisfaction. Our second contribution is to show that contract satisfaction can be defined using blame: satisfying programs are those that do not elicit blame when monitored. We define a series of properties mandating how contract satisfaction should compose, ensuring that a contract for a type behaves as one would expect for that type. Building on our technical developments, our third contribution is a practical evaluation of gradual typing using the DefinitelyTyped repository. We show that contracts can be used to enforce conformance to a definition file, detecting errors in the specification. Our evaluation also reveals that technical concerns associated with implementing contracts using JavaScript proxies are a problem in practice

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

Lucretia - intersection type polymorphism for scripting languages

Scripting code may present maintenance problems in the long run. There is, then, the call for methodologies that make it possible to control the properties of programs written in dynamic languages in an automatic fashion. We introduce Lucretia, a core language with an introspection primitive. Lucretia is equipped with a (retrofitted) static type system based on local updates of types that describe the structure of objects being used. In this way, we deal with one of the most dynamic features of scripting languages, that is, the runtime modification of object interfaces. Judgements in our systems have a Hoare-like shape, as they have a precondition and a postcondition part. Preconditions describe static approximations of the interfaces of visible objects before a certain expression has been executed and postconditions describe them after its execution. The field update operation complicates the issue of aliasing in the system. We cope with it by introducing intersection types in method signatures.Comment: In Proceedings ITRS 2014, arXiv:1503.0437

Gradual Program Analysis

Dataflow analysis and gradual typing are both well-studied methods to gain information about computer programs in a finite amount of time. The gradual program analysis project seeks to combine those two techniques in order to gain the benefits of both. This thesis explores the background information necessary to understand gradual program analysis, and then briefly discusses the research itself, with reference to publication of work done so far. The background topics include essential aspects of programming language theory, such as syntax, semantics, and static typing; dataflow analysis concepts, such as abstract interpretation, semilattices, and fixpoint computations; and gradual typing theory, such as the concept of an unknown type, liftings of predicates, and liftings of functions