Peter Thiemann

Type Analysis for JavaScript

JavaScript is the main scripting language for Web browsers, and it is essential to modern Web applications. Programmers have started using it for writing complex applications, but there is still little tool support available during development. We present a static program analysis infrastructure that can infer detailed and sound type information for JavaScript programs using abstract interpretation. The analysis is designed to support the full language as defined in the ECMAScript standard, including its peculiar object model and all built-in functions. The analysis results can be used to detect common programming errors --- or rather, prove their absence, and for producing type information for program comprehension. Preliminary experiments conducted on real-life JavaScript code indicate that the approach is promising regarding analysis precision on small and medium size programs, which constitute the majority of JavaScript applications. With potential for further improvement, we propose the analysis as a foundation for building tools that can aid JavaScript programmers.

Towards a type system for analyzing javascript programs

JavaScript is a popular language for client-side web scripting. It has a dubious reputation among programmers for two reasons. First, many JavaScript programs are written against a rapidly evolving API whose implementations are sometimes contradictory and idiosyncratic. Second, the language is only weakly typed and comes virtually without development tools. The present work is a first attempt to address the second point. It does so by defining a type system that tracks the possible traits of an object and flags suspicious type conversions. Because JavaScript is a classless, object-based language with first-class functions, the type system must include singleton types, subtyping, and first class record labels. The type system covers a representative subset of the language and there is a type soundness proof with respect to an operational semantics.

Functional and Logic Programming

proceedings of the 11th International symposium on Functional and Logic Programming (FLOPS 2012), held in Kobe, Japan, May 23-25, 2012

WASH/CGI: Server-Side Web Scripting with Sessions and Typed, Compositional Forms

The common gateway interface (CGI) is one of the prevalent methods to provide dynamic contents on the Web. Since it is cumbersome to use in its raw form, there are many libraries that make CGI programming easier.WASH/CGI is a Haskell library for server-side Web scripting. Its implementation relies on CGI, but its use avoids most of CGIs drawbacks. It incorporates the concept of a session, provides a typed, compositional approach to constructing interaction elements (forms), and relies on call-backs to specify control flow. From a programmers perspective, programming WASH/CGI is like programming a graphical user interface (GUI), where the layout is specified using HTML via a novel monadic interface.

Cogen in six lines

We have designed and implemented a program-generator generator (PGG) for an untyped higher-order functional programming language. The program generators perform continuation-based multi-level offline specialization and thus combine the most powerful and general offline partial evaluation techniques. The correctness of the PGG is ensured by deriving it from a multi-level specialize. Our PGG is extremely simple to implement due to the use of multi-level techniques and higher-order abstract syntax.

An Implementation of Session Types

A session type is an abstraction of a set of sequences of heterogeneous values sent and received over a communication channel. Session types can be used for specifying stream-based Internet protocols.

Single and loving it: must-alias analysis for higher-order languages

In standard control-flow analyses for higher-order languages, a single abstract binding for a variable represents a set of exact bindings, and a single abstract reference cell represents a set of exact reference cells. While such analyses provide useful may-alias information, they are unable to answer mustalias questions about variables and cells, as these questions ask about equality of specific bindings and references.In this paper, we present a novel program analysis for higher-order languages that answers must-alias questions. At every program point, the analysis associates with each variable and abstract cell a cardinality, which is either single or multiple. If variable x is single at program point p, then all bindings for x in the heap reachable from the environment at p hold the same value. If abstract cell r is single at p, then at most one exact cell corresponding to r is reachable from the environment at p.Must-alias information facilitates various program optimizations such as lightweight closure conversion [19]. In addition, must-alias information permits analyses to perform strong updates [3] on abstract reference cells known to be single. Strong updates improve analysis precision for programs that make significant use of state.A prototype implementation of our analysis yields encouraging results. Over a range of benchmarks, our analysis classifies a large majority of the variables as single.

An embedded domain-specific language for type-safe server-side web scripting

WASH/CGI is an embedded domain-specific language for server-side Web scripting. Due to its reliance on the strongly typed, purely functional programming language Haskell as a host language, it is highly flexible and---at the same time---it provides extensive guarantees due to its pervasive use of type information.WASH/CGI can be structured into a number of sublanguages addressing different aspects of the application. The document sublanguage provides tools for the generation of parameterized XHTML documents and forms. Its typing guarantees that almost all generated documents are valid XHTML documents. The session sublanguage provides a session abstraction with a transparent notion of session state and allows the composition of documents and Web forms to entire interactive scripts. Both are integrated with the widget sublanguage which describes the communication (parameter passing) between client and server. It imposes a simple type discipline on the parameters that guarantees that forms posted by the client are always understood by the server. That is, the server never asks for data not submitted by the client and the data submitted by the client has the type requested by the server. In addition, parameters are received in their typed internal representation, not as strings. Finally, the persistence sublanguage deals with managing shared state on the server side as well as individual state on the client side. It presents shared state as an abstract data type, where the script can control whether it wants to observe mutations due to concurrently executing scripts. It guarantees that states from different interaction threads cannot be confused.

Interprocedural analysis with lazy propagation

We propose lazy propagation as a technique for flow- and context-sensitive interprocedural analysis of programs with objects and first-class functions where transfer functions may not be distributive. The technique is described formally as a systematic modification of a variant of the monotone framework and its theoretical properties are shown. It is implemented in a type analysis tool for JavaScript where it results in a significant improvement in performance.

Combinators for program generation

We present a general method to transform a compositional specification of a specializer for a functional programming language into a set of combinators that can be used to perform the same specialization more efficiently. The main transformation steps are the transition to higher-order abstract syntax and untagging. All transformation steps are proved correct. The resulting combinators can be implemented in any functional language, typed or untyped, pure or impure. They may also be considered as forming a domain-specific language for meta-programming. We demonstrate the generality of the method by applying it to several specializers of increasing strength. We demonstrate its efficiency by comparing it with a traditional specialization system based on self-application.