Jay A. McCarthy

Implementation and use of the PLT scheme Web server

Abstract The PLT Scheme Web Server uses continuations to enable a natural, console-like program development style. We describe the implementation of the server and its use in the development of an application for managing conference paper reviews. In the process of developing this application, we encountered subtle forms of interaction not directly addressed by using continuations. We discuss these subtleties and offer solutions that have been successfully deployed in our application. Finally, we present some details on the server’s performance, which is comparable to that of the widely-used Apache Web server.

Temporal higher-order contracts

Behavioral contracts are embraced by software engineers because they document module interfaces, detect interface violations, and help identify faulty modules (packages, classes, functions, etc). This paper extends prior higher-order contract systems to also express and enforce temporal properties, which are common in software systems with imperative state, but which are mostly left implicit or are at best informally specified. The paper presents both a programmatic contract API as well as a temporal contract language, and reports on experience and performance results from implementing these contracts in Racket. Our development formalizes module behavior as a trace of events such as function calls and returns. Our contract system provides both non-interference (where contracts cannot influence correct executions) and also a notion of completeness (where contracts can enforce any decidable, prefix-closed predicate on event traces).

The Racket Manifesto.

The creation of a programming language calls for guiding principles that point the developers to goals. This article spells out the three basic principles behind the 20-year development of Racket. First, programming is about stating and solving problems, and this activity normally takes place in a context with its own language of discourse; good programmers ought to formulate this language as a programming language. Hence, Racket is a programming language for creating new programming languages. Second, by following this language-oriented approach to programming, systems become multi-lingual collections of interconnected components. Each language and component must be able to protect its specific invariants. In support, Racket offers protection mechanisms to implement a full language spectrum, from C-level bit manipulation to soundly typed extensions. Third, because Racket considers programming as problem solving in the correct language, Racket also turns extra-linguistic mechanisms into linguistic constructs, especially mechanisms for managing resources and projects. The paper explains these principles and how Racket lives up to them, presents the evaluation framework behind the design process, and concludes with a sketch of Rackets imperfections and opportunities for future improvements.

Deploying Exploring Computer Science Statewide

Exploring Computer Science (ECS) is a high school introductory computer science class designed to increase student interest in CS. Utah is the first state to offer ECS statewide and use it to meet a high school graduation requirement. Over the past four years, 150 teachers have been trained as Utah ECS teachers and over 10,000 Utah students have taken the class. The Utah initiative is unique because it is the first to deploy ECS in a non-urban environment and with a modified half-year curriculum that includes no additional equipment costs. This paper discusses how the Utah deployment was organized, reports its results and unique difficulties, and offers lessons for deployments with similar characteristics: statewide, rural, and limited resources.

Proving MCAPI executions are correct using SMT

Asynchronous message passing is an important paradigm in writing applications for embedded heterogeneous multicore systems. The Multicore Association (MCA), an industry consortium promoting multicore technology, is working to standardize message passing into a single API, MCAPI, for bare metal implementation and portability across platforms. Correctness in such an API is difficult to reason about manually, and testing against reference solutions is equally difficult as reference solutions implement an unknown set of allowed behaviors, and programmers have no way to directly control API internals to expose or reproduce errors. This paper provides a way to encode an MCAPI execution as a Satisfiability Modulo Theories (SMT) problem, which if satisfiable, yields a feasible execution schedule on the same trace, such that it resolves non-determinism in the MCAPI runtime in a way that it now fails user provided assertions. The paper proves the problem is NP-complete. The encoding is useful for test, debug, and verification of MCAPI program execution. The novelty in the encoding is the direct use of match pairs (potential send and receive couplings). Match-pair encoding for MCAPI executions, when compared to other encoding strategies, is simpler to reason about, results in significantly fewer terms in the SMT problem, and captures feasible behaviors that are ignored in previously published techniques. Further, to our knowledge, this is the first SMT encoding that is able to run in infinite-buffer semantics, meaning the runtime has unlimited internal buffering as opposed to no internal buffering. Results demonstrate that the SMT encoding, restricted to zero-buffer semantics, uses fewer clauses when compared to another zero-buffer technique, and it runs faster and uses less memory. As a result the encoding scales well for programs with high levels of non-determinism in how sends and receives may potentially match.

Cryptographic Protocol Explication and End-Point Projection

Cryptographic protocols are useful for engineering trust in transactions. There are several languages for describing these protocols, but these tend to capture the communications from the perspective of an individual role. In contrast, traditional protocol descriptions as found in a state of nature tend to employ a whole-protocol description, resulting in an impedance mismatch. In this paper we present two results to address this gap between human descriptions and deployable specifications. The first is an end-point projection technique that consumes an explicit whole-protocol description and generates specifications that capture the behavior of each participant role. In practice, however, many whole-protocol descriptions contain idiomatic forms of implicit specification. We therefore present our second result, a transformation that identifies and eliminates these implicit patterns, thereby preparing protocols for end-point projection. Concretely, our tools consume protocols written in our whole-protocol language, wppl , and generate role descriptions in the cryptographic protocol programming language, cppl . We have formalized and established properties of the transformations using the Coq proof assistant. We have validated our transformations by applying them successfully to most of the protocols in the spore repository.

Running Probabilistic Programs Backwards

Many probabilistic programming languages allow programs to be run under constraints in order to carry out Bayesian inference. Running programs under constraints could enable other uses such as rare event simulation and probabilistic verification—except that all such probabilistic languages are necessarily limited because they are defined or implemented in terms of an impoverished theory of probability. Measure-theoretic probability provides a more general foundation, but its generality makes finding computational content difficult.

Automatically RESTful web applications: marking modular serializable continuations

Continuation-based Web servers provide distinct advantages over traditional Web application development: expressive power and modularity. This power leads to fewer errors and more interesting applications. Furthermore, these Web servers are more than prototypes; they are used in some real commercial applications. Unfortunately, they pay a heavy price for the additional power in the form of lack of scalability. We fix this key problem with a modular program transformation that produces scalable, continuation-based Web programs based on the REST architecture. Our programs use the same features as non-scalable, continuation-based Web programs, so we do not sacrifice expressive power for performance. In particular, we allow continuation marks in Web programs. Our system uses 10 percent (or less) of the memory required by previous approaches.

A Coq Library for Internal Verification of Running-Times

This paper presents a Coq library that lifts an abstract yet precise notion of running-time into the type of a function. Our library is based on a monad that counts abstract steps, controlled by one of the monadic operations. The monad’s computational content, however, is simply that of the identity monad so programs written in our monad (that recur on the natural structure of their arguments) extract into idiomatic OCaml code. We evaluated the expressiveness of the library by proving that red-black tree insertion and search, merge sort, insertion sort, Fibonacci, iterated list insertion, BigNum addition, and Okasaki’s Braun Tree algorithms all have their expected running times.

The two-state solution: native and serializable continuations accord

Continuation-based Web servers provide advantages over traditional Web application development through the increase of expressive power they allow. This leads to fewer errors and more productivity for the programmers that adopt them. Unfortunately, existing implementation techniques force a hard choice between scalability and expressiveness. Our technique allows a smoother path to scalable, continuation-based Web programs. We present a modular program transformation that allows scalable Web applications to use third-party, higher-order libraries with higher-order arguments that cause Web interaction. Consequently, our system provides existing Web applications with more scalability through significantly less memory use than the traditional technique.