Search for: All records

Contract systems enable programmers to state specifications and have them enforced at run time. First-order contracts are expressed using ordinary code, while higher-order contracts are expressed using the notation familiar from type systems. Most interface descriptions, though, come with properties that involve not just assertions about single method calls, but entire call chains. Typical contract systems cannot express these specifications concisely. Such specifications demand domain-specific notations. In response, this paper proposes that contract systems abstract over the notation used for stating specifications. It presents an architecture for such a system, some illustrative examples, and an evaluation in terms of common notations from the literature. 

Software contracts empower programmers to describe functional properties of components. When it comes to constraining effects, though, the literature offers only one-off solutions for various effects. It lacks a universal principle. This paper presents the design of an effectful contract system in the context of effect handlers. A key metatheorem shows that contracts cannot unduly interfere with a program’s execution. An implementation of this design, along with an evaluation of its generality, demonstrates that the theory can guide practice. 

The literature presents many strategies for enforcing the integrity of types when typed code interacts with untyped code. This article presents a uniform evaluation framework that characterizes the differences among some major existing semantics for typed–untyped interaction. Type system designers can use this framework to analyze the guarantees of their own dynamic semantics. 

Abstract Behavioral software contracts allow programmers to strengthen the obligations and promises that they express with conventional types. They lack expressive power, though, when it comes to invariants that hold across several function calls. Trace contracts narrow this expressiveness gap. A trace contract is a predicate over the sequence of values that flow through function calls and returns. This paper presents a principled design, an implementation, and an evaluation of trace contracts. 

Eelco Visser envisioned a future where DSLs become a commonplace abstraction in software development. He took strides towards implementing this vision with the Spoofax language workbench. However, his vision is far from the mainstream of programming today. How will the many mainstream programmers encounter and adopt language workbench technology? We propose that the macro systems found in emerging industrial languages open a path towards delivering language workbenches as easy-to-adopt libraries. To develop the idea, we sketch an implementation of a language workbench as a macro-library atop Racket and identify the key features of the macro system needed to enable this evolution path. 

Sound migratory typing envisions a safe and smooth refactoring of untyped code bases to typed ones. However, the cost of enforcing safety with run-time checks is often prohibitively high, thus performance regressions are a likely occurrence. Additional types can often recover performance, but choosing the right components to type is difficult because of the exponential size of the migratory typing lattice. In principal though, migration could be guided by off-the-shelf profiling tools. To examine this hypothesis, this paper follows the rational programmer method and reports on the results of an experiment on tens of thousands of performance-debugging scenarios via seventeen strategies for turning profiler output into an actionable next step. The most effective strategy is the use of deep types to eliminate the most costly boundaries between typed and untyped components; this strategy succeeds in more than 50% of scenarios if two performance degradations are tolerable along the way. 

Equipping an existing programming language with a gradual type system requires two major steps. The first and most visible one in academia is to add a notation for types and a type checking apparatus. The second, highly practical one is to provide a type veneer for the large number of existing untyped libraries; doing so enables typed components to import pieces of functionality and get their uses type-checked, without any changes to the libraries. When programmers create such typed veneers for libraries, they make mistakes that persist and cause trouble. The question is whether the academically investigated run-time checks for gradual type systems assist programmers with debugging such mistakes. This paper provides a first, surprising answer to this question via a rational-programmer investigation: run-time checks alone are typically less helpful than the safety checks of the underlying language. Combining Natural run-time checks with blame, however, provides significantly superior debugging hints. 

Programming language theoreticians develop blame assignment systems and prove blame theorems for gradually typed programming languages. Practical implementations of gradual typing almost completely ignore the idea of blame assignment. This contrast raises the question whether blame provides any value to the working programmer and poses the challenge of how to evaluate the effectiveness of blame assignment strategies. This paper contributes (1) the first evaluation method for blame assignment strategies and (2) the results from applying it to three different semantics for gradual typing. These results cast doubt on the theoretical effectiveness of blame in gradual typing. In most scenarios, strategies with imprecise blame assignment are as helpful to a rationally acting programmer as strategies with provably correct blame.