Biblio

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2022-01-12
Coblenz, Michael, Kambhatla, Gauri, Koronkevich, Paulette, Wise, Jenna, Barnaby, Celeste, Aldrich, Jonathan, Sunshine, Joshua.  2021.  PLIERS: A Process that Integrates User-Centered Methods into Programming Language Design. ACM Transactions on Computer-Human Interaction (TOCHI).
Programming language design requires making many usability-related design decisions. However, existing HCI methods can be impractical to apply to programming languages: they have high iteration costs, programmers require significant learning time, and user performance has high variance. To address these problems, we adapted both formative and summative HCI methods to make them more suitable for programming language design. We integrated these methods into a new process, PLIERS, for designing programming languages in a user-centered way. We evaluated PLIERS by using it to design two new programming languages. Glacier extends Java to enable programmers to express immutability properties effectively and easily. Obsidian is a language for blockchains that includes verification of critical safety properties. Summative usability studies showed that programmers were able to program effectively in both languages after short training periods.
2021-03-09
Coblenz, Michael.  2020.  User-Centered Design of Principled Programming Languages. Computer Science Department. PhD:299.

Programming languages exist to enable people to create and maintain software as effectively as possible. They are subject to two very different sets of requirements: first, the need to provide strong safety guarantees to protect against bugs; and second, the need for users to effectively and efficiently write software that meets their functional and quality requirements. This thesis argues that fusing formal methods for reasoning about programming languages with user-centered design methods is a practical approach to designing languages that make programmers more effective. By doing so, designers can create safer languages that are more effective for programmers than existing languages. The thesis is substantiated by the introduction of PLIERS: Programming Language Iterative Evaluation and Refinement System. PLIERS is a process for designing programming languages that integrates formal methods with user-centered design. The hypothesis that PLIERS is beneficial is supported by two language design projects. In these projects, I show how PLIERS benefits the programming language design process. Glacier is an extension to Java that enforces transitive class immutability, which is a much stronger property than that provided by languages that are in use today. Although there are hundreds of possible ways of restricting changes to state in programming languages, Glacier occupies a point in the design space that was justified by interview studies with professional software engineers. I evaluated Glacier in case studies, showing that it is expressive enough for some real-world applications. I also evaluated Glacier in lab studies and compared it to Java’s final keyword, finding both that Glacier is more effective at expressing immutability than final and that Glacier detects bugs that users are likely to insert in code. Blockchains are a distributed computing platform that aim to enable safe computation among users who do not necessarily trust each other. To improve safety relative to existing languages, in which programmers have repeatedly deployed software with serious bugs, I designed Obsidian, a new programming language for blockchain application development. From observations about typical blockchain applications, I derived two features that motivated the design of Obsidian. First, blockchain applications typically implement state machines, which support different operations in different states. Obsidian uses typestate, which lifts dynamic state into static types, to provide static guarantees regarding object state. Second, blockchain applications frequently manipulate resources, such as virtual currency. Obsidian provides a notion of ownership, which distinguishes one reference from all others. Obsidian supports resources via linear types, which ensure that owning references to resources are not accidentally lost. The combination of resources and typestate results in a novel set of design problems for the type system; although each idea has been explored individually, combining them requires unifying different perspectives on linearity. Furthermore, no language with either one of these features has been designed in a user-centered way or evaluated with users. Typical typestate systems have a complex set of permissions that provides safety properties, but these systems focused on expressiveness rather than on usability. Obsidian integrates typestate and resources in a novel way, resulting in a new type system design with a focus on simplicity and usability while retaining the desired safety properties. Obsidian is based on a core calculus I designed, Silica, for which I proved type soundness. In order to make Obsidian as usable as possible, I based its design on the results of formative studies with programmers. I evaluated Obsidian with two case studies, showing that Obsidian can be used to implement relevant programs. I also conducted a randomized controlled trial comparing Obsidian to Solidity, a popular language for writing smart contracts. I found that most of the Obsidian participants learned Obsidian and completed programming tasks after only a short training period; further, in one task, 70% of the participants who used Solidity accidentally inserted bugs that Obsidian’s compiler would have detected. Finally, Obsidian participants completed significantly more tasks correctly than did Solidity participants.

Coblenz, Michael, Aldrich, Jonathan, Myers, Brad A., Sunshine, Joshua.  2020.  Can Advanced Type Systems Be Usable? An Empirical Study of Ownership, Assets, and Typestate in Obsidian ACM Journals: Proceedings of the ACM on Programming Languages. 4

Some blockchain programs (smart contracts) have included serious security vulnerabilities. Obsidian is a new typestate-oriented programming language that uses a strong type system to rule out some of these vulnerabilities. Although Obsidian was designed to promote usability to make it as easy as possible to write programs, strong type systems can cause a language to be difficult to use. In particular, ownership, typestate, and assets, which Obsidian uses to provide safety guarantees, have not seen broad adoption together in popular languages and result in significant usability challenges. We performed an empirical study with 20 participants comparing Obsidian to Solidity, which is the language most commonly used for writing smart contracts today. We observed that Obsidian participants were able to successfully complete more of the programming tasks than the Solidity participants. We also found that the Solidity participants commonly inserted asset-related bugs, which Obsidian detects at compile time.

Coblenz, Michael, Oei, Reed, Etzel, Tyler, Koronkevich, Paulette, Baker, Miles, Bloem, Yannick, Myers, Brad A., Aldrich, Jonathan, Sunshine, Joshua.  2020.  Obsidian: Typestate and Assets for Safer Blockchain Programming. ACM Journals: ACM Transactions on Programming Languages and Systems. 42

Blockchain platforms are coming into use for processing critical transactions among participants who have not established mutual trust. Many blockchains are programmable, supporting smart contracts, which maintain persistent state and support transactions that transform the state. Unfortunately, bugs in many smart contracts have been exploited by hackers. Obsidian is a novel programming language with a type system that enables static detection of bugs that are common in smart contracts today. Obsidian is based on a core calculus, Silica, for which we proved type soundness. Obsidian uses typestate to detect improper state manipulation and uses linear types to detect abuse of assets. We integrated a permissions system that encodes a notion of ownership to allow for safe, flexible aliasing. We describe two case studies that evaluate Obsidian’s applicability to the domains of parametric insurance and supply chain management, finding that Obsidian’s type system facilitates reasoning about high-level states and ownership of resources. We compared our Obsidian implementation to a Solidity implementation, observing that the Solidity implementation requires much boilerplate checking and tracking of state, whereas Obsidian does this work statically.

2020-03-27
Coblenz, Michael, Sunshine, Joshua, Aldrich, Jonathan, Myers, Brad A..  2019.  Smarter Smart Contract Development Tools. 2019 IEEE/ACM 2nd International Workshop on Emerging Trends in Software Engineering for Blockchain (WETSEB). :48–51.

Much recent work focuses on finding bugs and security vulnerabilities in smart contracts written in existing languages. Although this approach may be helpful, it does not address flaws in the underlying programming language, which can facilitate writing buggy code in the first place. We advocate a re-thinking of the blockchain software engineering tool set, starting with the programming language in which smart contracts are written. In this paper, we propose and justify requirements for a new generation of blockchain software development tools. New tools should (1) consider users' needs as a primary concern; (2) seek to facilitate safe development by detecting relevant classes of serious bugs at compile time; (3) as much as possible, be blockchain-agnostic, given the wide variety of different blockchain platforms available, and leverage the properties that are common among blockchain environments to improve safety and developer effectiveness.

2015-01-12
Coblenz, Michael, Aldrich, Jonathan, Myers, Bradley, Sunshine, Joshua.  2014.  Considering Productivity Effects of Explicit Type Declarations. Workshop on Evaluation and Usability of Programming Languages and Tools (PLATEAU), 2014.

Static types may be used both by the language implementation and directly by the user as documentation. Though much existing work focuses primarily on the implications of static types on the semantics of programs, relatively little work considers the impact on usability that static types pro- vide. Though the omission of static type information may decrease program length and thereby improve readability, it may also decrease readability because users must then frequently derive type information manually while reading programs. As type inference becomes more popular in languages that are in widespread use, it is important to consider whether the adoption of type inference may impact productivity of developers.