英语原文共 16 页

目 录

一、web编程.....................................................................................................................................................................17

二、概述.............................................................................................................................................................................19

三、JavaScript的连续性....................................................................................................................................................20

四、JavaScript的子语言....................................................................................................................................................24

五、执行控制.....................................................................................................................................................................25

六、评估.............................................................................................................................................................................27

七、相关研究.....................................................................................................................................................................31

八、结论.............................................................................................................................................................................33

致谢.....................................................................................................................................................................................33

Putting in All the Stops: Execution Control for JavaScript

Samuel Baxter

University of Massachusetts Amherst

Rachit Nigam

University of Massachusetts Amherst

Joe Gibbs Politz

University of California San Diego

Abstract

Shriram Krishnamurthi

Brown University

Arjun Guha

University of Massachusetts Amherst

viewed as presenting additional challenges to the creators of

Scores of compilers produce JavaScript, enabling program- mers to use many languages on the Web, reuse existing code, and even use Web IDEs. Unfortunately, most compilers inherit the browserrsquo;s compromised execution model, so long- running programs freeze the browser tab, infinite loops crash IDEs, and so on. The few compilers that avoid these problems suffer poor performance and are difficult to engineer.

This paper presents Stopify, a source-to-source compiler that extends JavaScript with debugging abstractions and blocking operations, and easily integrates with existing com- pilers. We apply Stopify to ten programming languages and develop a Web IDE that supports stopping, single-stepping, breakpointing, and long-running computations. For nine lan- guages, Stopify requires no or trivial compiler changes. For eight, our IDE is the first that provides these features. Two of our subject languages have compilers with similar features. Stopifyrsquo;s performance is competitive with these compilers and it makes them dramatically simpler.

Stopifyrsquo;s abstractions rely on first-class continuations, which it provides by compiling JavaScript to JavaScript. We also identify sub-languages of JavaScript that compilers im- plicitly use, and exploit these to improve performance. Fi- nally, Stopify needs to repeatedly interrupt and resume pro- gram execution. We use a sampling-based technique to esti- mate program speed that outperforms other systems.

1 Programming On the Web

Scores of programming languages now compile to JavaScript and run on the Web [23] and there are several Web IDEs in widespread use [4, 6–8, 48, 57, 69, 74, 78, 83]. This grow- ing audience for Web IDEs and languages that run in the browser includes both professionals and students. Unfortu- nately, JavaScript and the Web platform lack the abstractions necessary to build IDEs and serve as a complete compilation target for high-level languages. As a result, most compilers that produce JavaScript compromise on semantics and most Web IDEs compromise on basic debugging and usability fea- tures. Furthermore, as we explain in sect;7, new technologies such as WebAssembly and Web Workers do not address most of the problems that we address. Instead, our work may be

those technologies.

Limitations in Web IDEs The key problem facing a Web IDE is that JavaScript has a single-threaded execution envi- ronment. An IDE that wants to provide a “stop” button to halt runaway computation faces a nontrivial problem, be- cause the callback for that button gets queued for execution behind the running code—which is not going to terminate. Related to this, to make pages responsive, the browser threat- ens to interrupt computations that run longer than a few seconds. This means long-running computations need to be broken up into short events. In turn, because computations cannot run for very long, JavaScript engines in browsers tend to provide very shallow stacks, which is problematic for functional programs that rely on recursion.

These limitations are not at all hypothetical. For example,

Codecademy has a Web IDE for JavaScript and for Python. In response to several message board requests, Codecademy has a help article that explicitly addresses infinite loops that freeze the browser [81]: they suggest refreshing the page, whi

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