Triggered by the bridge parsing paper that Emma Nilsson-Nyman presented at SLE 2008, we started working in 2009 on error recovery for SGLR parsing in order to make Spoofax editors robust in the presence of syntactic errors. Most editor services, from syntax highlighting to code completion, depend on an abstract syntax tree. Since the programs are frequently in a syntactically incorrect state during editing, many editor services would break without parse error recovery.

Because of the parallel forking nature of GLR parsing, error recovery looked like an impossible problem to solve. We ended up developing an interesting mix of techniques consisting of permissive grammars, a back-tracking extension of SGLR, and layout-sensitive error region discovery that produce good error recovery without language designer intervention.

However, evaluating the quality of error recovery turned to be a laborious process with lots of pitfalls. In an ASE 2012 short paper that Maartje presented last week at the conference, a solution to this problem is presented. By generating programs with errors from correct programs, we cheaply get a large collection of test programs for which we know a good recovery. The generators randomly insert errors guided by rules about typical types of errors that occur during programming.

Maartje de Jonge, Eelco Visser. Automated Evaluation of Syntax Error Recovery. In 27th IEEE/ACM International Conference on Automated Software Engineering (ASE 2012), September 3-7, Essen, Germany. pages 322-325, ACM, 2012.

Abstract: Evaluation of parse error recovery techniques is an open problem. The community lacks objective standards and methods to measure the quality of recovery results. This paper proposes an automated technique for recovery evaluation that offers a solution for two main problems in this area. First, a representative testset is generated by a mutation based fuzzing technique that applies knowledge about common syntax errors. Secondly, the quality of the recovery results is automatically measured using an oracle-based evaluation technique. We evaluate the validity of our approach by comparing results obtained by automated evaluation with results obtained by manual inspection. The evaluation shows a clear correspondence between our quality metric and human judgement.

We have extended the Spoofax Language Workbench with a domain-specific language for specifying the name binding and scope rules of programming languages. Instead of programmatically encoding name resolution algorithms, as is standing practice, a language designer defines name binding in terms of four basic domain-specific concepts: 'definitions', 'references', 'scopes', and 'imports'. With these concepts a wide variety of name bindings can be expressed non-algorithmically. For example, the following rules define the binding of base class references (inheritance) and types to class definitions in C#:

    rules
      Class(NonPartial(), c, _, _) : 
        defines unique class c
      Class(Partial(), c, _, _) : 
        defines non−unique class c 
      Base(c) : 
        refers to class c
      ClassType(c) : 
        refers to class c

From such a definition we automatically derive a name resolution algorithm that is used as the basis for editor services such as reference resolution and code completion. Our hope is that NBL can play the role for name binding that BNF plays for syntax definition. That is, that a single declarative definition can be used as the basis for implementation and documentation.

NBL is already available in the nightly builds of Spoofax. In the coming months we will be spreading the word at various events. We will present a poster at SPLASH and SLE. I will present our NBL paper at SLE 2012, and the language will feature in my grammarware tutorial at SLE as well. I may be giving a talk in the Bay Area as well.

For a full account of NBL see our SLE 2012 paper:

Gabriël D. P. Konat, Lennart C. L. Kats, Guido Wachsmuth, Eelco Visser. Declarative Name Binding and Scope Rules. In Krzysztof Czarnecki, Görel Hedin, editors, Software Language Engineering, 5th International Conference, SLE 2012, Dresden, Germany, September 26-28, 2012, Revised Selected Papers. Volume 7745 of Lecture Notes in Computer Science, pages 311-331, 2013.

Abstract: In textual software languages, names are used to reference elements like variables, methods, classes, etc. Name resolution analyses these names in order to establish references between definition and use sites of elements. In this paper, we identify reoccurring patterns for name bindings in programming languages and introduce a declarative metalanguage for the specification of name bindings in terms of namespaces, definition sites, use sites, and scopes. Based on such declarative name binding specifications, we provide a language-parametric algorithm for static name resolution during compile-time. We discuss the integration of the algorithm into the Spoofax Language Workbench and show how its results can be employed in semantic editor services like reference resolution, constraint checking, and content completion.

Innovation emerges from the re-combination of existing ideas. Copyright and patent laws are detrimental to innovation. Argues Kirby Ferguson in his Embrace the Remix TED Talk.

Transformations and Abstractions | page 3 (20120907)

In April 2011 I launched this blog. It is a continuation of earlier blogs that ran on third party software. Last year I wrote my own application in WebDSL.

One of the greatest challenges in web programming I found to be the graphical design of sites. Achieving a web site with a modern look with CSS is much like programming a compiler in assembly language. It requires a considerable amount of low-level knowledge. So I was fairly satisfied with the design of my blog as seen in the screen shot on the right. The color scheme was based on Solarized, which was developed to optimize readability of code in editors. The font for the headers was some funny Google font.

However, there was one striking problem with the site. It was not very readable on mobile devices such as phones and tablets. So, I set out to develop a mobile web application in mobl. That project was orphaned when I had other projects to do. But I was also struggling with the motivation for the project; an offline-capable application for a blog/wiki site is not exactly what is needed.

For the WebLab project I had discovered Twitter Bootstrap, which makes web design a breeze. At least, it allows someone like me (with limited time to divulge in CSS hacking) to focus on the functional design and the high-level web design; as opposed to the ad hoc low-level CSS hacking that I was doing before. WebLab has a fixed size layout designed for a screen with 1024 pixels of resolution, as did my blog application.

Now, I've taken a dive into Bootstrap's features for responsive design, and redesigned this site. After porting the layout to Bootstrap, figuring out the drop down menus, and adapting the default Bootstrap CSS a bit, the main work was in making the header and footer menus configurable. The important difference with respect to the old design is that the sidebar is gone; the information has been moved to the header menus or to the itemized lists in the footer. In addition, I extended the wiki with a notion of groups, so that a set of wiki pages can share navigation menus.

You are looking at the result (unless you are reading this in a feed reader). Blog and wiki pages should be equally readable on phones, tables, laptops, and desktop computers. Even the images are automatically re-sized, without a minute of my time.

If you are interested in writing WebDSL applications with Bootstrap you will find my elib WebDSL library useful. It provides templates abstracting from common Bootstrap patterns. In addition it provides a bunch of other useful WebDSL utilities developed over the course of several projects.

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Last June I visited Ulrik Pagh Schultz in Odense, where TU Delft student Wouter van Koppen is doing his Master's thesis project on domain-specific languages for programming modular robots. Here are some Instagram impressions of the visit.

Untitled Untitled Untitled Untitled ulrik pagh schultz wouter van koppen wouter, ulrik, eelco Untitled odense odense