Abstract. Domain-specific languages (DSLs) are increasingly used as embedded languages within general-purpose host languages. DSLs provide a compact, dedicated syntax for specifying parts of an application related to specialized domains. Unfortunately, such language extensions typically do not integrate well with the development tools of the host language. Editors, compilers and debuggers are either unaware of the extensions, or must be adapted at a non-trivial cost. We present a novel approach to embed DSLs into an existing host language by leveraging the underlying representation of the host language used by these tools. Helvetia is an extensible system that intercepts the compilation pipeline of the Smalltalk host language to seamlessly integrate language extensions. We validate our approach by case studies that demonstrate three fundamentally different ways to extend or adapt the host language syntax and semantics. 1

Packrat parsing offers several advantages over other parsing techniques, such as the guarantee of linear parse times while supporting backtracking and unlimited look-ahead. Unfortunately, the limited support for left recursion in packrat parser implementations makes them difficult to use for a large class of grammars (Java’s, for example). This paper presents a modification to the memoization mechanism used by packrat parser implementations that makes it possible for them to support (even indirectly or mutually) left-recursive rules. While it is possible for a packrat parser with our modification to yield super-linear parse times for some left-recursive grammars, our experiments show that this is not the case for typical uses of left recursion. D.3.4 [Programming Lan-

The state of an imperative program—e.g., the values stored in global and local variables, objects’ instance variables, and arrays—changes as its statements are executed. These changes, or side effects, are visible globally: when one part of the program modifies an object, every other part that holds a reference to the same object (either directly or indirectly) is also affected. This paper introduces worlds, a language construct that reifies the notion of program state, and enables programmers to control the scope of side effects. We investigate this idea as an extension of JavaScript, and provide examples that illustrate some of the interesting idioms that it makes possible. 1

In this paper we present a macro system for the Fortress programming language. Fortress is a new programming language designed for scientific and high-performance computing. Features include: implicit parallelism, transactions, and concrete syntax that emulates mathematical notation. Fortress is intended to grow over time to accommodate the changing needs of its users. Our goal is to design and implement a macro system that allows for such growth. The main challenges are (1) to support extensions to a core syntax rich enough to emulate mathematical notation, (2) to support combinations of extensions from separately compiled macros, and (3) to allow new syntax that is indistinguishable from core language constructs. To emulate mathematical notation, Fortress syntax is specified as a parsing expression grammar (PEG), supporting unlimited lookahead. Macro definitions must be checked for well-formedness before they are expanded and macro uses must be well encapsulated (hygienic, composable, respecting referential transparency). Use sites must be parsed along with the rest of the program and expanded directly into abstract syntax trees. Syntax errors at use sites of a macro must refer to the unexpanded program at use sites, never to definition sites. Moreover, to allow for many common and important uses of macros, mutually recursive definitions should be supported. Our design meets these challenges. The result is a flexible system that allows us not only to support new language extensions, but also to move many constructs of the core language into libraries. New grammar productions are tightly integrated with the Fortress parser, and use sites expand into core abstract syntax trees. Our implementation is integrated into the open-source Fortress reference interpreter.To our knowledge, ours is the first implementation of a modular hygienic macro system based on parsing expression grammars.

Abstract. Software must be constantly adapted to changing requirements. The time scale, abstraction level and granularity of adaptations may vary from short-term, fine-grained adaptation to long-term, coarsegrained evolution. Fine-grained, dynamic and context-dependent adaptations can be particularly difficult to realize in long-lived, large-scale software systems. We argue that, in order to effectively and efficiently deploy such changes, adaptive applications must be built on an infrastructure that is not just model-driven, but is both model-centric and contextaware. Specifically, this means that high-level, causally-connected models of the application and the software infrastructure itself should be available at run-time, and that changes may need to be scoped to the run-time execution context. We first review the dimensions of software adaptation and evolution, and then we show how model-centric design can address the adaptation needs of a variety of applications that span these dimensions. We demonstrate through concrete examples how model-centric and context-aware designs work at the level of application interface, programming language and runtime. We then propose a research agenda for a model-centric development environment that supports dynamic software adaptation and evolution. 1

Does the use of DSL tools improve the maintainability of language implementations compared to implementations from scratch? We present empirical results on aspects of maintainability of six implementations of the same DSL using different languages (Java, JavaScript, C#) and DSL tools (ANTLR, OMeta, Microsoft “M”). Our evaluation indicates that the maintainability of language implementations is indeed higher when constructed using DSL tools. Categories and Subject Descriptors

Parsing Expression Grammar (PEG) is a recognition-based foundation for describing syntax that renewed interest in top-down parsing approaches. Generally, the implementation of PEGs is based on a recursive-descent parser, or uses a memoization algorithm. We present a new approach for implementing PEGs, based on a virtual parsing machine, which is more suitable for pattern matching. Each PEG has a corresponding program that is executed by the parsing machine, and new programs are dynamically created and composed. The virtual machine is embedded in a scripting language and used by a patternmatching tool. We give an operational semantics of PEGs used for pattern matching, then describe our parsing machine and its semantics. We show how to transform PEGs to parsing machine programs, and give a correctness proof of our transformation.

effort to create a large-scope-and-range software system in 3-4 orders of magnitude less code than current practice. The detailed STEPS proposal can be found at

Grammars for programming languages are traditionally specified statically. They are hard to compose and reuse due to ambiguities that inevitably arise. PetitParser combines ideas from scannerless parsing, parser combinators, parsing expression grammars and packrat parsers to model grammars and parsers as objects that can be reconfigured dynamically. Through examples and benchmarks we demonstrate that dynamic grammars are not only flexible but highly practical.

Parsing Expression Grammars (PEGs) are specifications of unambiguous recursive-descent style parsers. PEGs incorporate both lexing and parsing phases and have valuable properties, such as being closed under composition. In common with most recursive-descent systems, raw PEGs cannot handle left-recursion; traditional approaches to leftrecursion elimination lead to incorrect parses. In this paper, I show how the approach proposed for direct left-recursive Packrat parsing by Warth et al. can be adapted for ‘pure ’ PEGs. I then demonstrate that this approach results in incorrect parses for some PEGs, before outlining a restrictive subset of left-recursive PEGs which can safely work with this algorithm. Finally I suggest an alteration to Warth et al.’s algorithm that can correctly parse a less restrictive subset of directly recursive PEGs.