Towards User-Friendly Projectional Editors

Today's challenges for language development include language extension and composition, as well as the use of diverse notations. A promising approach is projectional editing, a technique to directly manipulate the abstract syntax tree of a program, without relying on parsers. Its potential lies in the ability to combine diverse notational styles - such as text, symbols, tables, and graphics - and the support for a wide range of composition techniques. However, projectional editing is often perceived as problematic for developers. Expressed drawbacks include the unfamiliar editing experience and challenges in the integration with existing infrastructure. In this paper we investigate the usability of projectional editors. We systematically identify usability issues resulting from the architecture. We use JetBrains Meta Programming System (MPS) as a case study. The case study discusses the concepts that MPS incorporates to address the identified issues, evaluates effectiveness of these concepts by surveying professional developers, and reports industrial experiences from realizing large-scale systems. Our results show that the benefits of flexible language composition and diverse notations come at the cost of serious usability issues - which, however, can be effectively mitigated with facilities that emulate editing experience of parser-based editors.