Institute for Software Research
School of Computer Science, Carnegie Mellon University


Exploring Language Support for Immutability

Michael Coblenz, Joshua Sunshine, Jonathan Aldrich,
Brad Myers, Sam Weber*, Forrest Shull*

May 2016

This is an extended version of a paper that was published in the
Proceedings of the International Conference on Software Engineering (ICSE 2016).


Keywords: Programming language design, Programming language usability, Immutability, Mutability, Programmer productivity, Empirical studies of programmers

Programming languages can restrict state change by preventing it entirely (immutability) or by restricting which clients may modify state (read-only restrictions). The benefits of immutability and read-only restrictions in software structures have been long-argued by practicing software engineers, researchers, and programming language designers. However, there are many proposals for language mechanisms for restricting state change, with a remarkable diversity of techniques and goals, and there is little empirical data regarding what practicing software engineers want in their tools and what would benefit them. We systematized the large collection of techniques used by programming languages to help programmers prevent undesired changes in state. We interviewed expert software engineers to discover their expectations and requirements, and found that important requirements, such as expressing immutability constraints, were not reflected in features available in the languages participants used. The interview results informed our design of a new language extension for specifying immutability in Java. Through an iterative, participatory design process, we created a tool that reflects requirements from both our interviews and the research literature.

19 pages

*Software Engineering Institute, Carnegie Mellon University

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