SASyLF: an educational proof assistant for language theory

  • Authors:

  • Jonathan Aldrich;Robert J. Simmons;Key Shin

  • Affiliations:

  • Carnegie Mellon University, Pittsburgh, PA, USA;Carnegie Mellon University, Pittsburgh, PA, USA;Microsoft Corporation, Redmond, WA, USA

  • Venue:
  • Proceedings of the 2008 international workshop on Functional and declarative programming in education
  • Year:
  • 2008

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Abstract

Teaching and learning formal programming language theory is hard, in part because it's easy to make mistakes and hard to find them. Proof assistants can help check proofs, but their learning curve is too steep to use in most classes, and is a barrier to researchers too. In this paper we present SASyLF, an LF-based proof assistant specialized to checking theorems about programming languages and logics. SASyLF has a simple design philosophy: language and logic syntax, semantics, and meta-theory should be written as closely as possible to the way it is done on paper. We describe how we designed the SASyLF syntax to be accessible to students learning type theory, and how students can understand its semantics directly in terms of the theory they are taught in class. SASyLF can express proofs typical of an introductory graduate type theory course. SASyLF proofs are generally very explicit, but its built-in support for variable binding provides substitution properties for free and avoids awkward variable encodings. We describe preliminary experience teaching with SASyLF.