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Authors Samuel Frontull , Georg Moser , Vincent van Oostrom

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Samuel Frontull
  • Universität Innsbruck, Austria
Georg Moser
  • Universität Innsbruck, Austria
Vincent van Oostrom
  • Barendrecht, The Netherlands

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Samuel Frontull, Georg Moser, and Vincent van Oostrom. α-Avoidance. In 8th International Conference on Formal Structures for Computation and Deduction (FSCD 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 260, pp. 22:1-22:22, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2023)


When substitutions and bindings interact, there is a risk of undesired side effects if the substitution is applied naïvely. The λ-calculus captures this phenomenon concretely, as β-reduction may require the renaming of bound variables to avoid variable capture. In this paper we introduce α-paths as an estimation for α-avoidance, roughly expressing that α-conversions are not required to prevent variable capture. These paths provide a novel method to analyse and predict the potential need for α in different calculi. In particular, we show how α-path characterises α-avoidance for several sub-calculi of the λ-calculus like (i) developments, (ii) affine/linear λ-calculi, (iii) the weak λ-calculus, (iv) μ-unfolding and (iv) finally the safe λ-calculus. Furthermore, we study the unavoidability of α-conversions in untyped and simply-typed λ-calculi and prove undecidability of the need of α-conversions for (leftmost-outermost reductions) in the untyped λ-calculus. To ease the work with α-paths, we have implemented the method and the tool is publicly available.

Subject Classification

ACM Subject Classification
  • Theory of computation → Program analysis
  • λ-calculus
  • variable capture
  • α-conversion
  • developments
  • safe λ-calculus
  • undecidability


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