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Strict Ideal Completions of the Lambda Calculus

Author Patrick Bahr



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Patrick Bahr
  • IT University of Copenhagen, Denmark

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Patrick Bahr. Strict Ideal Completions of the Lambda Calculus. In 3rd International Conference on Formal Structures for Computation and Deduction (FSCD 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 108, pp. 8:1-8:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)
https://doi.org/10.4230/LIPIcs.FSCD.2018.8

Abstract

The infinitary lambda calculi pioneered by Kennaway et al. extend the basic lambda calculus by metric completion to infinite terms and reductions. Depending on the chosen metric, the resulting infinitary calculi exhibit different notions of strictness. To obtain infinitary normalisation and infinitary confluence properties for these calculi, Kennaway et al. extend beta-reduction with infinitely many `bot-rules', which contract meaningless terms directly to bot. Three of the resulting Böhm reduction calculi have unique infinitary normal forms corresponding to Böhm-like trees. In this paper we develop a corresponding theory of infinitary lambda calculi based on ideal completion instead of metric completion. We show that each of our calculi conservatively extends the corresponding metric-based calculus. Three of our calculi are infinitarily normalising and confluent; their unique infinitary normal forms are exactly the Böhm-like trees of the corresponding metric-based calculi. Our calculi dispense with the infinitely many bot-rules of the metric-based calculi. The fully non-strict calculus (called 111) consists of only beta-reduction, while the other two calculi (called 001 and 101) require two additional rules that precisely state their strictness properties: lambda x.bot -> bot (for 001) and bot M -> bot (for 001 and 101).

Subject Classification

ACM Subject Classification
  • Theory of computation → Rewrite systems
Keywords
  • lambda calculus
  • infinitary rewriting
  • Böhm trees
  • meaningless terms
  • confluence

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References

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