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DOI: 10.4230/LIPIcs.TYPES.2024.5

Implementing a Type Theory with Observational Equality, Using Normalisation by Evaluation

Authors: Matthew Sirman, Meven Lennon-Bertrand, and Neel Krishnaswami

Published in: LIPIcs, Volume 336, 30th International Conference on Types for Proofs and Programs (TYPES 2024)

Abstract

We report on an experimental implementation in Haskell of a dependent type theory featuring an observational equality type, based on Pujet et al.’s CCobs. We use normalisation by evaluation to produce an efficient normalisation function, which is used to implement a bidirectional type checker. To allow for greater expressivity, we extend the core CCobs calculus with quotient types and inductive types. To make the system usable, we explore various proof-assistant features, notably a rudimentary version of a "hole" system similar to Agda’s. While rather crude, this experience should inform other, more substantial implementation efforts of observational equality.

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Matthew Sirman, Meven Lennon-Bertrand, and Neel Krishnaswami. Implementing a Type Theory with Observational Equality, Using Normalisation by Evaluation. In 30th International Conference on Types for Proofs and Programs (TYPES 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 336, pp. 5:1-5:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{sirman_et_al:LIPIcs.TYPES.2024.5,
  author =	{Sirman, Matthew and Lennon-Bertrand, Meven and Krishnaswami, Neel},
  title =	{{Implementing a Type Theory with Observational Equality, Using Normalisation by Evaluation}},
  booktitle =	{30th International Conference on Types for Proofs and Programs (TYPES 2024)},
  pages =	{5:1--5:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-376-8},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{336},
  editor =	{M{\o}gelberg, Rasmus Ejlers and van den Berg, Benno},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TYPES.2024.5},
  URN =		{urn:nbn:de:0030-drops-233673},
  doi =		{10.4230/LIPIcs.TYPES.2024.5},
  annote =	{Keywords: Dependent type theory, Bidirectional typing, Observational equality, Normalisation by evaluation}
}

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Implementing a Type Theory with Observational Equality, Using Normalisation by Evaluation

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