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We present normalization for intuitionistic combinatorial proofs (ICPs) and relate it to the simply-typed lambda-calculus. We prove confluence and strong normalization. Combinatorial proofs, or "proofs without syntax", form a graphical semantics of proof in various logics that is canonical yet complexity-aware: they are a polynomial-sized representation of sequent proofs that factors out exactly the non-duplicating permutations. Our approach to normalization aligns with these characteristics: it is canonical (free of permutations) and generic (readily applied to other logics). Our reduction mechanism is a canonical representation of reduction in sequent calculus with closed cuts (no abstraction is allowed below a cut), and relates to closed reduction in lambda-calculus and supercombinators. While we will use ICPs concretely, the notion of reduction is completely abstract, and can be specialized to give a reduction mechanism for any representation of typed normal forms.
@InProceedings{heijltjes_et_al:LIPIcs.FSCD.2022.19,
author = {Heijltjes, Willem B. and Hughes, Dominic J. D. and Stra{\ss}burger, Lutz},
title = {{Normalization Without Syntax}},
booktitle = {7th International Conference on Formal Structures for Computation and Deduction (FSCD 2022)},
pages = {19:1--19:19},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-233-4},
ISSN = {1868-8969},
year = {2022},
volume = {228},
editor = {Felty, Amy P.},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2022.19},
URN = {urn:nbn:de:0030-drops-163004},
doi = {10.4230/LIPIcs.FSCD.2022.19},
annote = {Keywords: combinatorial proofs, intuitionistic logic, lambda-calculus, Curry-Howard, proof nets}
}