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Functional Kan simplicial sets are simplicial sets in which the horn-fillers required by the Kan extension condition are given explicitly by functions. We show the non-constructivity of the following basic result: if B and A are functional Kan simplicial sets, then A^B is a Kan simplicial set. This strengthens a similar result for the case of non-functional Kan simplicial sets shown by Bezem, Coquand and Parmann [TLCA 2015, v. 38 of LIPIcs]. Our result shows that-from a constructive point of view-functional Kan simplicial sets are, as it stands, unsatisfactory as a model of even simply typed lambda calculus. Our proof is based on a rather involved Kripke countermodel which has been encoded and verified in the Coq proof assistant.
@InProceedings{parmann:LIPIcs.TYPES.2015.8,
author = {Parmann, Erik},
title = {{Functional Kan Simplicial Sets: Non-Constructivity of Exponentiation}},
booktitle = {21st International Conference on Types for Proofs and Programs (TYPES 2015)},
pages = {8:1--8:25},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-030-9},
ISSN = {1868-8969},
year = {2018},
volume = {69},
editor = {Uustalu, Tarmo},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TYPES.2015.8},
URN = {urn:nbn:de:0030-drops-84787},
doi = {10.4230/LIPIcs.TYPES.2015.8},
annote = {Keywords: constructive logic, simplicial sets, semantics of simple types}
}