In this paper we introduce several quantitative methods for the lambda-calculus based on partial metrics, a well-studied variant of standard metric spaces that have been used to metrize non-Hausdorff topologies, like those arising from Scott domains. First, we study quantitative variants, based on program distances, of sensible equational theories for the λ-calculus, like those arising from Böhm trees and from the contextual preorder. Then, we introduce applicative distances capturing higher-order Scott topologies, including reflexive objects like the D_∞ model. Finally, we provide a quantitative insight on the well-known connection between the Böhm tree of a λ-term and its Taylor expansion, by showing that the latter can be presented as an isometric transformation.
@InProceedings{maestracci_et_al:LIPIcs.CSL.2025.34, author = {Maestracci, Valentin and Pistone, Paolo}, title = {{The Lambda Calculus Is Quantifiable}}, booktitle = {33rd EACSL Annual Conference on Computer Science Logic (CSL 2025)}, pages = {34:1--34:23}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-362-1}, ISSN = {1868-8969}, year = {2025}, volume = {326}, editor = {Endrullis, J\"{o}rg and Schmitz, Sylvain}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2025.34}, URN = {urn:nbn:de:0030-drops-227911}, doi = {10.4230/LIPIcs.CSL.2025.34}, annote = {Keywords: Lambda-calculus, Scott semantics, Partial metric spaces, B\"{o}hm trees, Taylor expansion} }
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