Computational Paths. The Calculus of Equality (Paperback or Softback)

When are two proofs of the same proposition equal? This book answers through computational paths - explicit syntactic witnesses that record how one proof-term rewrites into another. Continuing the programme begun in The Functional Interpretation of Logical Deduction (2011), it extends the Curry-Howard correspondence and the tradition of labelled deductive systems into a rewrite calculus of proof equalities.

Part I develops a 77-rule rewrite system on paths, proves it confluent and terminating, and derives decidable path equivalence from unique normal forms. Part II uncovers the higher-dimensional structure latent in this calculus: paths assemble into a weak ω-groupoid, with 2-cells as derivations between paths, 3-cells as coherences between derivations, and the Eckmann-Hilton argument recovered directly from rewrite combinatorics. Part III turns outward - to path induction and the J-eliminator, the failure of UIP and the resulting proof-relevance, fundamental groupoids of combinatorial spaces, and the framework's relationship to Homotopy Type Theory, category theory, and higher algebra.

The approach shares conceptual ground with HoTT but diverges in its commitments: where HoTT posits univalence and higher inductive types, computational paths offer explicit rewrite witnesses and effective normalisation, with path equivalence rendered decidable rather than postulated. A companion Lean 4 formalization machine-checks the core constructions and major theorems.

Aimed at logicians, type theorists, and mathematicians concerned with the computational content of equality, the volume is self-contained - appendices cover term rewriting and higher groupoids - while charting a research programme that reaches toward dependent types, directed rewriting, and homotopical semantics.