Confluence results for the pure strong categorical logic CCL. λ-calculi as subsystems of CCL
Theoretical Computer Science
The Strong Categorical Combinatory Logic (CCL, CCLpqSP), developed by Curien (1986) is, when typed and augmented with a rule defining a terminal object, a presentation of Cartesian Closed Categories. Furthermore, it is equationally equivalent to the Lambda-calculus with explicit couples and Surjective Pairing. Here we study the confluence properties of (CCL, CCLpqSP) and of several of its subsystems, and the relationship between untyped Lambda-calculi and (CCL, CCLpqSP) as rewriting systems.
... prove that there exists a subset 9 of CCL, and a subsystem SLP of CCLfiqSP confluent on 9, a very simple isomorphism between A, the classical Lambda-calculus, and a subset 99, of g5 which is extended between P-derivations of A and a class of derivations of S&l. Substitution, which is a one-step operation belonging to the metalanguage of A, is now described by rewritings with SLP and calculations between several substitutions launched at the same time may be performed by SLP. This point is a real increase in the calculation capacities of Lambda-calculus (same results for 9). The same result holds for the Lambda-calculus with couples and projection rules (without Surjective Pairing). .The locally confluent subsystem CCLPSP (that is SLfl+(SP)) is not confluent. This result is obtained by firstly designing a new counter-example (different from J. W. Klop's one) for confluence of the Lambda-calculus with couples and Surjective Pairing and then translating it into CCL. However, CCLPSP is shown to be confluent on the set derived from YB*. These results cannot be obtained with classical methods of confluence and we designed a new method called Interpretation Method based on this trick: a given relation R is confluent on a set X if and only if a relation 8(R) induced by R on a set of regularized terms 8(X) I$ confluent.