Currying Second-Order Unification Problems

• Authors: Jordi Levy;Mateu Villaret
• Venue: RTA '02 Proceedings of the 13th International Conference on Rewriting Techniques and Applications
• Year: 2002

The Curry form of a term, like f(a, b), allows us to write it, using just a single binary function symbol, as @(@(f, a), b). Using this technique we prove that the signature is not relevant in second-order unification, and conclude that one binary symbol is enough. By currying variable applications, like X(a), as @(X, a), we can transform second-order terms into first-order terms, but we have to add beta-reduction as a theory. This is roughly what it is done in explicit unification. We prove that by currying only constant applications we can reduce second-order unification to second-order unification with just one binary function symbol. Both problems are already known to be undecidable, but applying the same idea to context unification, for which decidability is still unknown, we reduce the problem to context unification with just one binary function symbol.We also discuss about the difficulties ofapplying the same ideas to third or higher order unification.