Extracting Recursion Operators in Nuprl’s Type Theory

In this paper we describe the extraction of “efficient” recursion schemes from proofs of well-founded induction principles. This is part of a larger methodology; when these well-founded induction principles are used in proofs, the structure of the program extracted from the proof is determined by the recursion scheme inhabiting the induction principle. Our development is based on Paulson’s paper Constructing recursion operators in intuitionistic type theory, but we specifically address two possibilities raised in the conclusion of his paper: the elimination of non-computational content from the recursion schemes themselves and, the use of the Y combinator to allow the recursion schemes to be extracted directly from the proofs of well-founded relations.