Solving type equations by graph rewriting

• Hassan Ait-Kaci
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 202)

Abstract

I have described a syntactic calculus of partially ordered structures and its application to computation. A syntax of record-like terms and a "type subsumption" ordering were defined and shown to form a lattice structure. A simple "type-as-set" interpretation of these term structures extends this lattice to a distributive one, and in the case of finitary terms, to a complete Brouwerian lattice. As a result, a method for solving systems of type equations by iterated rewriting of type symbols was proposed which defines an operational semantics for KBL — a Knowledge Base Language. It was shown that a KBL program can be seen as a system of equations. Thanks to the lattice properties of finite structures, a system of equations admits a least fixed-point solution. The particular order of computation of KBL, the "fan-out computation order", which rewrites symbols closer to the root first was formally defined and shown to be maximal. Unfortunately, the complete "correctness" of KBL is not yet established. That is, it is not known at this point whether the normal form of a term is equal to the fixed-point solution. However, as steps in this direction, two technical lemmas were conjectured to which a proof of the correctness is corollary.

Keywords

Distributive Lattice Term Structure Operational Semantic Algebraic Semantic Regular Term
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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