In, Harper, Honsell, and Plotkin present LF (the Logical Framework) as a general framework for the definition of logics. LF provides a uniform way of encoding a logical language, its inference rules and its proofs. In, Avron, Honsell, and Mason give a variety of examples for encoding logics in LF. In this paper we describe Elf, a meta-language intended for environments dealing with deductive systems represented in LF.

While this paper is intended to include a full description of the Elf core language, we only state, but do not prove here the most important theorems regarding the basic building blocks of Elf. These proofs are left to a future, paper. A preliminary account of Elf can be found in. The range of applications of Elf includes theorem proving and proof transformation in various logics, definition and execution of structured operational and natural semantics for programming languages, type checking and type inference, etc. The basic idea behind Elf is to unify logic definition (in the style of LF) with logic programming (in the style of λProlog, see). It achieves this unification by giving types an operational interpretation, much the same way that Prolog gives certain formulas (Horn-clauses) an operational interpretation. An alternative approach to logic programming in LF has been developed independently by Pym.

Here are some of the salient characteristics of our unified approach to logic definition and meta-programming.