A Complete Proof Synthesis Method for the Cube of Type Systems

We present a complete proof synthesis method for the eight type systems of Barendregt's cube extended with $\eta$-conversion. Because these systems verify the proofs-as-objects paradigm, the proof synthesis method is a one level process merging unification and resolution. Then we present a variant of this method, which is incomplete but much more efficient. At last we show how to turn this algorithm into a unification algorithm

Higher Order Unification via Explicit Substitutions

AbstractHigher order unification is equational unification for βη-conversion. But it is not first order equational unification, as substitution has to avoid capture. Thus, the methods for equational unification (such as narrowing) built upon grafting (i.e., substitution without renaming) cannot be used for higher order unification, which needs specific algorithms. Our goal in this paper is to reduce higher order unification to first order equational unification in a suitable theory. This is achieved by replacing substitution by grafting, but this replacement is not straightforward as it raises two major problems. First, some unification problems have solutions with grafting but no solution with substitution. Then equational unification algorithms rest upon the fact that grafting and reduction commute. But grafting and βη-reduction do not commute in λ-calculus and reducing an equation may change the set of its solutions. This difficulty comes from the interaction between the substitutions initiated by βη-reduction and the ones initiated by the unification process. Two kinds of variables are involved: those of βη-conversion and those of unification. So, we need to set up a calculus which distinguishes between these two kinds of variables and such that reduction and grafting commute. For this purpose, the application of a substitution of a reduction variable to a unification one must be delayed until this variable is instantiated. Such a separation and delay are provided by a calculus of explicit substitutions. Unification in such a calculus can be performed by well-known algorithms such as narrowing, but we present a specialised algorithm for greater efficiency. At last we show how to relate unification in λ-calculus and in a calculus with explicit substitutions. Thus, we come up with a new higher order unification algorithm which eliminates some burdens of the previous algorithms, in particular the functional handling of scopes. Huet's algorithm can be seen as a specific strategy for our algorithm, since each of its steps can be decomposed into elementary ones, leading to a more atomic description of the unification process. Also, solved forms in λ-calculus can easily be computed from solved forms in λσ-calculus

The Coq Proof Assistant : Reference Manual : Version 7.2

Coq is a proof assistant based on a higher-order logic. Coq allows to handle calculus mathematical assertions and to check mechanically proofs of these assertions. It helps to find formal proofs, and allows extraction of a certified program from the constructive proof of its formal specification. This document is the reference manual for the version V7.2 of Coq which is available from http://coq.inria.fr

The Coq Proof Assistant : Reference Manual : Version 7.2

Coq is a proof assistant based on a higher-order logic. Coq allows to handle calculus mathematical assertions and to check mechanically proofs of these assertions. It helps to find formal proofs, and allows extraction of a certified program from the constructive proof of its formal specification. This document is the reference manual for the version V7.2 of Coq which is available from http://coq.inria.fr

A Complete Proof Synthesis Method for the Cube of Type Systems

We present a complete proof synthesis method for the eight type systems of Barendregt's cube extended with η-conversion. Because these systems verify the proofs-as-objects paradigm, the proof synthesis method is a one level process merging unification and resolution. Then we present a variant of this method, which is incomplete but much more efficient. At last we show how to turn this algorithm into a unification algorithm