## Archive for the 'Theorem proving' Category

### Higher-order representations of substructural logics

Now I’ve settled in to Italy (and my new job), it’s time to resurrect the blog.

By Karl Crary, from ICFP 2010, available from Karl Crary’s website:

We present a technique for higher-order representation of substructural logics such as linear or modal logic. We show that such logics can be encoded in the (ordinary) Logical Framework, without any linear or modal extensions. Using this encoding, metatheoretic proofs about such logics can easily be developed in the Twelf proof assistant.

Associated Twelf code can be found here.

### Proof pearl: A new foundation for Nominal Isabelle

Ack!  Sorry for the severe lack of updates lately.  The blog hasn’t died (I’m in the process of moving countries).  Anyway…

By Christian Urban and Brian Huffman, from ITP 2010, available from Christian Urban’s website:

Pitts et al introduced a beautiful theory about names and binding based on the notions of permutation and support. The engineering challenge is to smoothly adapt this theory to a theorem prover environment, in our case Isabelle/HOL. We present a formalisation of this work that differs from our earlier approach in two important respects: First, instead of representing permutations as lists of pairs of atoms, we now use a more abstract representation based on functions. Second, whereas the earlier work modeled different sorts of atoms using different types, we now introduce a unified atom type that includes all sorts of atoms. Interestingly, we allow swappings, that is permutations build from two atoms, to be ill-sorted. As a result of these design changes, we can iron out inconveniences for the user, considerably simplify proofs and also drastically reduce the amount of custom ML-code. Furthermore we can extend the capabilities of Nominal Isabelle to deal with variables that carry additional information. We end up with a pleasing and formalised theory of permutations and support, on which we can build an improved and more powerful version of Nominal Isabelle.

### Hybrid: A Defintional Two Level Approach to Reasoning with Higher-Order Abstract Syntax

By Amy Felty and Alberto Monigliano, accepted in the Journal of Automated Reasoning 2010, available from Amy Felty’s website:

Combining higher-order abstract syntax and (co)-induction in a logical framework is well known to be problematic. We describe the theory and the practice of a tool called Hybrid, within Isabelle/HOL and Coq, which aims to address many of these difficulties. It allows object logics to be represented using higher-order abstract syntax, and reasoned about using tactical theorem proving and principles of (co)induction. Moreover, it is definitional, which guarantees consistency within a classical type theory. The idea is to have a de Bruijn representation of λ-terms providing a definitional layer that allows the user to represent object languages using higher-order abstract syntax, while offering tools for reasoning about them at the higher level. In this paper we describe how to use Hybrid in a multi-level reasoning fashion, similar in spirit to other systems such as Twelf and Abella. By explicitly referencing provability in a middle layer called a specification logic, we solve the problem of reasoning by (co)induction in the presence of non-stratifiable hypothetical judgments, which allow very elegant and succinct specifications of object logic inference rules. We first demonstrate the method on a simple example, formally proving type soundness (subject reduction) for a fragment of a pure functional language, using a minimal intuitionistic logic as the specification logic. We then prove an analogous result for a continuation-machine presentation of the operational semantics of the same language, encoded this time in an ordered linear logic that serves as the specification layer. This example demonstrates the ease with which we can incorporate new specification logics, and also illustrates a significantly more complex object logic whose encoding is elegantly expressed using features of the new specification logic.

### Five axioms of alpha-conversion

Again, sorry for the delay in updates to the site.  I handed in my thesis this morning, so updates should now become much more frequent.

By Andrew D. Gordon and Tom Melham, from TPHOLs 1996, available from CiteSeer X:

We present five axioms of name-carrying lambda-terms identified up to alpha-conversion—that is, up to renaming of bound variables. We assume constructors for constants, variables, application and lambda-abstraction. Other constants represent a function Fv that returns the set of free variables in a term and a function that substitutes a term for a variable free in another term. Our axioms are (1) equations relating Fv and each constructor, (2) equations relating substitution and each constructor, (3) alpha-conversion itself, (4) unique existence of functions on lambda-terms defined by structural iteration, and (5) construction of lambda-abstractions given certain functions from variables to terms. By building a model from de Bruijn’s nameless lambda-terms, we show that our five axioms are a conservative extension of HOL. Theorems provable from the axioms include distinctness, injectivity and an exhaustion principle for the constructors, principles of structural induction and primitive recursion on lambda-terms, Hindley and Seldin’s substitution lemmas and the existence of their length function. These theorems and the model have been mechanically checked in the Cambridge HOL system.

### Reasoning with higher-order abstract syntax and contexts

By Amy Felty and Brigitte Pientka, from ITP 2010, available from Amy Felty’s website:

A variety of logical frameworks support the use of higher-order abstract syntax (HOAS) in representing formal systems given via axioms and inference rules and reasoning about them. In such frameworks, object-level binding is encoded directly using meta-level binding. Although these systems seem superficially the same, they differ in a variety of ways; for example, in how they handle a context of assumptions and in what theorems about a given formal system can be expressed and proven. In this paper, we present several case studies which highlight a variety of different aspects of reasoning using HOAS, with the intention of providing a basis for qualitative comparison of different systems. We then carry out such a comparison among three systems: Twelf, Beluga, and Hybrid. We also develop a general set of criteria for comparing such systems. We hope that others will implement these challenge problems, apply these criteria, and further our understanding of the trade-offs involved in choosing one system over another for this kind of reasoning.

Associated proof scripts can be found here.

### Higher-order term indexing using substitution trees

By Brigitte Pientka, from ACM Transactions on Computational Logic, available from Brigitte Pientka’s website:

We present a higher-order term indexing strategy based on substitution trees for simply typed lambda-terms. There are mainly two problems in adapting first-order indexing techniques. First many operations used in building an efficient term index and retrieving a set of candidate terms from a large collection are undecidable in general for higher-order terms. Second, the scoping of variables and binders in the higher-order case presents challenges. The approach taken in this paper is to reduce the problem to indexing linear higher-order
patterns, a decidable fragment of higher-order terms, and delay solving terms outside of this fragment. We present insertion of terms into the index based on computing the most specific linear generalization of two linear higher-order patterns, and retrieval based on matching two linear higher-order patterns. Our theoretical framework maintains that terms are in βη-normal form, thereby eliminating the need to re-normalize and raise terms during insertion and retrieval. Finally, we prove correctness of our presented algorithms. This indexing structure is implemented as part of the Twelf system to speed up the execution of the tabled higher-logic programming interpreter.

### First-order logic a la carte

What’s covered in today’s post isn’t strictly a trick for handling binding (in this case, binding is handled by HOAS, though the method doesn’t appear to be limited to using HOAS).  However, the approach that’s summaried, datatypes a la carte, is a neat trick that I’ve only recently come across.  In short, the trick can be boiled down to: “build larger datatypes from smaller datatypes by taking their coproduct”.

The following article demonstrates one benefit of doing this: building a datatype representing first-order formulae, we can then define a series of transformation steps (at each stage eliminating a logical connective) which transforms an arbitrary FOL formula into implicative normal form.  What’s really unique about this approach is that every step of the transformation is guaranteed, by the type system, to eliminate the corresponding connective from the input formula.