Jean-Yves Girard

The system F of variable types, fifteen years later

The semantic study of system F stumbles on the problem of variable types for which there was no convincing interpretation; we develop here a semantics based on the category-theoretic idea of direct limit, so that the behaviour of a variable type on any domain is determined by its behaviour on finite ones, thus getting rid of the circularity of variable types. To do so, one has first to simplify somehow the extant semantic ideas, replacing Scott domains by the simpler and more finitary qualitative domains. The interpretation obtained is extremely compact, as shown on simple examples. The paper also contains the definitions of a very small `universal model? of lambda-calculus, and investigates the concept totality.

A new constructive logic: classic logic

There are two ways to present this work; the most efficient is of course to start with the main syntactical definitions, and to end with semantics: this is the presentation that we follow in the body of the text: section 1, syntex; section 2, semantics. Another possibility is to follow the order of discovery of the concepts, which (as expected) starts with the semantics and ends with the syntex; we adopt this second way for our introduction, hoping that this orthogonal look at the same object will help to apprehend the concepts.

Locus Solum: From the rules of logic to the logic of rules

Go back to An-fang, the Peace Square at An-Fang, the Beginning Place at An-Fang, where all things start (…) An-Fang was near a city, the only living city with a pre-atomic name (…) The headquarters of the People Programmer was at An-Fang, and there the mistake happened: A ruby trembled. Two tourmaline nets failed to rectify the laser beam. A diamond noted the error. Both the error and the correction went into the general computer. Cordwainer Smith The Dead Lady of Clown Town, 1964.

Bounded linear logic: a modular approach to polynomial-time computability

Abstract Usual typed lambda-calculi yield input/output specifications; in this paper the authors show how to extend this paradigm to complexity specifications. This is achieved by means of a restricted version of linear logic in which the use of exponential connectives is bounded in advance. This bounded linear logic naturally involves polynomials in its syntax and dynamics. It is then proved that any functional term of appropriate type actually encodes a polynomial-time algorithm and that conversely any polynomial-time function can be obtained in this way.

Geometry of Interaction 1: Interpretation of System F

Publisher Summary This chapter describes the development of a semantics of computation free from the twin drawbacks of reductionism (that leads to static modification) and subjectivism (that leads to syntactical abuses, in other terms, bureaucracy). The new approach initiated in this chapter rests on the use of a specific C*-algebra Λ* that has the distinguished property of bearing a (non associative) inner tensor product. The chapter describes that a representative class of algorithms can be modelized by means of standard mathematics.

Linear logic and lazy computation

Recently, J.Y. Girard discovered that usual logical connectors such as ⇒ (implication) could be broken up into more elementary linear connectors. This provided a new linear logic [Girard86] where hypothesis are (in some sense) used once and only once. The most surprising is that all the power of the usual logic can be recovered by means of recursive logical operators (connector “of course”).

Geometry of interaction III: accommodating the additives

The paper expounds geometry of interaction, for the first time in the full case, i.e. for all connectives of linear logic, including additives and constants. The interpretation is done within a C∗-algebra which is induced by the rule of resolution of logic programming, and therefore the execution formula can be presented as a simple logic programming loop. Part of the data is public (shared channels) but part of it can be viewed as private dialect (defined up to isomorphism) that cannot be shared during interaction, thus illustrating the theme of communication without understanding. One can prove a nilpotency (i.e. termination) theorem for this semantics, and also its soundness w.r.t. a slight modification of familiar sequent calculus in the case of exponential-free conclusions.

Normal functors, power series and λ-calculus

On presente une nouvelle approche des fonctionnelles continues comportant les modeles de differents lambda calculs

The Blind Spot

These lectures on logic, more specifically proof theory, are basically intended for postgraduate students and researchers in logic. The question at stake is the nature of mathematical knowledge and the difference between a question and an answer, i.e., the implicit and the explicit. The problem is delicate mathematically and philosophically as well: the relation between a question and its answer is a sort of equality where one side is “more equal than the other”: one thus discovers essentialist blind spots. Starting with Godel’s paradox (1931) – so to speak, the incompleteness of answers with respect to questions – the book proceeds with paradigms inherited from Gentzen’s cut-elimination (1935). Various settings are studied: sequent calculus, natural deduction, lambda calculi, category-theoretic composition, up to geometry of interaction (GoI), all devoted to explicitation, which eventually amounts to inverting an operator in a von Neumann algebra. Mathematical language is usually described as referring to a preexisting reality. Logical operations can be given an alternative procedural meaning: typically, the operators involved in GoI are invertible, not because they are constructed according to the book, but because logical rules are those ensuring invertibility. Similarly, the durability of truth should not be taken for granted: one should distinguish between imperfect (perennial) and perfect modes. The procedural explanation of the infinite thus identifies it with the unfinished, i.e., the perennial. But is perenniality perennial? This questioning yields a possible logical explanation for algorithmic complexity. This highly original course on logic by one of the world’s leading proof theorists challenges mathematicians, computer scientists, physicists and philosophers to rethink their views and concepts on the nature of mathematical knowledge in an exceptionally profound way.

On the Meaning of Logical Rules I: Syntax Versus Semantics

« Oui c’est imbecile ce que je dis ! Seulement je ne sais pas comment concilier tout ca. Il est sur que je ne me sens libre que parce que j’ai fait mes classes et que je ne sors de la fugue que parce que je la sais. »