Section: Software

http://coq.inria.fr COQ

Participants : Bruno Barras [TypiCal team, Saclay] , Yves Bertot [Marelle team, Sophia] , Pierre Boutillier, Xavier Clerc [SED team] , Pierre Courtieu [CNAM] , Maxime Dénès [Marelle team, Sophia] , Julien Forest [CNAM] , Stéphane Glondu [CARAMEL team, Nancy Grand Est] , Benjamin Grégoire [Marelle team, Sophia] , Vincent Gross [Consultant at NBS Systems] , Hugo Herbelin [correspondant] , Pierre Letouzey, Assia Mahboubi [TypiCal team, Saclay] , Julien Narboux [University of Strasbourg] , Jean-Marc Notin [TypiCal team, Saclay] , Christine Paulin [Proval team, Saclay] , Pierre-Marie Pédrot, Loïc Pottier [Marelle team, Sophia] , Matthias Puech, Yann Régis-Gianas, François Ripault, Matthieu Sozeau, Arnaud Spiwack, Pierre-Yves Strub [Formes team, Beijing] , Enrico Tassi [TypiCal team, Saclay] , Benjamin Werner [TypiCal team, Saclay] .

Version 8.4

Version 8.4 was released in August 2012. It introduced a new proof engine designed and implemented by Arnaud Spiwack and a new extensive modular library of arithmetic contributed by Pierre Letouzey. It also included an extension of the underlying logic with “$\eta$-conversion” by Hugo Herbelin and “commutative-cuts compliant guard condition” by Pierre Boutillier, an extension of the pattern-matching compilation algorithm by Hugo Herbelin, an extension of the procedure of simplification of polynomial expressions by Loïc Pottier, a refinement of the type classes mechanism by Matthieu Sozeau, a new communication model by Vincent Gross for the graphical user interface CoqIDE, that Pierre Letouzey, Pierre Boutillier and Pierre-Marie Pédrot further extended.

Several users gracefully contributed improvements of various features (Tom Prince, Enrico Tassi, Daniel Grayson, Hendrik Tews, ...).

Graphical user interface

Pierre Letouzey has finalised and extended the work initiated by Vincent Gross (former ADT engineer) concerning the CoqIDE user interface: CoqIDE and Coq are now separate Unix processes, enhancing the reliability and improving the user experience.

In Fall 2012, Pierre Letouzey also revised the event infrastructure of CoqIDE, from a thread-based model to pure GTK event-loop. This way, CoqIDE is more reactive, less subject to deadlocks (especially under Windows), and the source code is more idiomatic and easier to understand. Interestingly, this work takes advantage of deeper notions such as C.P.S. (continuation passing style).

Pierre Boutillier and Pierre-Marie Pédrot built an abstract communication interface between Coq and CoqIDE based on XML syntax. They also refined the ability to customise CoqIDE. Pierre Boutillier made CoqIDE rely on Gtksourceview.

Proof engine

Arnaud Spiwack has proposed an extension of the expressiveness of tactics based on his previous work for a new proof engine. It allows for more atomic tactics, has a primitive support for backtracking, and allows for tactics which manipulate several goals.

Evaluation algorithms

Pierre Boutillier has proposed a new unfolding algorithm for global constants so that the definition of these ones are unfolded only if it triggers extra reductions. This helps users to keep goals concise during interactive proofs.

Type classes, internal representation

Matthieu Sozeau is adapting the type-classes mechanism to benefit from the new tactic engine and avoid reimplementing a whole proof-search algorithm with backtracking on top of the tactic language. This will bring high benefits in terms of efficiency and ease of use to the users. Forward proof-search for type classes was stabilised and is now used in libraries for better control on the search space, notably in the MathClasses library developed in Nijmegen [69] .

An important shortcoming of type classes is the verbosity of the representation of projections from a class, as was illustrated in François Garillot's PhD thesis [48] . Matthieu Sozeau has developed a branch of Coq supporting an efficient representation of these projections based on the idea of bidirectional type checking which is now under stabilisation. This support will also enhance the performance of the assistant on developments using regular parameterised records and dependent sums like the HoTT library on homotopy type theory and the Forcing plugin developed by Sozeau et al [32] .

Universes

While visiting the Institute for Advanced Study, Matthieu Sozeau implemented a new system of universe polymorphism that makes it possible to develop highly generic theories in the Coq system. Based on ideas from Harper and Pollack's [53] design of polymorphism as an elaboration in the Lego theorem prover, he developed an original algorithm for type inference of universes and implemented it in Coq. Its first application is inside the Homotopy Type Theory (HoTT) research program, as the formalisation of HoTT requires a high level of polymorphism that was not available before. Many other theories will benefit from this, including Sozeau's work on Forcing, B. Barras' (Typical) work on models of type theory or in the Math Classes library mentioned before. It also opens up possibilities to formalise category-theoretic notions without being limited by the universe system, a long-standing barrier in the Coq proof assistant.

The Equations plugin

Matthieu Sozeau continued the maintenance of the Equations plugin and developed a new Forcing plugin for Coq (see below).

Tools

Pierre-Marie Pédrot has written a program using the internal representation of libraries to compact Coq object files. It is based on well-known automata algorithms, representing memory as transition systems. The idea underlying this program is generalisable to any OCaml data structure, provided some conditions on its use are satisfied, and was formalised in a paper that was accepted at JFLA 2013.

Internal architecture of the Coq software

Pierre Letouzey continued a large reorganisation of the internal components of Coq, since these components are currently too much interdependent. This work brought better isolation between some of the Coq components and explicit interfaces between them. This allowed to simplify the compilation of Coq, since it is now easier to build the OCaml syntax extension used when compiling many advanced parts of Coq. Moreover, this clearer architecture should also help new contributors when they discover and interact with this large and complex code-base.

Pierre-Marie Pédrot also made some reorganisations of the code. This includes a clean generic library superseding the one of OCaml, pushing the CAMLP4/5 dependent parts out of the lower strata, as well as benefiting from the OCaml module system to get more uniformity in the naming of usual data structures.

Pierre Letouzey proposed a nicer backtracking infrastructure to Coq, used when the user wants to cancel some recent commands and go back before them. This new infrastructure unifies and improves what was used earlier by ProofGeneral and CoqIDE, the two main user interfaces for Coq.

Pierre Letouzey also dedicated many efforts to improve the support of the Windows platform by Coq.

Efficiency

Pierre-Marie Pédrot has been trying to optimise various parts of the Coq system, including the new tactical system designed by Arnaud Spiwack. Some neat tricks on garbage collection permitted to reach a substantial time improvement in compilation of object files. Various architectural modifications were also made in the process, like trying to get rid of the generic comparison in the code base.

Pierre Letouzey investigated an alternative implementation of the code dealing with Coq universes. These universes are a critical part of Coq: they have direct consequences on Coq safety, and handling them is time-consuming (between 10% to 20% depending on the Coq usage). This alternative implementation looks promising, but still requires some more work and stress-tests before being integrated in mainstream Coq.

Documentation generation

François Ripault and Yann Régis-Gianas developed a new version of coqdoc, the documentation generator of Coq. This new implementation is based on the interaction protocol with the Coq system and should be more robust with respect to the evolution of Coq.

General maintenance

Pierre Letouzey has been the main maintainer of Coq with extra contributions from Hugo Herbelin, Pierre Boutillier, Matthieu Sozeau, Pierre-Marie Pédrot, ...

Development Action

An “Action de Développement Technologique” about Coq started September 2011 and continued this year. It gathers the $\pi r^2$ team, the Marelle team and the CPR team from CNAM, Hugo Herbelin acting as the coordinator. It supports visits and meetings between developers and aims at strengthening the community of Coq users and contributors.

Yann Régis-Gianas set up an “osqa” server for Frequently Asked Questions.

The ADT Coq supported the internship of François Ripault.

Hugo Herbelin formalised a type-theoretic construction of semi-simplicial sets answering a problem raised early this year by Steve Awodey, Peter LeFanu Lumsdaine and others, in relation with the homotopy models of type theory.