MetaCoq

MetaCoq is a project formalizing Coq in Coq and providing tools for manipulating Coq terms and developing certified plugins (i.e. translations, compilers or tactics) in Coq.

Quick jump

• Getting started
• Installation instructions
• Documentation
• Overview of the project
• Papers
• Team & Credits
• Bugs

Getting started

• You may want to start with a demo.

• The current branch documentation (as light coqdoc files).

• The overview of the different parts of the project.

Installation instructions

See INSTALL.md

Documentation

See DOC.md

Overview of the project

At the center of this project is the Template-Coq quoting library for Coq. The project currently has a single repository extending Template-Coq with additional features. Each extension is in a dedicated folder. The dependency graph might be useful to navigate the project. Statistics: ~150kLoC of Coq, ~30kLoC of OCaml.

Template-Coq

Template-Coq is a quoting library for Coq. It takes Coq terms and constructs a representation of their syntax tree as an inductive data type. The representation is based on the kernel's term representation.

After importing MetaCoq.Template.Loader there are commands MetaCoq Test Quote t., MetaCoq Quote Definition name := (t). and MetaCoq Quote Recursively Definition name := (t). as well as a tactic quote_term t k, where in all cases t is a term and k a continuation tactic.

In addition to this representation of terms, Template Coq includes:

• Reification of the environment structures, for constant and inductive declarations along with their universe structures.

• Denotation of terms and global declarations.

• A monad for querying the environment, manipulating global declarations, calling the type checker, and inserting them in the global environment, in the style of MTac. Monadic programs p : TemplateMonad A can be run using MetaCoq Run p.

• A formalisation of the typing rules reflecting the ones of Coq, covering all of Coq except eta-expansion and template polymorphism.

PCUIC

PCUIC, the Polymorphic Cumulative Calculus of Inductive Constructions is a cleaned up version of the term language of Coq and its associated type system, shown equivalent to the one of Coq. This version of the calculus has proofs of standard metatheoretical results:

• Weakening for global declarations, weakening and substitution for local contexts.

• Confluence of reduction using a notion of parallel reduction

• Context cumulativity / conversion and validity of typing.

• Subject Reduction (case/cofix reduction excluded)

• Principality: every typeable term has a smallest type.

• Bidirectional presentation: an equivalent presentation of the system that enforces directionality of the typing rules. Strengthening follows from this presentation.

• Elimination restrictions: the elimination restrictions ensure that singleton elimination (from Prop to Type) is only allowed on singleton inductives in Prop.

• Canonicity: The weak head normal form of a term of inductive type is a constructor application.

• Consistency under the assumption of strong normalization

• Weak call-by-value standardization: Normal forms of terms of first-order inductive type can be found via weak call-by-value evaluation.

See the PCUIC README for a detailed view of the development.

Safe Checker

Implementation of a fuel-free and verified reduction machine, conversion checker and type checker for PCUIC. This relies on a postulate of strong normalization of the reduction relation of PCUIC on well-typed terms. The checker is shown to be correct and complete w.r.t. the PCUIC specification. The extracted safe checker is available in Coq through a new vernacular command:

MetaCoq SafeCheck <term>

After importing MetaCoq.SafeChecker.Loader.

To roughly compare the time used to check a definition with Coq's vanilla type-checker, one can use:

MetaCoq CoqCheck <term>

This also includes a verified, efficient re-typing procedure (useful in tactics) in MetaCoq.SafeChecker.PCUICSafeRetyping.

See the SafeChecker README for a detailed view of the development.

Erasure

An erasure procedure to untyped lambda-calculus accomplishing the same as the type and proof erasure phase of the Extraction plugin of Coq. The extracted safe erasure is available in Coq through a new vernacular command:

MetaCoq Erase <term>

After importing MetaCoq.Erasure.Loader.

The erasure pipeline includes verified optimizations to remove lets in constructors, remove cases on propositional terms, switch to an unguarded fixpoint reduction rule and transform the higher-order constructor applications to first-order blocks for easier translation to usual programming languages. See the erasure README for a detailed view of the development.

Translations

Examples of translations built on top of this:

• a parametricity plugin in translations/param_original.v

• a plugin to negate functional extensionality in translations/times_bool_fun.v

Examples

• An example Coq plugin built on the Template Monad, which can be used to add a constructor to any inductive type is in examples/add_constructor.v

• An example extracted Coq plugin built on the extractable Template Monad, which can be used to derive lenses associated to a record type is in test-suite/plugin-demo. The plugin runs in OCaml and is a template for writing extracted plugins.

• An example constructor tactic written using the Template Monad is in examples/constructor_tac.v, and a more elaborate verified tautology checker is in examples/tauto.v.

• The test-suite files test-suite/erasure_test.v and test-suite/safechecker_test.v show example uses (and current limitations of) the extracted verified checker and erasure.

• The test-suite/self_erasure.v file checks that erasure works on the verified typechecking and erasure programs themselves.

• The test-suite file test-suite/erasure_live_test.v shows uses of the verified erasure running inside Coq.

Papers

• "The Curious Case of Case" Matthieu Sozeau, Meven Lennon-Bertrand and Yannick Forster. WITS 2022 presentation, Philadelphia. This presents the challenges around the representation of cases in Coq and PCUIC.

• "Bidirectional Typing for the Calculus of Inductive Constructions" Meven Lennon-Bertrand, PhD thesis, June 2022. Part 2 describes in detail the bidirectional variant of typing and its use to verify correctness and completeness of the type checker.

• "Coq Coq Correct! Verification of Type Checking and Erasure for Coq, in Coq" Matthieu Sozeau, Simon Boulier, Yannick Forster, Nicolas Tabareau and Théo Winterhalter. POPL 2020, New Orleans.

• "Formalisation and meta-theory of type theory" Théo Winterhalter, PhD thesis, September 2020. Part 3 describes in detail the verified reduction, conversion and type checker.

• "Coq Coq Codet! Towards a Verified Toolchain for Coq in MetaCoq" Matthieu Sozeau, Simon Boulier, Yannick Forster, Nicolas Tabareau and Théo Winterhalter. Abstract and presentation given at the Coq Workshop 2019, September 2019.

• "The MetaCoq Project" Matthieu Sozeau, Abhishek Anand, Simon Boulier, Cyril Cohen, Yannick Forster, Fabian Kunze, Gregory Malecha, Nicolas Tabareau and Théo Winterhalter. JAR, February 2020. Extended version of the ITP 2018 paper.

  This includes a full documentation of the Template Monad and the typing rules of PCUIC.

• A certifying extraction with time bounds from Coq to call-by-value λ-calculus. Yannick Forster and Fabian Kunze. ITP 2019. Example

• "Towards Certified Meta-Programming with Typed Template-Coq" Abhishek Anand, Simon Boulier, Cyril Cohen, Matthieu Sozeau and Nicolas Tabareau. ITP 2018.

• The system was presented at Coq'PL 2018

Team & Credits

MetaCoq is developed by (left to right) Abhishek Anand, Danil Annenkov, Simon Boulier, Cyril Cohen, Yannick Forster, Meven Lennon-Bertrand, Kenji Maillard, Gregory Malecha, Jakob Botsch Nielsen, Matthieu Sozeau, Nicolas Tabareau and Théo Winterhalter.

Copyright (c) 2014-2022 Gregory Malecha
Copyright (c) 2015-2022 Abhishek Anand, Matthieu Sozeau
Copyright (c) 2017-2022 Simon Boulier, Nicolas Tabareau, Cyril Cohen
Copyright (c) 2018-2022 Danil Annenkov, Yannick Forster, Théo Winterhalter
Copyright (c) 2020-2022 Jakob Botsch Nielsen, Meven Lennon-Bertrand
Copyright (c) 2022      Kenji Maillard

This software is distributed under the terms of the MIT license. See LICENSE for details.

Bugs

Please report any bugs or feature requests on the github issue tracker.