Notes on the prehistory of Coq

This archive contains the sources of the CONSTR ancestor of the Coq proof
assistant. CONSTR, then Coq, was designed and implemented in the Formel team,
joint between the INRIA Rocquencourt laboratory and the Ecole Normale Supérieure
of Paris, from 1984 onwards.

Version 1

This software is a prototype type-checker for a higher-order logical formalism
known as the Theory of Constructions, presented in his PhD thesis by
Thierry Coquand, with influences from Girard's system F and de Bruijn's Automath.
The metamathematical analysis of the system is the
PhD work of Thierry Coquand. The software is mostly the work of Gérard Huet.
Most of the mathematical examples verified with the software are due
to Thierry Coquand.

The programming language of the CONSTR software (as it was called at the time)
is a version of ML issued from the Edinburgh LCF system and running on
a LISP backend. The main improvements from the original LCF ML are that ML
is compiled rather than interpreted (Gérard Huet building on the original
translator by Lockwood Morris), and that it is enriched by recursively
defined types (work of Guy Cousineau). This ancestor of CAML was used
and improved by Larry Paulson for his implementation of Cambridge LCF.

Software developments of this prototype occurred from late 1983 to early 1985.

Version 1.10 was frozen on December 22nd 1984. It is the version used for the
examples in Thierry Coquand's thesis, defended on January 31st 1985.
There was a unique binding operator, used both for universal quantification
(dependent product) at the level of types and functional abstraction (lambda)
at the level of terms/proofs, in the manner of Automath. Substitution
(lambda reduction) was implemented using de Bruijn's indexes.

Version 1.11 was frozen on February 19th, 1985. It is the version used for the
examples in the paper:
Th. Coquand, G. Huet. Constructions: A Higher Order Proof System for Mechanizing
Mathematics. Invited paper, EUROCAL85, April 1985, Linz, Austria. Springer Verlag
LNCS 203, pp. 151-184.

Christine Paulin joined the team at this point, for her DEA research internship.
In her DEA memoir (August 1985) she presents developments for the lambo function
computing the minimal m such that f(m) is greater than n, for f an increasing
integer function, a challenge for constructive mathematics. She also encoded
the majority voting algorithm of Boyer and Moore.

Version 2

The formal system, now renamed as the "Calculus of Constructions", was presented
with a proof of consistency and comparisons with proof systems of Per
Martin Löf, Girard, and the Automath family of N. de Bruijn, in the paper:
T. Coquand and G. Huet. The Calculus of Constructions.
Submitted on June 30th 1985, accepted on December 5th, 1985,
Information and Computation. Preprint as Rapport de Recherche Inria n°530,
Mai 1986. Final version in Information and Computation 76,2/3, Feb. 88.

An abstraction of the software design, in the form of an abstract machine
for proof checking, and a fuller sequence of mathematical developments was
presented in:
Th. Coquand, G. Huet. Concepts Mathématiques et Informatiques Formalisés dans le Calcul des Constructions. Invited paper, European Logic Colloquium, Orsay,
July 1985. Preprint as Rapport de recherche INRIA n°463, Dec. 85.
Published in Logic Colloquium 1985, North-Holland, 1987.

Version 2.8 was frozen on December 16th, 1985, and served for developing
the exemples in the above papers.

This calculus was then enriched in version 2.9 with a cumulative hierarchy of
universes. Universe levels were initially explicit natural numbers.
Another improvement was the possibility of automatic synthesis of implicit
type arguments, relieving the user of tedious redundant declarations.

Christine Paulin wrote an article "Algorithm development in the Calculus of
Constructions", preprint as Rapport de recherche INRIA n°497, March 86.
Final version in Proceedings Symposium on Logic in Computer Science, Cambridge,
MA, 1986 (IEEE Computer Society Press). Besides lambo and majority,
she presents quicksort and a text formatting algorithm.

Version 2.13 of the calculus of constructions with universes was frozen
on June 25th, 1986.

A synthetic presentation of type theory along constructive lines with ML
algorithms was given by Gérard Huet in his May 1986 CMU course notes
"Formal Structures for Computation and Deduction". Its chapter
"Induction and Recursion in the Theory of Constructions" was presented
as an invited paper at the Joint Conference on Theory and Practice of Software
Development TAPSOFT’87 at Pise in March 1987, and published as
"Induction Principles Formalized in the Calculus of Constructions" in
Programming of Future Generation Computers, Ed. K. Fuchi and M. Nivat,
North-Holland, 1988.

Version 3

This version saw the beginning of proof automation, with a search algorithm
inspired from PROLOG and the applicative logic programming programs
of the course notes "Formal structures for computation and deduction".
The search algorithm was implemented in ML by Thierry Coquand.
The proof system could thus be used in two modes: proof verification and
proof synthesis, with tactics such as "AUTO".

The implementation language was now called CAML, for "categorical abstract
machine language". It used as backend the LLM3 virtual machine of Le Lisp
by Jérôme Chailloux. The main developers of CAML were Michel Mauny,
Ascander Suarez and Pierre Weis.

V3.1 was started in the summer of 1986, V3.2 was frozen at the end of November
1986. V3.4 was developed in the first half of 1987.

Thierry Coquand held a post-doctoral position in Cambrige University in 1986-87,
where he developed a variant implementation in SML, with which he wrote
some developments on fixpoints in Scott's domains.

Version 4

This version saw the beginning of program extraction from proofs, with
two varieties of the type Prop of propositions, indicating constructive intent.
The proof extraction algorithms were implemented by Christine Paulin-Mohring.

V4.1 was frozen on July 24th, 1987. It had a first identified library of
mathematical developments (directory exemples), with libraries Logic
(containing impredicative encodings of intuitionistic logic and algebraic
primitives for booleans, natural numbers and list), Peano developing second-order
Peano arithmetic, Arith defining addition, multiplication, euclidean division
and factorial. Typical developments were the Knaster-Tarski theorem
and Newman's lemma from rewriting theory.

V4.2 was a joint development of a team consisting of Thierry Coquand, Gérard
Huet and Christine Paulin-Mohring. A file V4.2.log records the log of changes.
It was frozen on September 1987 as the last version implemented in CAML 2.3,
and V4.3 followed on CAML 2.5, a more stable development system.

V4.3 saw the first top-level of the system. Instead of evaluating explicit
quotations, the user could develop his mathematics in a high-level language
called the mathematical vernacular (following Automath terminology).
The user could develop files in the vernacular notation (with .v extension)
which were now separate from the ml sources of the implementation.
Gilles Dowek joined the team to develop the vernacular language as his
DEA internship research.

A notion of sticky constant was introduced, in order to keep names of lemmas
when local hypotheses of proofs were discharged. This gave a notion
of global mathematical environment with local sections.

Another significant practical change was that the system, originally developped
on the VAX central computer of our lab, was transferred on SUN personal
workstations, allowing a level of distributed development.
The extraction algorithm was modified, with three annotations Pos, Null and
Typ decorating the sorts Prop and Type.

Version 4.3 was frozen at the end of November 1987, and was distributed to an
early community of users (among those were Hugo Herbelin and Loic Colson).

V4.4 saw the first version of (encoded) inductive types.
Now natural numbers could be defined as:
Inductive NAT : Prop = O : NAT | Succ : NAT->NAT.
These inductive types were encoded impredicatively in the calculus,
using a subsystem "rec" due to Christine Paulin.
V4.4 was frozen on March 6th 1988.

Version 4.5 was the first one to support inductive types and program extraction.
Its banner was "Calcul des Constructions avec Realisations et Synthese".
The vernacular language was enriched to accommodate extraction commands.

The verification engine design was presented as:
G. Huet. The Constructive Engine. Version 4.5. Invited Conference, 2nd European
Symposium on Programming, Nancy, March 88.
The final paper, describing the V4.9 implementation, appeared in:
A perspective in Theoretical Computer Science, Commemorative Volume in memory
of Gift Siromoney, Ed. R. Narasimhan, World Scientific Publishing, 1989.

Version 4.5 was demonstrated in June 1988 at the YoP Institute on Logical
Foundations of Functional Programming organized by Gérard Huet at Austin, Texas.

Version 4.6 was started during summer 1988. Its main improvement was the
complete rehaul of the proof synthesis engine by Thierry Coquand, with
a tree structure of goals.

Its source code was communicated to Randy Pollack on September 2nd 1988.
It evolved progressively into LEGO, proof system for Luo's formalism
of Extended Calculus of Constructions.

The discharge tactic was modified by G. Huet to allow for inter-dependencies
in discharged lemmas. Christine Paulin improved the inductive definition scheme
in order to accommodate predicates of any arity.

Version 4.7 was started on September 6th, 1988.

This version starts exploiting the CAML notion of module in order to improve the
modularity of the implementation. Now the term verifier is identified as
a proper module Machine, which the structure of its internal data structures
being hidden and thus accessible only through the legitimate operations.
This machine (the constructive engine) was the trusted core of the
implementation. The proof synthesis mechanism was a separate proof term
generator. Once a complete proof term was synthesized with the help of tactics,
it was entirely re-checked by the engine. Thus there was no need to certify
the tactics, and the system took advantage of this fact by having tactics ignore
the universe levels, universe consistency check being relegated to the final
type-checking pass. This induced a certain puzzlement of early users who saw
their successful proof search ended with QED, followed by silence, followed by
a failure message of universe inconsistency rejection...

The set of examples comprise set theory experiments by Hugo Herbelin,
and notably the Schroeder-Bernstein theorem.

Version 4.8, started on October 8th, 1988, saw a major re-implementation of the
abstract syntax type constr, separating variables of the formalism and
metavariables denoting incomplete terms managed by the search mechanism.
A notion of level (with three values TYPE, OBJECT and PROOF) is made explicit
and a type judgement clarifies the constructions, whose implementation is now
fully explicit. Structural equality is speeded up by using pointer equality,
yielding spectacular improvements. Thierry Coquand adapts the proof synthesis
to the new representation, and simplifies pattern matching to 1st order
predicate calculus matching, with important performance gain.

A new representation of the universe hierarchy is then defined by G. Huet.
Universe levels are now implemented implicitly, through a hidden graph
of abstract levels constrained with an order relation.
Checking acyclicity of the graph insures well-foundedness of the ordering,
and thus consistency. This was documented in a memo
"Adding Type:Type to the Calculus of Constructions" which was never published.

The development version is released as a stable 4.8 at the end of 1988.

Version 4.9 is released on March 1st 1989, with the new "elastic"
universe hierarchy.

The spring 89 saw the first attempt at documenting the system usage,
with a number of papers describing the formalism:
- Metamathematical Investigations of a Calculus of Constructions, by
Thierry Coquand (INRIA Research Report N°1088, Sept. 1989, published in
Logic and Computer Science, ed. P.G. Odifreddi, Academic Press, 1990)
- Inductive definitions in the Calculus of Constructions, by
Christine Paulin-Mohring,
- Extracting Fomega's programs from proofs in the Calculus of Constructions, by
Christine Paulin-Mohring (published in POPL'89)
- The Constructive Engine, by Gérard Huet
as well as a number of user guides:
- A short user's guide for the Constructions Version 4.10, by Gérard Huet
- A Vernacular Syllabus, by Gilles Dowek.
- The Tactics Theorem Prover, User's guide, Version 4.10, by Thierry Coquand.

Stable V4.10, released on May 1st, 1989, was then a mature system,
distributed with CAML V2.6.

In the mean time, Thierry Coquand and Christine Paulin-Mohring
had been investigating how to add native inductive types to the
Calculus of Constructions, in the manner of Per Martin-Löf's Intuitionistic
Type Theory. The impredicative encoding had already been presented in:
F. Pfenning and C. Paulin-Mohring. Inductively defined types in the Calculus
of Constructions. Preprint technical report CMU-CS-89-209, final version in
Proceedings of Mathematical Foundations of Programming Semantics,
volume 442, Lecture Notes in Computer Science. Springer-Verlag, 1990.
An extension of the calculus with primitive inductive types appeared in:
Th. Coquand and C. Paulin-Mohring. Inductively defined types.
In P. Martin-Löf and G. Mints, editors, Proceedings of Colog'88, volume 417,
Lecture Notes in Computer Science. Springer-Verlag, 1990.

This lead to the Calculus of Inductive Constructions, logical formalism
implemented in Versions 5 upward of the system, and documented in:
C. Paulin-Mohring. Inductive Definitions in the System Coq - Rules and
Properties. In M. Bezem and J.-F. Groote, editors, Proceedings of the conference
Typed Lambda Calculi and Applications, volume 664, Lecture Notes in Computer
Science, 1993.

The last version of CONSTR is Version 4.11, which was last distributed
in Spring 1990. It was demonstrated at the first workshop of the European
Basic Research Action Logical Frameworks In Sophia Antipolis in May 1990.

At the end of 1989, Version 5.1 was started, and renamed as the system Coq
for the Calculus of Inductive Constructions. It was then ported to the new
stand-alone implementation of ML called Caml-light.

In 1990 many changes occurred. Thierry Coquand left for Chalmers University
in Göteborg. Christine Paulin-Mohring took a CNRS researcher position
at the LIP laboratory of Ecole Normale Supérieure de Lyon. Project Formel
was terminated, and gave rise to two teams: Cristal at INRIA-Roquencourt,
that continued developments in functional programming with Caml-light then
Ocaml, and Coq, continuing the type theory research, with a joint team
headed by Gérard Huet at INRIA-Rocquencourt and Christine Paulin-Mohring
at the LIP laboratory of CNRS-ENS Lyon.

Chetan Murthy joined the team in 1991 and became the main software architect
of Version 5. He completely rehauled the implementation for efficiency.
Versions 5.6 and 5.8 were major distributed versions, with complete
documentation and a library of users' developements. The use of the RCS
revision control system, and systematic ChangeLog files, allow a more
precise tracking of the software developments.

Developments from Version 6 upwards are documented in the credits section of
Coq's Reference Manual.

September 2015
Thierry Coquand, Gérard Huet and Christine Paulin-Mohring.