plugin_tutorial

How to write plugins in Coq

This document describes how to extend Coq by writing plugins in the OCaml functional programming language. Before writing a plugin, you should consider whether easier approaches can achieve your goal. It is often easier to use an extension language such as Ltac2 or Elpi. Ltac2 is documented in the refman and Elpi comes with excellent tutorials. Plugin development is harder due to the lower level OCaml API it uses. It is also more maintenance intensive because the OCaml API can change from release to release without any backward compatibility (it's not uncommon for plugins to only work with a given version of Coq).

Working environment

In addition to installing OCaml and Coq, you need to make sure that you also have the development headers for Coq, because you will need them to compile extensions. If you installed Coq from source or from OPAM, you already have the required headers. If you installed them from your system package manager, there may be a separate package which contains the development headers (for example, in Ubuntu they are contained in the package libcoq-ocaml-dev). It can help to install several tools for development.

Tuareg and Merlin

These instructions use OPAM

opam install merlin               # prints instructions for vim and emacs
opam install tuareg               # syntax highlighting for OCaml
opam user-setup install           # automatically configures editors for merlin

Adding this line to your .emacs helps Tuareg recognize the .mlg extension:

(add-to-list 'auto-mode-alist '("\\.mlg$"      . tuareg-mode) t)

If you are using vscoq, you will need to ensure that vscoq loads the _CoqProject file for the extension you are working on. You can do this by opening Visual Studio Code with the _CoqProject file in the project root directory, or by editing the coqtop.coqProjectRoot setting for vscoq.

This tutorial

cd plugin_tutorials/tuto0
make .merlin                # run before opening .ml files in your editor
make                        # build

tuto0 : basics of project organization

package an mlg file in a plugin, organize a Makefile, _CoqProject

• Example of syntax to add a new toplevel command
• Example of function call to print a simple message
• Example of function call to print a simple warning
• Example of function call to raise a simple error to the user
• Example of syntax to add a simple tactic (that does nothing and prints a message)
• To use it:

    cd tuto0; make
    coqtop -I src -R theories Tuto0

In the Coq session type:

    Require Import Tuto0.Loader. HelloWorld.

You can also modify and run theories/Demo.v.

tuto1 : OCaml to Coq communication

Explore the memory of Coq, modify it

• Commands that take arguments: strings, integers, symbols, expressions of the calculus of constructions
• Examples of using environments correctly
• Examples of using state (the evar_map) correctly
• Commands that interact with type-checking in Coq
• A command that checks convertibility between two terms
• A command that adds a new definition or theorem
• A command that uses a name and exploits the existing definitions or theorems
• A command that exploits an existing ongoing proof
• A command that defines a new tactic

Compilation and loading must be performed as for tuto0.

tuto2 : OCaml to Coq communication

A more step by step introduction to writing commands

• Explanation of the syntax of entries
• Adding a new type to and parsing to the available choices
• Handling commands that store information in user-chosen registers and tables

Compilation and loading must be performed as for tuto0.

tuto3 : manipulating terms of the calculus of constructions

Manipulating terms, inside commands and tactics.

• Obtaining existing values from memory
• Composing values
• Verifying types
• Using these terms in commands
• Using these terms in tactics
• Automatic proofs without tactics using type classes and canonical structures

compilation and loading must be performed as for tuto0.