package coq-serapi

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Sexp-based serialization library and protocol for machine interaction with the Coq proof assistant


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Sexp-based serialization library and protocol for machine interaction with the Coq proof assistant

Published: 28 Jan 2019


The Coq Se(xp)rialized Protocol

$ opam install coq-serapi
$ sertop --help

SerAPI is a library for machine-to-machine interaction with the Coq proof assistant, with particular emphasis on IDE support and code analysis tools. SerAPI provides automatic serialization of Ocaml datatypes from/to S-expressions.

SerAPI is a proof-of-concept and should be considered alpha-quality. However, it is fully functional and supports, among other things, asynchronous proof checking, full-document parsing, and serialization of Coq's core datatypes. SerAPI can also be run as WebWorker thread, providing a self-contained Coq system inside the browser. Typical load times in Google Chrome are less than a second.

The main design philosophy of SerAPI is to make clients life easy, trying to provide a convenient, robust interface that hides away most of the scary details involved in interacting with Coq.

Feedback from Coq users and developers is very welcome and intrinsic to the project. We are open to implementing new features and exploring new use cases, let us know what you think via the mailing list, the issue tracker, or our Gitter chat channel. See also SerAPI's FAQ.


SerAPI 0.6.x is based on Coq 8.9. These days, most work related to SerAPI is directly happening over Coq's upstream itself. The main objective is to improve the proof-document model; building a rich query language will be next.

Quick Overview and Documentation:

SerAPI can be installed as the OPAM package coq-serapi. See build instructions for manual installation. The experimental in-browser version is also online.

The principal entry point in SerAPI is the sertop REPL, a basic toplevel that reads and executes commands (S-Expressions) from stdin to stdout in both machine or human-friendly format. See sertop --help for an overview of the main options. Ctrl-C will interrupt a busy Coq process in the same way coqtop does.

We recommend rlwrap or the emacs mode for direct interaction.

Protocol Description

API WARNING: The protocol is experimental and may change often.

The canonical documentation for the protocol is the interface file itself; the data types exposed there are serialized in an automatic way so any change in that file will imply a change in the actual protocol.

Coq Objects:

An important part of the SerAPI protocol is the CoqObject data type, a union type that encapsulates most core Coq objects.


Interaction with sertop is done using commands, which can be optionally tagged in the form of (tag cmd); otherwise, an automatic tag will be assigned. For every command, SerAPI will always reply with (Answer tag Ack) to indicate that the command was successfully parsed and delivered to Coq, or with a SexpError if parsing failed.

There are three categories of commands:

  • Document manipulation: Add, Cancel, Exec, ...: these commands instruct Coq to perform some action on the current document. Every command will produce zero or more different tagged answers, and a final answer (Answer tag Completed), indicating that there won't be more output.

    SerAPI document commands are an evolution of the OCaml STM API, here and here you can find a few informal notes on how it works. We are working on a more detailed specification, for now you can get some more details in the issue tracker.

  • Queries: (Query ((opt value) ...) kind):

    Queries stream Coq objects of type kind. This can range from options, goals and hypotheses, tactics, etc... The first argument is a list of options: preds is a list of conjunctive filters, limit specifies how many values the query may return. pp controls the output format: PpSer for full serialization, or PpStr for "pretty printing". For instance:

    (tag (Query ((preds (Prefix "Debug")) (limit 10) (pp PpSexp)) Option))

    will stream all Coq options that start with "Debug", limiting to the first 10 and printing the full internal Coq datatype:

    (CoqOption (Default Goal Selector)
       ((opt_sync true) (opt_depr false) (opt_name "default goal selector")
       (opt_value (StringValue 1))))

    Options can be omitted, as in: (tag (Query ((limit 10)) Option)), and currently supported queries can be seen here

  • Printing: (Print opts obj): The Print command provides access to the Coq pretty printers. Its intended use is for printing (maybe IDE manipulated) objects returned by Query.

Quick demo (not always up to date)

$ rlwrap sertop --printer=human
(Add () "Lemma addn0 n : n + 0 = n. Proof. now induction n. Qed.")
  > (Answer 0 Ack)
  > (Answer 0 (Added 2 ((fname "") (line_nb 1) (bol_pos 0) (line_nb_last 1) (bol_pos_last 0) (bp 0) (ep 26))
  >            NewTip))
  > ...
  > (Answer 0 (Added 5 ... NewTip))
  > (Answer 0 Completed)

(Exec 5)
  > (Answer 1 Ack)
  > (Feedback ((id 5) (route 0) (contents (ProcessingIn master))))
  > ...
  > (Feedback ((id 5) (route 0) (contents Processed)))
  > (Answer 1 Completed)

(Query ((sid 3)) Goals)
  > (Answer 2 Ack)
  > (Answer 2
  >  (ObjList ((CoqGoal ((fg_goals (((name 5) (ty (App (Ind ...))))
                         (bg_goals ()) (shelved_goals ()) (given_up_goals ()))))))
  > (Answer 2 Completed)

(Query ((sid 3) (pp ((pp_format PpStr)))) Goals)
  > (Answer 3 Ack)
  > (Answer 3 (ObjList ((CoqString
  >   "\
  >    \n  n : nat\
  >    \n============================\
  >    \nn + 0 = n"))))
  > (Answer 3 Completed)

(Query ((sid 4)) Ast)
  > (Answer 4 Ack)
  > (Answer 4 (ObjList ((CoqAst ((((fname "") (line_nb 1) (bol_pos 0) (line_nb_last 1)
  >                                (bol_pos_last 0) (bp 34) (ep 50)))
  > ...
  >            ((Tacexp
  >              (TacAtom
  >                (TacInductionDestruct true false
  > ...
  > (Answer 4 Completed)

(pp_ex (Print () (CoqConstr (App (Rel 0) ((Rel 0))))))
  > (Answer pp_ex Ack)
  > (Answer pp_ex(ObjList((CoqString"(_UNBOUND_REL_0 _UNBOUND_REL_0)"))))

(Query () (Vernac "Print nat. "))
  > (Answer 6 Ack)
  > (Feedback ((id 5) (route 0) (contents
  >    (Message Notice ()
  >    ((Pp_box (Pp_hovbox 0) ...)
  > (Answer 6 (ObjList ()))
  > (Answer 6 Completed)

(Query () (Definition nat))
  > (Answer 7 Ack)
  > (Answer 7 (ObjList ((CoqMInd (Mutind ....)))))
  > (Answer 7 Completed)

Technical Report

There is a brief technical report with some details at


SerAPI has been developed at the Centre de Recherche en Informatique of MINES ParisTech (former École de Mines de Paris) and partially supported by the FEEVER project.

Clients using SerAPI

  • jsCoq allows you run Coq in your browser. JsCoq is the predecessor of SerAPI and will be shortly fully based on it.

  • elcoq, an emacs technology demo based on SerAPI by Clément Pit--Claudel. elcoq is not fully functional but illustrates some cool features of SerAPI.

  • peacoq, a Coq IDE for the browser has an experimental branch that uses SerAPI.

Developer information

Technical details

Coq SerAPI has three main components:

  • serapi: an extended version of the current IDE protocol,

  • serlib a library providing automatic de/serialization of most Coq data structures using ppx_conv_sexp. This should be eventually incorporated into Coq itself. Support for ppx_deriving_yojson is work in progress,

  • sertop, sertop_js, toplevels offering implementation of the protocol.

Building your own toplevels using serlib and serapi is encouraged.

Advanced use cases

With a bit more development effort, you can also:

  • use SerAPI as an Ocaml library. The low-level serialization library serlib/ and the higher-level SerAPI protocol in serapi/serapi_protocol.mli can be linked standalone,

  • use SerAPI's web worker JavaScript Worker from your web/node application. In this model, you communicate with SerAPI using the typical onmessage/postMessage worker API. Ready-to-use builds may be found at here, we provide an example REPL at:

  • we would also like to provide a Jupyter/IPython kernel.

Developer/Users Mailing List

SerApi development is main discussed in GitHub and in the Gitter channel. You can lso use the jsCoq mailing list, subscribe at:

The list archives should be also available at the Gmane group: You can post to the list using nntp.

Dependencies (9)

  1. ppx_sexp_conv build & >= "v0.11.0" & < "v0.15"
  2. ppx_deriving build & >= "4.2.1"
  3. ppx_import build & >= "1.5-3" & < "2.0"
  4. dune >= "1.2.0"
  5. sexplib >= "v0.11.0" & < "v0.15"
  6. ocamlfind >= "1.8.0"
  7. cmdliner >= "1.0.0"
  8. coq >= "8.9.0" & < "8.10"
  9. ocaml >= "4.06.0"

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