package gospel

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A tool-agnostic formal specification language for OCaml


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A tool-agnostic formal specification language for OCaml.


Gospel is a behavioural specification language for OCaml program. It provides developers with a non-invasive and easy-to-use syntax to annotate their module interfaces with formal contracts that describe type invariants, mutability, function pre-conditions and post-conditions, effects, exceptions, and much more!

We designed Gospel to provide a tool-agnostic frontend for bringing formal methods into the OCaml ecosystem, meaning that we make no assumptions on the future use of the specifications.

You can use Gospel specifications to complete and precise your documentation comments with non-ambiguous annotations and use a type-checker to ensure they always remain in sync. Other tools also rely on these annotations to provide additional features such as automated deductive verification or runtime assertion checking.

Please feel free to visit the project page if you wish to learn more about Gospel!

Getting Started


Gospel is not yet available on Opam repositories. You can install it via pinning:

$ opam pin add
$ opam install gospel

This will install the gospel tool binary, as well as the developer library if you wish to build your software on top of Gospel. You may check the installation with.

$ gospel --version
gospel version xxxxxx


Gospel contracts live in OCaml interface files (.mli), as special comments starting with the @ symbol:

val max_array: int array -> int
(*@ m = max_array a
    requires Array.length a > 0
    ensures forall i. 0 <= i < Array.length a -> a.(i) <= m
    ensures exists i. 0 <= i < Array.length a /\ a.(i) = m *)

Gospel provides a type-checker that ensures that your specifications are well-formed, well-typed, and remain in sync with the interface they annotate!

$ gospel check max_array.mli

Tools using Gospel

At the moment, three tools leverage Gospel specifications to provide more guarantees to your programs:

  • Cameleer. A tool that extends Gospel to implementation files to provide semi-automated deductive verification of OCaml programs.

  • Ortac. A runtime assertion checking tool based that generates verifying code for your test suites or programs monitors.

  • Why3gospel. A Why3 plugin that lets you verify that a program proof refines the Gospel specifications before extracting it to OCaml.


This project is licensed under the MIT license.

See the LICENSE file for more information.


Gospel was initially developed by Cláudio Lourenço (LRI postdoctorate).

It is now maintained by Clément Pascutto, Mário Pereira, and Jean-Christophe Filliâtre.

The full list of contributors is available here.


This project is part of a collaboration between the LMF laboratory, Tarides, and NOVA LINCS.

The development is supported by:

  • The VOCaL project. ANR grant No. ANR-15-CE25-0008, 1/10/2015 - 31/3/2021.

  • The HORIZON 2020 Cameleer project (Marie Skłodowska-Curie grant agreement ID:897873) and NOVA LINCS (Ref. UIDB/04516/2020).