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!
type 'a t
(*@ model capacity : integer
mutable model contents : 'a sequence
with s
invariant s.capacity > 0
invariant Sequence.length s.contents <= s.capacity *)
exception Full
exception Empty
val create : int -> 'a t
(*@ s = create n
requires n > 0
ensures s.capacity = n
ensures s.contents = Sequence.empty *)
val is_empty : 'a t -> bool
(*@ b = is_empty s
pure
ensures b <-> s.contents = Sequence.empty *)
val clear : 'a t -> unit
(*@ clear s
modifies s.contents
ensures s.contents = Sequence.empty *)
val push : 'a -> 'a t -> unit
(*@ push a s
modifies s.contents
ensures s.contents = Sequence.cons a (old s.contents)
raises Full -> Sequence.length (old s.contents) = s.capacity
/\ s.contents = old s.contents *)
val pop : 'a t -> 'a
(*@ a = pop s
modifies s.contents
ensures a = Sequence.hd (old s.contents)
ensures s.contents = Sequence.tl (old s.contents)
raises Empty -> old s.contents = Sequence.empty = s.contents *)
val pop_opt : 'a t -> 'a option
(*@ o = pop_opt s
modifies s.contents
ensures match o with
| None -> old s.contents = Sequence.empty = s.contents
| Some a -> old s.contents = Sequence.cons a s.contents *)
val top : 'a t -> 'a
(*@ a = top s
ensures a = Sequence.hd s.contents
raises Empty -> s.contents = Sequence.empty *)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!
Gospel is available on Opam repositories. Installing it is straightforward:
$ opam install gospelThis 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 0.2.0Gospel 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.mliGospel also provides a ppx rewriter to allow odoc to display the contents of gospel specifications and declarations as documentation. This ppx rewriter works under the assumption that the source preprocessor has been run first. The dune stanza reads as follows:
(library
(name lib_name)
(preprocess
(pps gospel.ppx -- -pp "gospel pps")))Be aware that it is the user's responsability to run the gospel type-checker when needed.
You are using Gospel as a frontend? Let us know!
At the moment, three tools leverage Gospel specifications to provide more guarantees to your programs:
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 currently maintained by Jean-Christophe Filliâtre, Samuel Hym, Nicolas Osborne, Clément Pascutto, and Mário Pereira.
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: