VOCaL -- a Verified OCaml Library

VOCaL is a general-purpose data structure and algorithm OCaml library.

What distinguishes this library from many other is the fact that its
implementation has been formally verified. The verification includes
absence of run-time errors and functional correctness. The latter uses
formal specifications inserted in .mli files with special comments
starting with (*@. The specification language for OCaml is still
under development and a quick documentation is given below.
The .mli files also contain traditional, informal documentation,
similar to that of any other OCaml library.

The github repository also contains some of the source files for the
proofs of correctness (eventually, it will contain all of them). Yet
these proofs are not needed to compile and use the VOCaL library.
Have a look in proofs/ if you are curious.

OCaml Interface Specification Language

We briefly describe the specification language using an example,
taken from Vector.mli.

The abstract type t of vectors below is identified as ephemeral (elements
can be mutated in-place) and is modeled using a polymorphic sequence, introduced
using the mutable model syntax. Properties about the data type and associated
models can be captured using the invariant keyword.

(** The polymorphic type of vectors.
    This is a mutable data type. *)
type 'a t
(*@ ephemeral *)
(*@ mutable model view: 'a seq *)
(*@ invariant length view <= Sys.max_array_length *)

To provide specification for function declarations, the parameters and the
returned value must be named first. Preconditions are stated in requires
clause, while postconditions are introduced after ensures.

val create: ?capacity:int -> dummy:'a -> 'a t
(*@ a = create capacity dummy
      requires let capacity = match capacity with
                 | None -> 0 | Some c -> c in
               0 <= capacity <= Sys.max_array_length
      ensures  length a.view = 0 *)

Whenever type int is mentioned, it refers to the OCaml type int
of native machine integer (e.g. 63-bit signed integers on a 64-bit
platform). A type integer for mathematical integers is also
provided. Here is an example:

val make: ?dummy:'a -> int -> 'a -> 'a t
(*@ a = make ?dummy n x
      requires 0 <= n <= Sys.max_array_length
      ensures  length a.view = n
      ensures  forall i: integer. 0 <= i < n -> a.view[i] = x *)

Whenever a function has side effects, this is indicated using a
modifies clause. Here is an example:

val resize: 'a t -> int -> unit
(*@ resize a n
      checks   0 <= n <= Sys.max_array_length
      modifies a
      ensures  length a.view = n
      ensures  forall i. 0 <= i < min (length (old a.view)) n ->
                 a.view[i] = (old a.view)[i] *)

This last example also features the checks clause. This is an
alternative to requires. Contrary to the latter, a checks clause
is checked at run-time, and raises an Invalid_argument exception
when it is not satisfied.

Last, an equivalent clause is sometimes used to describe the
behavior of an OCaml function using an equivalent piece of OCaml
code. Here is an example:

val iter : ('a -> unit) -> 'a t -> unit
(*@ iter f a
      equivalent "for i = 0 to length a - 1 do f (get a i) done" *)

A forthcoming documentation of this specification language (work in
progress) will hopefully provide more details and clarify the semantics.

Copyright

See the enclosed file LICENSE.

Bugs report

Please use the github issues
to report bugs.