`with` is optional for type invariants

[n-osborne]

Gospel parser allows for:

type t
(*@ invariant ... *)

and

type t
(*@ with x invariant ... *)

Is the first form necessary? My feeling is that without naming a value of type t, there is not much content to give to a type invariant. But maybe I am missing something?

This then make the identifier optional in the AST, hence an additional layer of destruction (so to speak) when exploring a type specification and a decision to make about what to do when there is no name (which is useless work if this never happen, and hard to do without having an idea of a potential use case).

Also, I believe the documentation is not up to date: https://github.com/ocaml-gospel/gospel/blob/main/docs/docs/language/type-specifications.md?plain=1#L33

[shym]

I imagine the specification dates back from a previous version in which it was possible to write:

type 'a t
(** The type for containers. *)
(*@ model capacity: int
    mutable model contents: 'a set
    invariant capacity > 0
    invariant Set.cardinal contents <= capacity *)

though this has not been valid syntax for quite some time I think. (I don’t see an example in which it would be still interesting not to name the t in the invariant at the moment)