Module Base.Invariant

This module defines signatures that are to be included in other signatures to ensure a consistent interface to invariant-style functions. There is a signature (S, S1, S2, S3) for each arity of type. Usage looks like:

type t
include Invariant.S with type t := t

or

type 'a t
include Invariant.S1 with type 'a t := 'a t
type nonrec 'a t = 'a -> unit
module type S = sig ... end
module type S1 = sig ... end
module type S2 = sig ... end
module type S3 = sig ... end
val invariant : Caml.Lexing.position -> 'a -> ('a -> Sexp.t) -> (unit -> unit) -> unit

invariant here t sexp_of_t f runs f (), and if f raises, wraps the exception in an Error.t that states "invariant failed" and includes both the exception raised by f, as well as sexp_of_t t. Idiomatic usage looks like:

invariant [%here] t [%sexp_of: t] (fun () ->
  ... check t's invariants ... )

For polymorphic types:

let invariant check_a t =
  Invariant.invariant [%here] t [%sexp_of: _ t] (fun () -> ... )

It's okay to use [%sexp_of: _ t] because the exceptions raised by check_a will show the parts that are opaque at top-level.

val check_field : 'a -> 'b t -> ('a, 'b) Field.t -> unit

check_field is used when checking invariants using Fields.iter. It wraps an exception raised when checking a field with the field's name. Idiomatic usage looks like:

type t =
  { foo : Foo.t;
    bar : Bar.t;
  }
[@@deriving fields]

let invariant t : unit =
  Invariant.invariant [%here] t [%sexp_of: t] (fun () ->
    let check f = Invariant.check_field t f in
    Fields.iter
      ~foo:(check Foo.invariant)
      ~bar:(check Bar.invariant))
;;