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.

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))
  ;;