This module can be used to safely expose functions and values in signatures that can only be used in unit tests.

Under the hood, 'a t = (unit -> 'a) and the only thing that ever forces them is the run_tests_and_exit function below which actually runs the unit tests.

For example, suppose in some module, type t is actually an int. You want to keep the type definition opaque, but use the underlying representation in unit tests. You could write in the ml:

let test_to_int t = Only_in_test.return t
    [let test_of_int n = Only_in_test.return n

You would then expose in the mli:

type t
    ... functions for use in regular programming...
    val test_to_int : t -> int Only_in_test.t
    val test_of_int : int -> t Only_in_test.t

Finally, if you have specific values that you might want to use in unit tests, but that have top-level side-effects or take too long to compute, you can delay the side-effects or computation until the unit tests are run by writing, e.g.:

let (test_special_value : t Only_in_test.t) = Only_in_test.of_thunk (fun () -> (factorial 100))

instead of

let (test_special_value : t Only_in_test.t) = Only_in_test.return (factorial 100)

type 'a t
include Monad.S with type t := 'a t
val of_thunk : (unit -> 'a) -> 'a t
val force : 'a t -> 'a