# Module Only_in_test

    module Only_in_test: sig .. end

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

Under the hood, `'a t = 'a Lazy.t` and the only thing that ever forces them is the `force` function below which should only be called in 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

    val of_thunk : (unit -> 'a) -> 'a t

    val force : 'a t -> 'a