# Module `Core_kernel.Only_in_test`

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
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)
```

`include Core_kernel__.Import.Monad.S with type 'a t := 'a t`

`include Base__.Monad_intf.S_without_syntax with type 'a t := 'a t`

`module Monad_infix : Base__.Monad_intf.Infix with type 'a t := 'a t`

`val return : 'a -> 'a t`

`return v`

returns the (trivial) computation that returns v.

`val of_thunk : (Core_kernel__.Import.unit -> 'a) -> 'a t`

`val force : 'a t -> 'a`