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)

type 'a t
include Core_kernel__.Import.Monad.S with type 'a t := 'a t
type 'a t
include Base__.Monad_intf.S_without_syntax with type 'a t := 'a t
type 'a t
include Base__.Monad_intf.Infix with type 'a t := 'a t
type 'a t
val (>>=) : 'a t -> ('a -> 'b t) -> 'b t

t >>= f returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t to yield a value v, and then runs the computation returned by f v.

val (>>|) : 'a t -> ('a -> 'b) -> 'b t

t >>| f is t >>= (fun a -> return (f a)).

module Monad_infix : Base__.Monad_intf.Infix with type 'a t := 'a t
val bind : 'a t -> f:('a -> 'b t) -> 'b t

bind t ~f = t >>= f

val return : 'a -> 'a t

return v returns the (trivial) computation that returns v.

val map : 'a t -> f:('a -> 'b) -> 'b t

map t ~f is t >>| f.

val join : 'a t t -> 'a t

join t is t >>= (fun t' -> t').

val ignore_m : 'a t -> unit t

ignore_m t is map t ~f:(fun _ -> ()). ignore_m used to be called ignore, but we decided that was a bad name, because it shadowed the widely used Caml.ignore. Some monads still do let ignore = ignore_m for historical reasons.

val all : 'a t list -> 'a list t
val all_unit : unit t list -> unit t

Like all, but ensures that every monadic value in the list produces a unit value, all of which are discarded rather than being collected into a list.

val all_ignore : unit t list -> unit t
include Base__.Monad_intf.Syntax with type 'a t := 'a t
type 'a t
module Let_syntax : sig ... end
val of_thunk : (Core_kernel__.Import.unit -> 'a) -> 'a t
val force : 'a t -> 'a