The main purpose of this module is to be used with parametric types such as ('a, Empty.t) result. Such type is actually isomorphic to 'a (see get_ok function). This is useful if a module signature expects a generic ('a,'b) result type, but for some instantiation, 'b is actually empty. Here is a small example how such module can be used:
module type S = sig
type error
type 'a t
val return : 'a -> 'a t
val fail : error -> 'a t
val bind : 'a t -> ('a -> 'a t) -> (error -> 'a t) -> 'a t end
module M : S with type error = Empty.t and type 'a t = ('a,
Empty.t) result = struct
type error = Empty.t
type 'a t = ('a, Empty.t) result
let return = fun x -> Ok x
let fail = Empty.absurd
let bind = fun data left _right -> match data with | Ok x ->
left x | Error err -> Empty.absurd err end
let _ = let data = M.bind (M.return 5) (fun x -> M.return (x +
2)) (Empty.absurd) in assert (data |> Empty.get_ok = 7) This type has no canonical inhabitant (there is no terminating OCaml value which inhabits this type).
get_ok r eliminates the impossible case. This function is total and does not raise any exception.
val absurd : t -> 'aabsurd empty allows constructing any type from the absurd case. This is useful to typecheck impossible branches, e.g.
Result.fold ~ok:(fun i -> i + 1) ~error:Empty.absurd (Ok 41)Note that this particular example using `result` can be rewritten as:
let i = Empty.get_ok (Ok 41) in i + 1