Here is another variant using normal values.
The advantage is that it does no tricks with bits, and supports
marshaling.
It is less efficient because the search is linear, but by using as
tag (lazy_tag -1) we can avoid being too inefficient in most cases.
Note however that after marshaling the values are going to
have the same tag, so this is going to be much less efficient.
Jacques
module Sopt : sig
type +'a t
val none : 'a t
val some : 'a -> 'a t
val is_none : 'a t -> bool
val arg : 'a t -> 'a
val depth : 'a t -> int
end = struct
type 'a t = Obj.t
let sopt_tag = Obj.lazy_tag - 1
let none = Obj.new_block sopt_tag 0
let last = 255
let area = Array.create (last+1) none
let () =
for i = 1 to last do
let stub = Obj.new_block sopt_tag 1 in
Obj.set_field stub 0 area.(i-1);
area.(i) <- stub
done
let is_none x = (x == none)
let rec some_aux x i =
if i < last then
if x == area.(i) then area.(i+1) else some_aux x (i+1)
else (* i = last *)
if x == area.(last) then invalid_arg "Sopt.some" else x
let some (x : 'a) : 'a t =
let x = Obj.repr x in
if Obj.is_int x || Obj.tag x <> sopt_tag then Obj.obj x
else Obj.obj (some_aux x 0)
let rec arg_aux x i =
if i <= last then
if x == area.(i) then area.(i-1) else arg_aux x (i+1)
else
if x == area.(0) then invalid_arg "Sopt.arg" else x
let arg (x : 'a t) : 'a =
let x = Obj.repr x in
if Obj.is_int x || Obj.tag x <> sopt_tag then Obj.obj x
else Obj.obj (arg_aux x 1)
let rec depth_aux x i =
if i <= last then
if x == area.(i) then i else depth_aux x (i+1)
else -1
let depth x = depth_aux (Obj.repr x) 0
end
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