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open Std
type 'a state =
| Value of 'a
| Waiting of (unit Promise.u -> unit)
| Running of unit Promise.t
| Failed of Exn.with_bt
type 'a t = 'a state Atomic.t
let init = Waiting (fun _ -> assert false)
let from_fun ~cancel fn =
let state = Atomic.make init in
let rec force r =
match
if cancel = `Protect then Cancel.protect fn else fn ()
with
| x ->
Atomic.set state (Value x);
Promise.resolve r ()
| exception ex ->
let bt = Printexc.get_raw_backtrace () in
match ex with
| Cancel.Cancelled _ when cancel = `Restart && Fiber.is_cancelled () ->
Atomic.set state (Waiting force);
Promise.resolve r ();
Fiber.check ()
| _ ->
Atomic.set state (Failed (ex, bt));
Promise.resolve r ();
Printexc.raise_with_backtrace ex bt
in
Atomic.set state @@ Waiting force;
state
let from_val v = Atomic.make (Value v)
let rec force t =
match Atomic.get t with
| Value v -> v
| Failed (ex, bt) -> Printexc.raise_with_backtrace ex bt
| Running p -> Promise.await p; force t
| Waiting fn as prev ->
let p, r = Promise.create () in
if Atomic.compare_and_set t prev (Running p) then fn r;
force t