package picos

yield () asks the current fiber to be rescheduled.

ℹ️ The behavior is that

• on OCaml 5, yield perform the Yield effect, and
• on OCaml 4, yield will call the yield operation of the current handler.

sleep ~seconds suspends the current fiber for the specified number of seconds.

Represents a fiber or an independent thread of execution.

⚠️ Unlike with most other concepts of Picos, operations on fibers are typically not concurrency or parallelism safe, because the fiber is considered to be owned by a single thread of execution.

Type alias for Fiber.t within this signature.

current () returns the current fiber.

⚠️ Extra care should be taken when storing the fiber object in any shared data structure, because, aside from checking whether two fibers are equal, it is generally unsafe to perform any operations on foreign fibers.

ℹ️ The behavior is that

• on OCaml 5, current performs the Current effect, and
• on OCaml 4, current will call the current operation of the current handler.

⚠️ The current operation must always resume the fiber without propagating cancelation. A scheduler may, of course, decide to reschedule the current fiber to be resumed later.

Because the scheduler does not discontinue a fiber calling current, it is recommended that the caller checks the cancelation status at the next convenient moment to do so.

has_forbidden fiber determines whether the fiber forbids or permits the scheduler from propagating cancelation to it.

ℹ️ This is mostly useful in the effect handlers of schedulers.

⚠️ There is no "reference count" of how many times a fiber has forbidden or permitted propagation of cancelation. Calls to forbid and permit directly change a single boolean flag.

⚠️ It is only safe to call has_forbidden from the fiber itself.

forbid fiber thunk tells the scheduler that cancelation must not be propagated to the fiber during the execution of thunk.

The main use case of forbid is the implementation of concurrent abstractions that may have to await for something, or may need to perform other effects, and must not be canceled while doing so. For example, the wait operation on a condition variable typically reacquires the associated mutex before returning, which may require awaiting for the owner of the mutex to release it.

ℹ️ forbid does not prevent the fiber or the associated computation from being canceled. It only tells the scheduler not to propagate cancelation to the fiber when it performs effects.

⚠️ It is only safe to call forbid from the fiber itself.

permit fiber thunk tells the scheduler that cancelation may be propagated to the fiber during the execution of thunk.

It is possible to spawn a fiber with cancelation forbidden, which means that cancelation won't be propagated to fiber unless it is explicitly permitted by the fiber at some point.

⚠️ It is only safe to call permit from the fiber itself.

is_canceled fiber is equivalent to

not (Fiber.has_forbidden fiber) &&
let (Packed computation) =
  Fiber.get_computation fiber
in
Computation.is_canceled computation

ℹ️ This is mostly useful in the effect handlers of schedulers.

⚠️ It is only safe to call is_canceled from the fiber itself.

canceled fiber is equivalent to:

if Fiber.has_forbidden fiber then
  None
else
  let (Packed computation) =
    Fiber.get_computation fiber
  in
  Computation.canceled computation

ℹ️ This is mostly useful in the effect handlers of schedulers.

⚠️ It is only safe to call canceled from the fiber itself.

check fiber is equivalent to:

if not (Fiber.has_forbidden fiber) then
  let (Packed computation) =
    Fiber.get_computation fiber
  in
  Computation.check computation

ℹ️ This is mostly useful for periodically polling the cancelation status during CPU intensive work.

⚠️ It is only safe to call check from the fiber itself.

exchange fiber ~forbid sets the bit that tells the scheduler whether to propagate cancelation or not and returns the previous state.

set fiber ~forbid sets the bit that tells the scheduler whether to propagate cancelation or not.

Fiber local storage

create_packed ~forbid packed creates a new fiber record.

create ~forbid computation is equivalent to create_packed ~forbid (Computation.Packed computation).

spawn fiber main starts a new fiber by performing the Spawn effect.

⚠️ Fiber records must be unique and the caller of spawn must make sure that a specific fiber record is not reused. Failure to ensure that fiber records are unique will break concurrent abstractions written on top the the Picos interface.

⚠️ If the main function raises an exception it is considered a fatal error. A fatal error should effectively either directly exit the program or stop the entire scheduler, without discontinuing existing fibers, and force the invocations of the scheduler on all domains to exit. What this means is that the caller of spawn should ideally arrange for any exception to be handled by main, but, in case that is not practical, it is also possible to allow an exception to propagate out of main, which is then guaranteed to, one way or the other, to stop the entire program. It is not possible to recover from a fatal error.

ℹ️ The behavior is that

• on OCaml 5, spawn performs the Spawn effect, and
• on OCaml 4, spawn will call the spawn operation of the current handler.

get_computation fiber returns the computation that the fiber is currently associated with.

set_computation fiber packed associates the fiber with the specified computation.

equal fiber1 fiber2 is physical equality for fibers, i.e. it determines whether fiber1 and fiber2 are one and the same fiber.

ℹ️ One use case of equal is in the implementation of concurrent primitives like mutexes where it makes sense to check that acquire and release operations are performed by the same fiber.

An unboxed optional fiber.

try_suspend fiber trigger x y resume tries to suspend the fiber to await for the trigger to be signaled. If the result is false, then the trigger is guaranteed to be in the signaled state and the fiber should be eventually resumed. If the result is true, then the fiber was suspended, meaning that the trigger will have had the resume action attached to it and the trigger has potentially been attached to the computation of the fiber.

unsuspend fiber trigger makes sure that the trigger will not be attached to the computation of the fiber. Returns false in case the fiber has been canceled and propagation of cancelation is not forbidden. Otherwise returns true.

⚠️ The trigger must be in the signaled state!

resume fiber k is equivalent to Effect.Deep.continue k (Fiber.canceled t).

resume_with fiber k h is equivalent to Effect.Shallow.continue_with k (Fiber.canceled t) h.

continue fiber k v is equivalent to:

match Fiber.canceled fiber with
| None -> Effect.Deep.continue k v
| Some (exn, bt) ->
  Effect.Deep.discontinue_with_backtrace k exn bt

continue_with fiber k v h is equivalent to:

match Fiber.canceled fiber with
| None -> Effect.Shallow.continue_with k v h
| Some (exn, bt) ->
  Effect.Shallow.discontinue_with_backtrace k exn bt h

Schedulers must handle the Current effect to implement the behavior of current.

⚠️ The scheduler must eventually resume the fiber without propagating cancelation. This is necessary to allow a fiber to control the propagation of cancelation through the fiber.

The scheduler is free to choose which ready fiber to resume next. However, in typical use cases of current it makes sense to give priority to the fiber performing Current, but this is not required.

Schedulers must handle the Yield effect to implement the behavior of yield.

In case the fiber permits propagation of cancelation and the computation associated with the fiber has been canceled the scheduler is free to discontinue the fiber immediately.

The scheduler should give priority to running other ready fibers before resuming the fiber performing Yield. A scheduler that always immediately resumes the fiber performing Yield may prevent an otherwise valid program from making progress.

Schedulers must handle the Spawn effect to implement the behavior of spawn.

In case the fiber permits propagation of cancelation and the computation associated with the fiber has been canceled the scheduler is free to discontinue the fiber immediately before spawning new fibers.

The scheduler is free to run the newly created fiber on any domain and decide which fiber to give priority to.

⚠️ The scheduler should guarantee that, when Spawn returns normally, the given main will eventually be called by the scheduler and, when Spawn raises an exception, the main will not be called. In other words, Spawn should check cancelation just once and be all or nothing. Furthermore, in case a newly spawned fiber is canceled before its main is called, the scheduler must still call the main. This allows a program to ensure, i.e. keep track of, that all fibers it spawns are terminated properly and any resources transmitted to spawned fibers will be disposed properly.