package async_kernel

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Module type
Class type

A value that will become determined asynchronously.

A deferred can be "undetermined" or "determined". A deferred that is undetermined may at some point become determined with value v, and will henceforth always be determined with value v.

type +'a t = 'a Async_kernel__.Types.Deferred.t
val sexp_of_t : ('a -> Sexplib0.Sexp.t) -> 'a t -> Sexplib0.Sexp.t
include Core.Invariant.S1 with type 'a t := 'a t
val invariant : ('a -> unit) -> 'a t -> unit

sexp_of_t t f returns a sexp of the deferred's value, if it is determined, or an informative string otherwise.

This is just for display purposes. There is no t_of_sexp.

val create : ('a Ivar.t -> unit) -> 'a t

create f calls f i, where i is an empty ivar. create returns a deferred that becomes determined when f fills i.

val upon : 'a t -> ('a -> unit) -> unit

upon t f will run f v at some point after t becomes determined with value v.

val peek : 'a t -> 'a option

peek t returns Some v iff t is determined with value v.

val value_exn : 'a t -> 'a

value_exn t returns v if t is determined with value v, and raises otherwise.

val is_determined : 'a t -> bool

is_determined t returns true iff t is determined.

Deferreds form a monad.

let%bind v = t in f v returns a deferred t' that waits until t is determined with value v, at which point it waits for f v to become determined with value v', to which t' will become determined.

return v returns a deferred that is immediately determined with value v.

Note that:

upon t f 

is more efficient than:

ignore (let%bind a = t in f a; return ()) 

because upon, unlike let%bind, does not create a deferred to hold the result.

For example, one can write a loop that has good constant factors with:

let rec loop () =
  upon t (fun a -> ... loop () ... ) 

although often forever or repeat_until_finished is more clear.

The same loop written with let%bind would allocate deferreds that would be immediately garbage collected. (In the past, this loop would have also used linear space in recursion depth!)

In general, for deferreds that are allocated by let%bind to be garbage collected quickly, it is sufficient that the allocating bind be executed in tail-call position of the right-hand side of an outer bind.

include Core.Monad with type 'a t := '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 : sig ... end
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 Stdlib.ignore. Some monads still do let ignore = ignore_m for historical reasons.

module Let_syntax : sig ... end

These are convenient to have in scope when programming with a monad:

module Infix : sig ... end
val unit : unit t

unit is a deferred that is always determined with value ()

val never : unit -> _ t

never () returns a deferred that never becomes determined.

val both : 'a t -> 'b t -> ('a * 'b) t

both t1 t2 becomes determined after both t1 and t2 become determined.

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

all ts returns a deferred that becomes determined when every t in ts is determined. The output is in the same order as the input.

val all_unit : unit t list -> unit t

Like all, but ignores results of the component deferreds.

val any : 'a t list -> 'a t

any ts returns a deferred that is determined when any of the underlying deferreds is determined.

val any_unit : unit t list -> unit t

any_unit is like any, but ignores results of the component deferreds.

val don't_wait_for : unit t -> unit

don't_wait_for t ignores t. It is like Fn.ignore, but is more constrained because it requires a unit Deferred.t.

Rather than ignore (t : _ t), do don't_wait_for (Deferred.ignore_m t).

We chose to give don't_wait_for type unit t rather than _ t to catch errors where a value is accidentally ignored.

module Choice : sig ... end

A Choice.t is used to produce an argument to enabled or choose. See below.

val choice : 'a t -> ('a -> 'b) -> 'b Choice.t

choice t g corresponds to one alternative to use in choose.

One needs to be mindful of memory leaks and effect leaks when using this interface: if the corresponding choice is not taken, then t is simply discarded (as if don't_wait_for was called), but any work that was scheduled to make t determined won't be cancelled automatically.

This means that something as simple as choice (map d ~f) g creates a leak: it adds a handler f to d, which will still run if d is filled (effect leak), or stay in memory until the subroutine responsible for filling this ivar drops it (memory leak).

To avoid this memory leak it's sufficient to choose on an pre-existing ivar, whose lifetime is controlled separately. For example:

  • use Pipe.read_choice, not,
  • use Clock.Event.fired, not Clock.after (then cancel the event).
val enabled : 'b Choice.t list -> (unit -> 'b list) t

enabled [choice t1 f1; ... choice tn fn;] returns a deferred d that becomes determined when any of the ti becomes determined. The value of d is a function f that when called, for each ti that is enabled, applies fi to ti, and returns a list of the results. It is guaranteed that the list is in the same order as the choices supplied to enabled, but of course it may be shorter than the input list if not all ti are determined.

val choose : 'b Choice.t list -> 'b t
choose [ choice t1 f1
       ; ...
         ; choice tn fn ] 

returns a deferred t that becomes determined with value fi ai after some ti becomes determined with value ai. It is guaranteed that choose calls at most one of the fis, the one that determines its result. There is no guarantee that the ti that becomes determined earliest in time will be the one whose value determines the choose. Nor is it guaranteed that the value in t is the first value (in place order) from choices that is determined at the time t is examined.

For example, in:

choose [ choice t1 (fun () -> `X1)
       ; choice t2 (fun () -> `X2) ]
>>> function
| `X1 -> e1
| `X2 -> e2 

it may be the case that both t1 and t2 become determined, yet e2 actually runs.

It is guaranteed that if multiple choices are determined with no intervening asynchrony, then the earliest choice in the list will become the value of the choose.

val for_ : int -> to_:int -> do_:(int -> unit t) -> unit t

for_ start ~to_:stop ~do_:f is the deferred analog of:

for i = start to stop do
  f i;
val repeat_until_finished : 'state -> ('state -> [ `Repeat of 'state | `Finished of 'result ] t) -> 'result t

repeat_until_finished initial_state f repeatedly runs f until f returns `Finished. The first call to f happens immediately when repeat_until_finished is called.

val forever : 'state -> ('state -> 'state t) -> unit

forever initial_state f repeatedly runs f, supplying the state returned to the next call to f.

val ok : 'a t -> ('a, _) Core.Result.t t

Useful for lifting values from the Deferred.t monad to the Result.t Deferred.t monad.

Deferred collections

These contain operations for iterating in a deferred manner over different collection types.

module Array : sig ... end
module List : sig ... end
module Map : sig ... end
module Memo : sig ... end
module Queue : sig ... end

All Deferred_queue iteration functions first copy the queue (to a list) and then start calling the user function f. So, if f modifies the queue, that will have no effect on the iteration.

module Sequence : sig ... end
module Set : sig ... end

Error-carrying deferreds

These contain interfaces for working with deferred type containing error-aware types, like 'a Option.t Deferred.t, or 'a Or_error.t Deferred.t. These all include support for monadic programming.

module Option : sig ... end
module Or_error : sig ... end

The deferred analog of Core.Or_error. It is exposed in as Deferred.Or_error.

module Result : sig ... end
module For_tests : sig ... end

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