Module Deferred

module Deferred: Deferred

type 'a t = ('a, Execution_context.t) Raw_deferred.t 
A deferred is 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.
val sexp_of_t : ('a -> Sexplib.Sexp.t) -> 'a t -> Sexplib.Sexp.t
type 'a detailed = 'a t 
val sexp_of_detailed : ('a -> Sexplib.Sexp.t) -> 'a detailed -> Sexplib.Sexp.t

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 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 t.
val is_determined : 'a t -> bool
is_determined t returns true iff t is determined.
include Monad
Deferreds form a monad.

t >>= f returns a deferred t' that waits until t is determined to have 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 (t >>= (fun a -> f a; Deferred.unit))

because upon, unlike >>= 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 () ... )

The same loop written with >>= 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 >>= 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.

module Infix: sig .. end
val unit : unit t
unit is a deferred that is always determined with value ()
val never : unit -> 'a 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
module Array: Deferred_sequence  with type 'a t = 'a array
module List: Deferred_sequence  with type 'a t = 'a list
module Queue: Deferred_sequence  with type 'a t = 'a Queue.t
module Map: Deferred_map  with type ('k, 'v) t = ('k, 'v) Map.Poly.t
val whenever : unit t -> unit
whenever t ignores t completely. It is like Fn.ignore, but is more constrained because it requires a deferred.

If you want to ignore t : 'a t, then do whenever (Deferred.ignore t).

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

type 'a choice 
choice is used to produce an argument to enabled or choose. See below.
val choice : 'a t -> ('a -> 'b) -> 'b choice
val enabled : 'b choice list -> (unit -> 'b list) t
enabled [choice t1 f1; ... choice tn fn;] returns a deferred d that becomes determined when any of the ti become 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 list -> 'b t
choose choices is enabled choices >>| (fun f -> List.hd_exn (f ())). That is:

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. 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, if you write

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

It may be the case that both d1 and d2 become determined, yet the code e2 actually runs.

val choose_ident : 'a t list -> 'a t
choose_ident l = choose ( l ~f:(fun t -> choice t ident)). That is, choose_ident is a choice among deferreds of the same type.
val repeat_until_finished : 'state ->
('state -> [ `Finished of 'result | `Repeat of 'state ] 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 debug_space_leaks : int option Pervasives.ref
Set debug_space_leaks to Some n to trigger assertion failures when single deferred has more than n handlers waiting for it to be filled. Note that if n is less than 2, we may not trigger all assertion failures.