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__.Deferred1.t```
`include sig ... end`
`val sexp_of_t : ('a ‑> Base.Sexp.t) ‑> 'a t ‑> Base.Sexp.t`
`include Core_kernel.Invariant.S1 with type a t := a t`
`type 'a t`````
`val invariant : 'a Base__.Invariant_intf.inv ‑> 'a t Base__.Invariant_intf.inv`

`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_kernel.Monad with type a t := a t`
`type 'a t`````
`include Base__.Monad_intf.S_without_syntax with type a t := a t`
`type 'a t`````
`include Base__.Monad_intf.Infix with type a t := a t`
`type '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 : Base__.Monad_intf.Infix with type a t := a t`
`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 `Pervasives.ignore`. Some monads still do `let ignore = ignore_m` for historical reasons.

`val all : 'a t list ‑> 'a list t`
`val all_unit : unit t list ‑> unit t`
`val all_ignore : unit t list ‑> unit t`

- Deprecated [since 2018-02] Use [all_unit]

`include Base__.Monad_intf.Syntax with type a t := a t`
`type 'a t`````
`module Let_syntax : sig ... end`
`module Infix : sig ... end`
`val unit : unit t`

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

`val ignore : _ t ‑> unit t`
`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 `t`s 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 : 'a 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 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.

`type 'a choice``= 'a Choice.t```
`val choice : 'a t ‑> ('a ‑> 'b) ‑> 'b Choice.t`
`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 `fi`s, 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;
          done

`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_kernel.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 = Async_kernel__.Deferred_array`
`module List = Async_kernel__.Deferred_list`
`module Map = Async_kernel__.Deferred_map`
`module Memo = Async_kernel__.Deferred_memo`
`module Queue = Async_kernel__.Deferred_queue`
`module Sequence = Async_kernel__.Deferred_sequence`

### 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 = Async_kernel__.Deferred_option`
`module Or_error = Async_kernel__.Deferred_or_error`
`module Result = Async_kernel__.Deferred_result`