Module Bus

A Bus is a publisher/subscriber system within the memory space of the program. A bus has a mutable set of subscribers, which can be modified using subscribe_exn and unsubscribe.

create returns a Bus.Read_write.t, which you can use to write values to the bus. write calls the callbacks of all current subscribers before returning.

In a ('callback, 'phantom) Bus.t, 'phantom is a read-write phantom type that controls whether one can read values from or write values to the bus. The phantom type states the capabilities one could ever have access to, not the capabilities that are immediately available. In particular, if one wants to subscribe to a Bus.Read_write.t, one must call read_only on it in order to get a Bus.Read_only.t that can be passed to subscribe_exn. This is deliberate, and is meant to avoid unintentional reads from code that should only be writing.

module Callback_arity : sig ... end

Callback_arity states the type of callbacks stored in a bus. Using Callback_arity is an implementation technique that allows callbacks to be defined as ordinary n-ary curried functions (e.g., a1 -> a2 -> a3 -> r), instead of forcing n-ary-variadic callbacks to use tuples (e.g., a1 * a2 * a3 -> r). This also avoids extra allocation.

type ('callback, 'phantom) t
val sexp_of_t : ('callback -> Ppx_sexp_conv_lib.Sexp.t) -> ('phantom -> Ppx_sexp_conv_lib.Sexp.t) -> ('callback, 'phantom) t -> Ppx_sexp_conv_lib.Sexp.t
type ('callback, 'phantom) bus = ('callback, 'phantom) t
module Read_write : sig ... end
module Read_only : sig ... end
module On_subscription_after_first_write : sig ... end
val read_only : ('callback, _) t -> 'callback Read_only.t
val create : ?⁠name:Core_kernel.Info.t -> Core_kernel.Source_code_position.t -> 'callback Callback_arity.t -> on_subscription_after_first_write:On_subscription_after_first_write.t -> on_callback_raise:(Core_kernel.Error.t -> unit) -> 'callback Read_write.t

In create [%here] ArityN ~on_subscription_after_first_write ~on_callback_raise, [%here] is stored in the resulting bus, and contained in %sexp_of: t, which can help with debugging.

If on_subscription_after_first_write is Raise, then subscribe_exn will raise if it is called after write has been called the first time. If on_subscription_after_first_write is Allow_and_send_last_value, then the bus will remember the last value written and will send it to new subscribers.

If a callback raises, on_callback_raise is called with an error containing the exception.

If on_callback_raise raises, then the exception is raised to write and the bus is closed.

val callback_arity : ('callback, _) t -> 'callback Callback_arity.t
val num_subscribers : (_, _) t -> int
val is_closed : (_, _) t -> bool
val close : 'callback Read_write.t -> unit

close disallows future writes -- once close t is called, all further calls to write t will raise. close is idempotent. If close is called from within a callback, the current message will still be sent to all subscribed callbacks that have not yet seen it before the close takes effect.

val write : ('a -> unit) Read_write.t -> 'a -> unit
val write2 : ('a -> 'b -> unit) Read_write.t -> 'a -> 'b -> unit
val write3 : ('a -> 'b -> 'c -> unit) Read_write.t -> 'a -> 'b -> 'c -> unit
val write4 : ('a -> 'b -> 'c -> 'd -> unit) Read_write.t -> 'a -> 'b -> 'c -> 'd -> unit
val write5 : ('a -> 'b -> 'c -> 'd -> 'e -> unit) Read_write.t -> 'a -> 'b -> 'c -> 'd -> 'e -> unit
module Subscriber : sig ... end
val subscribe_exn : ?⁠extract_exn:bool -> ?⁠on_callback_raise:(Core_kernel.Error.t -> unit) -> ?⁠on_close:(unit -> unit) -> ('callback, [> Core_kernel.read ]) t -> Core_kernel.Source_code_position.t -> f:'callback -> 'callback Subscriber.t

subscribe_exn t [%here] ~f adds the callback f to the set of t's subscribers, and returns a Subscriber.t that can later be used to unsubscribe. [%here] is stored in the Subscriber.t, and contained in %sexp_of: Subscriber.t, which can help with debugging. If subscribe_exn t is called by a callback in t, i.e., during write t, the subscription takes effect for the next write, but does not affect the current write. subscribe_exn takes amortized constant time.

If on_callback_raise is supplied, then it will be called by write whenever f raises; only if that subsequently raises will t's on_callback_raise be called. If on_callback_raise is not supplied, then t's on_callback_raise will be called whenever f raises.

If on_callback_raise is supplied and extract_exn is set to true, then the error passed to the on_callback_raise method will contain only the exception raised by f without any additional information about the bus subscription or backtrace.

on_close is called if you are still subscribed when Bus.close is called.

val iter_exn : ('callback, [> Core_kernel.read ]) t -> Core_kernel.Source_code_position.t -> f:'callback -> unit

iter_exn t [%here] ~f is ignore (subscribe_exn t [%here] ~callback:f). This captures the common usage in which one never wants to unsubscribe from a bus.

module Fold_arity : sig ... end
val fold_exn : ('callback, [> Core_kernel.read ]) t -> Core_kernel.Source_code_position.t -> ('callback, 'f, 's) Fold_arity.t -> init:'s -> f:'f -> unit

fold_exn t [%here] arity ~init ~f folds over the bus events, threading a state value to every call. It is otherwise similar to iter_exn.

val unsubscribe : ('callback, [> Core_kernel.read ]) t -> 'callback Subscriber.t -> unit

unsubscribe t subscriber removes the callback corresponding to subscriber from t. unsubscribe never raises and is idempotent. As with subscribe_exn, unsubscribe t during write t takes effect after the current write finishes. Also like subscribe_exn, unsubscribe takes amortized constant time.