compose

The (a-sync compose) library file provides a compose-a-sync macro interface for the a-sync procedure provided by the (a-sync coroutines) library file.

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(compose-a-sync [loop] ((var await-exp0) ...) await-exp1 await-exp2 ...)

This library file provides the compose-a-sync macro. This does two things: first, it calls a-sync for you and deals with the resulting 'await' and 'resume' procedures without exposing them, which for many simple uses makes a-sync easier to use; and secondly it enables asynchronous tasks to be more easily composed on an event loop with intermediate results, by using a let* type syntax (in fact, let* is used internally).

The 'loop' argument of compose-a-sync is optional. If an event loop constructed by make-event-loop is passed to 'loop', then that is the main loop on which the tasks will be composed, otherwise if there is no 'loop' argument given, the default main loop will be used. This is followed by bindings which are optional (there need not be any), each of which must be initialised by an expression comprising the application of a 'compose-a-sync'-capable procedure, and following the bindings there must be a body of 'compose-a-sync'-capable procedures executed solely for the purpose of asynchronous side effects (this macro does not, and cannot, return a value because as soon as the first await is made control is passed to the event loop). As in the case of let*, unlike the bindings the body cannot be empty - there must be at least one expression comprising the application of a 'compose-a-sync'-capable procedure in the body.

As in the case of let*, each 'compose-a-sync'-capable procedure initializing a binding can see the values of the initializations made prior to it. Unlike let*, a compose-a-sync block cannot be nested within another compose-a-sync block unless the nested block is placed within a 'no-await' expression or is within a callback or other procedure. Furthermore, within a compose-a-sync block, the result obtained from a 'compose-a-sync'-capable procedure cannot be passed directly as an argument to another 'compose-a-sync'-capable procedure: the intermediate result must be stored as the value of a binding in the compose-a-sync block.

A 'compose-a-sync'-capable procedure is one which takes an 'await' and 'resume' procedure from a-sync as its first and second arguments, and (if the optional 'loop' argument of this macro is used) takes the event loop as its third argument, followed by such further arguments as it requires. All of the await-task!, await-task-in-thread!, await-task-in-event-loop!, await-yield!, await-generator!, await-generator-in-thread!, await-generator-in-event-loop!, await-timeout!, await-sleep!, await-getline!, await-geteveryline!, await-getsomelines!, await-getblock!, await-geteveryblock!, await-getsomeblocks!, await-put-bytevector!, await-put-string!, await-task-in-thread-pool! and await-generator-in-thread-pool! procedures provided by the (a-sync event-loop) and (a-sync thread-pool) library files are 'compose-a-sync'-capable. In addition, to make an ordinary body of code which does not block (and which does not need to invoke a-sync's await procedure) usable by compose-a-sync, the no-await macro can be used to generate a 'compose-a-sync'-capable procedure for it (see below).

Each binding is initialized as if sequentially (although it is done asynchronously on the relevant event loop). An initialization does not begin until an earlier one has completed. In addition, each clause in the body is executed sequentially in turn, but does so asynchronously on the event loop using 'await' semantics.

When calling a 'compose-a-sync'-capable procedure within a 'compose-a-sync' block (including when initializing its bindings), the 'await' and 'yield' and event-loop arguments are not explicitly passed to it. The compose-a-sync macro will do it for you.

(set-default-event-loop!) ;; if none has yet been set
(compose-a-sync ([line (await-getline! (open-input-file "/dev/tty"))])
              ((no-await (format #t
                                 "The line was: ~A\n"
                                 line))))
(event-loop-run!)

The meeting-send and meeting-receive procedures in the (a-sync meeting) library file also meet the 'compose-a-sync'-capable requirements. Here is an example using them:

(set-default-event-loop!) ;; if none has yet been set
(define m1 (make-meeting))
(compose-a-sync ((datum (meeting-receive m1)))
                ((no-await (display datum)(newline))))
(compose-a-sync ()
                (meeting-send m1 100))
(event-loop-run!)

Each block of code within a compose-a-sync block will run independently of (and concurrently with) code in other compose-a-sync blocks. Asynchronous operations are only serialized within any one compose-a-sync block. As soon as any code calls a-sync's 'await' procedure in a compose-a-sync block, compose-a-sync will return and begin executing whatever follows it, and further execution of the compose-a-sync block will occur within the event loop concerned.

Other examples of the use of this macro are given in the documentation of the (a-sync coroutines) library file.

This macro must (like the a-sync procedure) be called in the same thread as that in which the event loop runs.

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(no-await body0 body1 ...)

This macro will generate a 'compose-a-sync'-capable procedure from a body of code which does not block. It can be passed to compose-a-sync, either for use as an initializer or as a clause in its body. When used as an initializer, it evaluates to the value of the last expression in the 'no-await' body.

If the body throws an exception which is not caught locally, it will propagate out of event-loop-run!.

Here is an example of the use of no-await:

(var ((no-await (+ a b))))