11.2.4 Buffered Asynchronous Channels

package: base

The bindings documented in this section are provided by the racket/async-channel library, not racket/base or racket.

11.2.4.1 Creating and Using Asynchronous Channels

See also Thread Mailboxes.

An asynchronous channel is like a channel, but it buffers values so that a send operation does not wait on a receive operation.

In addition to its use with procedures that are specific to asynchronous channels, an asynchronous channel can be used as a synchronizable event (see Events). An asynchronous channel is ready for synchronization when async-channel-get would not block; the asynchronous channel’s synchronization result is the same as the async-channel-get result.

procedure
(async-channel? v) → boolean?
v : any/c

Returns #t if v is an asynchronous channel, #f otherwise.

procedure
(make-async-channel [limit]) → async-channel?
limit : (or/c exact-positive-integer? #f) = #f

Returns an asynchronous channel with a buffer limit of limit items. A get operation blocks when the channel is empty, and a put operation blocks when the channel has limit items already. If limit is #f, the channel buffer has no limit (so a put never blocks).

procedure
(async-channel-get ach) → any/c
ach : async-channel?

Blocks until at least one value is available in ach, and then returns the first of the values that were put into async-channel.

procedure
(async-channel-try-get ach) → any/c
ach : async-channel?

If at least one value is immediately available in ach, returns the first of the values that were put into ach. If async-channel is empty, the result is #f.

procedure
(async-channel-put ach v) → void?
ach : async-channel?
v : any/c

Puts v into ach, blocking if ach’s buffer is full until space is available.

procedure
(async-channel-put-evt ach v) → evt?
ach : async-channel?
v : any/c

Returns a synchronizable event that is ready for synchronization when (async-channel-put ach v) would return a value (i.e., when the channel holds fewer values already than its limit); the synchronization result of a asynchronous channel-put event is the asynchronous channel-put event itself.

Examples:

(define (server input-channel output-channel)
  (thread (lambda ()
            (define (get)
              (async-channel-get input-channel))
            (define (put x)
              (async-channel-put output-channel x))
            (define (do-large-computation)
              (sqrt 9))
            (let loop ([data (get)])
              (case data
                [(quit) (void)]
                [(add) (begin
                         (put (+ 1 (get)))
                         (loop (get)))]
                [(long) (begin
                          (put (do-large-computation))
                          (loop (get)))])))))
(define to-server (make-async-channel))
(define from-server (make-async-channel))

> (server to-server from-server)
#<thread>
> (async-channel? to-server)
#t
> (printf "Adding 1 to 4\n")
Adding 1 to 4
> (async-channel-put to-server 'add)
> (async-channel-put to-server 4)
> (printf "Result is ~a\n" (async-channel-get from-server))
Result is 5
> (printf "Ask server to do a long computation\n")
Ask server to do a long computation
> (async-channel-put to-server 'long)
> (printf "I can do other stuff\n")
I can do other stuff
> (printf "Ok, computation from server is ~a\n"
          (async-channel-get from-server))
Ok, computation from server is 3
> (async-channel-put to-server 'quit)

11.2.4.2 Contracts and Impersonators on Asynchronous Channels

procedure
(async-channel/c c) → contract?
c : contract?

Returns a contract that recognizes asynchronous channels. Values put into or retrieved from the channel must match c.

If the c argument is a flat contract or a chaperone contract, then the result will be a chaperone contract. Otherwise, the result will be an impersonator contract.

When an async-channel/c contract is applied to an asynchronous channel, the result is not eq? to the input. The result will be either a chaperone or impersonator of the input depending on the type of contract.

procedure
(impersonate-async-channel channel
get-proc
put-proc
prop
prop-val ...
...)
→ (and/c async-channel? impersonator?)
  channel : async-channel?
  get-proc : (any/c . -> . any/c)
  put-proc : (any/c . -> . any/c)
  prop : impersonator-property?
  prop-val : any

Returns an impersonator of channel, which redirects the async-channel-get and async-channel-put operations.

The get-proc must accept the value that async-channel-get produces on channel; it must produce a replacement value, which is the result of the get operation on the impersonator.

The put-proc must accept the value passed to async-channel-put called on channel; it must produce a replacement value, which is the value passed to the put procedure called on the original channel.

The get-proc and put-proc procedures are called for all operations that get or put values from the channel, not just async-channel-get and async-channel-put.

Pairs of prop and prop-val (the number of arguments to impersonate-async-channel must be odd) add impersonator properties or override impersonator property values of channel.

procedure
(chaperone-async-channel channel
get-proc
put-proc
prop
prop-val ...
...)
→ (and/c async-channel? chaperone?)
  channel : async-channel?
  get-proc : (any/c . -> . any/c)
  put-proc : (any/c . -> . any/c)
  prop : impersonator-property?
  prop-val : any

Like impersonate-async-channel, but the get-proc procedure must produce the same value or a chaperone of the original value, and put-proc must produce the same value or a chaperone of the original value.

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1	Language Model
2	Notation for Documentation
3	Syntactic Forms
4	Datatypes
5	Structures
6	Classes and Objects
7	Units
8	Contracts
9	Pattern Matching
10	Control Flow
11	Concurrency and Parallelism
12	Macros
13	Input and Output
14	Reflection and Security
15	Operating System
16	Memory Management
17	Unsafe Operations
18	Running Racket
	Bibliography
	Index

11.1	Threads
11.2	Synchronization
11.3	Thread-Local Storage
11.4	Futures
11.5	Places
11.6	Engines
11.7	Machine Memory Order

11.2.1	Events
11.2.2	Channels
11.2.3	Semaphores
11.2.4	Buffered Asynchronous Channels

11.2.4.1	Creating and Using Asynchronous Channels
11.2.4.2	Contracts and Impersonators on Asynchronous Channels