Arie van Wingerden writes:

> Hi Christopher,
> 2018-05-11 15:31 GMT+02:00 Christopher Lemmer Webber <
>> In comparison to Syndicate, Goblins is less a new language and more a
>> lightweight library for actors that interfaces nicely with "#lang
>> racket" type code
>> ​..
> For me personally Goblin would be the better choice I think.​

If you do think that's the case (again, with the warning that it's
pre-alpha), I'd encourage you to look at the goblins/demos/ directory
for some examples.  But here is the main API:

racket@> (require goblins)

;; Spawning an actor from a procedure
racket@> (define counting-actor
            (let ([counter 0])
              (lambda ()
                (set! counter (+ counter 1))
                (format "I've been called ~a times" counter)))))

;; this sends a message, but we don't get a response...
;; "fire and forget"
racket@> (<- counting-actor)

;; this sends a message, but we do wait for a response...
;; "resume the continuation with a response"
racket@> (<<- counting-actor)
"I've been called 2 times"

;; Note that doing <<- outside an actor context will block the current
;; thread until a response comes in.  Doing <<- within an actor will
;; stuff this into a queue we're "waiting on responses" from, but
;; the actor will robustly continue to respond to messages.

;; We can also spawn Racket's classes as actors fairly easily:
racket@> (define greeter%
           (class object%
             (define/public (greet name)
               (string-append "Hello, " name "!"))))
;; And we can send messages to their public methods
racket@> (<<- (spawn-new greeter%) 'greet "Bert")
"Hello, Bert!"

;; Actors are shut down when all references to them are garbage
;; collected, so we don't need to worry about doing a one-off
;; spawn as we did above.

;; Subclassing of course works too.
racket@> (define groucher%
           (class greeter%
             (init [annoyed-by "my back"])
             (define am-annoyed-by annoyed-by)
             (define/override (greet name)
               (string-append "Grumble grumble... "
                              (super greet name)
                              "... "
                              am-annoyed-by " is irritating me..."))))
racket@> (<<- (spawn-new groucher%
                         [annoyed-by "your hair"])
              'greet "Grover")
"Grumble grumble... Hello, Grover!... your hair is irritating me..."

;; Here's a more complex example that demonstrates using our own address
;; with (self)
racket@> (let* ([narrator
                  (lambda (msg)
                    (displayln msg)))]
                 (spawn-new (class object%
                              (define listeners '())
                              (define/public (subscribe address)
                                ;; We don't care about the order of this
                                ;; so fire and forget
                                (<- narrator "gossiper> Got a new subscriber!")
                                (set! listeners (cons address listeners)))
                              (define/public (spread-gossip msg)
                                (<- narrator
                                    (format "gossiper> Spreading juicy gossip: 
                                (for ([listener listeners])
                                  (<- listener 'receive-gossip msg)))))]
                 (class object%
                   (init name)
                   (define our-name name)
                   (define/public (start-listening)
                     ;; We're going to wait here because we don't want to return
                     ;; until this completes
                     ;; (self) is our own actor address and is bound dynamically
                     ;; in a message handling context
                     (<<- gossiper 'subscribe (self)))
                   (define/public (receive-gossip msg)
                     (<- narrator
                         (format "~a> Ooh, I just heard that ~a"
                                 our-name msg))))]
                [sam (spawn-new listener% [name "sam"])]
                [pat (spawn-new listener% [name "pat"])])
           (<<- sam 'start-listening)
           (<<- gossiper 'spread-gossip "Tim has great hair")
           (<<- pat 'start-listening)
           (<<- gossiper 'spread-gossip "Samantha has a nice car"))
gossiper> Got a new subscriber!
gossiper> Spreading juicy gossip: Tim has great hair
sam> Ooh, I just heard that Tim has great hair
gossiper> Got a new subscriber!
gossiper> Spreading juicy gossip: Samantha has a nice car
pat> Ooh, I just heard that Samantha has a nice car
racket@> sam> Ooh, I just heard that Samantha has a nice car

;; In fact you can see on that last one that since talking to the
;; gossiper is asynchronous with <-, it clobbered my racket prompt ;)

====== End tutorial ======

Hope that was interesting!
As you can see, it tries very hard to look a lot like racket code.
I used to have addresses even be callable as if
(address args ...) was implicitly the same as (<<- address args)
but since Goblins actors are "robust", I didn't want people to be
surprised that their actors may respond to other messages while waiting
for replies to their continuation with <<-.
But when I had it like that it didn't even look like message passing,
just normal procedure calls, which in a way was fun :)

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