[plt-scheme] newbie question: returning values from threads

You're missing some syncronization. The lookup of result is happening
before the assignment. Rather than trying to use set! like that, it's
probably better to do this:

(define (thread/result thnk)
  (let ([c (make-channel)])
    (thread (lambda () (channel-put c (thnk))))
    c))

Then, when you want to get the values from the threads, use something
like this:

  (define chans (map get-host hosts))
  (define hosts (map channel-get hosts))

where get-host uses thread/result instead of thread.

hth,
Robby

At Tue, 15 Jun 2004 16:15:16 -0700 (PDT), Kirk Miller wrote:
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> I am trying to access multiple xml-rpc servers in
> parallel since each call takes a long time. I don't 
> know how to propagate the answer back out of a thread.
> I make a call starting a thread for each host.
> Then I wait for the thread to join and return a
> list of strings. But in this case "result" is always
> '(). I think its a synchronization thing. In previous
> attempts I have pre-created a list
> of outports to hold displayed data, but I want to know
> how to get a list back instead of having to read
> ports. (the program should be more generally useful)
> see
> http://www.cs.utah.edu/plt/mailarch/plt-scheme-2000/msg00137.html
> 
>   (define result '())
>   
>   (define (get-host host) 
>     ; in= string: host
>     ; out=(cons (tid los))
>   
>       (cons
>        (thread
>         (lambda ()
>           (let (
>                 (server (xml-rpc-server host 8000
> "/servlets/dpmon-server")))
>             (set! result (cons host
>                                ((server 'sm50)))))))
>        result)
>       )
> 
>   (define (sm66 hostlist)
>     (let ((tid-los
>            (map 
>             (lambda (h)
>               (get-host h))
>             hostlist)))
>       (map
>        (lambda (t)
>          (thread-wait (car t))
>          (cdr t))
>        tid-los)))
> 
> 
> 
> 
> 
> 		
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