[plt-scheme] Creating a suspended thread

Marc,

local comment: I don't understand why you lock the done-mutex twice.

On Apr 13, 2009, at 4:39 PM, Marc Feeley wrote:

>     (define (amb thunk1 thunk2)
>       (let ((result #f)
>             (result-mutex (make-mutex))
>             (done-mutex (make-mutex)))
>         (letrec ((child1
>                   (make-thread
>                     (lambda ()
>                       (let ((x (thunk1)))
>                         (mutex-lock! result-mutex #f #f)
>                         (set! result x)
>                         (thread-terminate! child2)
>                         (mutex-unlock! done-mutex)))))
>                  (child2
>                   (make-thread
>                     (lambda ()
>                       (let ((x (thunk2)))
>                         (mutex-lock! result-mutex #f #f)
>                         (set! result x)
>                         (thread-terminate! child1)
>                         (mutex-unlock! done-mutex))))))
>           (mutex-lock! done-mutex #f #f)
>           (thread-start! child1)
>           (thread-start! child2)
>           (mutex-lock! done-mutex #f #f)
>           result)))
>
> Delayed activation is essential for creating mutually recursive  
> threads.

global comment: I'd program this kind of situation with cml-style  
sync primitives (available in PLT Scheme) not with mutexes and an  
imperative style.

-- Matthias