[plt-scheme] Could you give an example of the need of the second if in make-promise ?

If a continuation jumps back into the evaluation of the call to `p'
after that call has returned once, that test will go the other way.

Robby

At Wed, 23 Aug 2006 12:49:31 -0400, "Andre Mayers" wrote:
> The following is an extract from TSPL3
> (http://www.scheme.com/tspl3/control.html#./control:h6)
> ---
> (define-syntax delay
>   (syntax-rules ()
>     ((_ exp) (make-promise (lambda () exp))))) 
> 
> where make-promise is defined as 
> 
> (define make-promise
>   (lambda (p)
>     (let ((val #f) (set? #f))
>       (lambda ()
>         (if (not set?)
>             (let ((x (p)))
>               (if (not set?)
>                   (begin (set! val x)
>                          (set! set? #t)))))
>         val)))) 
> 
> With this definition of delay, force simply invokes the promise to force
> evaluation or to retrieve the saved value. 
> 
> (define force
>   (lambda (promise)
>     (promise))) 
> 
> The second test of the variable set? in make-promise is necessary in the
> unlikely event that, as a result of applying p, the promise is recursively
> forced. Since a promise must always return the same value, the result of the
> first application of p to complete is returned. 
> 
>  
> 
> ----
> 
> André Mayers, Ph.D., M. Ps.
> professeur agrégé
> Département d'informatique
> Université de Sherbrooke
> Sherbrooke (Québec) Canada J1K 2R1
> tél: +1 819-821-8000 poste 62041
> fax: +1 819-821-8200
> andre.mayers at usherbrooke.ca
> http://www.dmi.usherb.ca/~amayers
> 
> 
> 
> 
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