Re: feature request from r5rs

1st caveat I agree with but the other two caveats are also true for

(+ (force (delay (* 3 7)) 13)      ===> 34

Matthias Radestock <[email protected]> writes:

> Narang wrote:
> > From r5rs: * Some implementations may implement ``implicit forcing,''
> > where the value of a promise is forced by primitive procedures like
> > cdr and +:
> > (+ (delay (* 3 7)) 13)          ===>  34
> > Can this feature be added to sisc? thoughts.
> 
> It wouldn't be hard to add, with three caveats:
> 
> * preventing it from slowing down *everything* is difficult,
> 
> * synchronization is required in order to implement the correct
> semantics in the presence of multiple threads,
> 
> * care needs to be taken to ensure the correct call/cc semantics is
> implemented.
> 
> R5RS doesn't say anything explictly about the last point. However, it
> is clear from the examples that capturing a k inside a delayed
> expression and subsequently invoking it will
> 
> * (partially) re-evaluate the delayed expression and re-set the
> memoized value,
> 
> * resume computation at the point where the force (explicit or
> implicit) occured.
> 
> 
> Matthias.



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