Re: feature request from r5rs

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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