Re: [9fans] Concurrency and message passing with Newsqueak

["David Leimbach" <leimy2k@gmail.com>]

On 5/22/07, Bakul Shah <bakul+plan9@bitblocks.com> wrote:
> > > http://en.literateprograms.org/Sieve_of_Eratosthenes_%28Haskell%29
>
> > I'm not sure that's even valid Haskell though.   Looks like two type
> > definitions for the same function, and even if you correct that I
> > don't think it produces what one might think it does.  At least not on
> > GHC :-)
>
> The second type definition on the referred page is not needed
> (actually you don't need either defn).  The function works
> but you have to give it [2..], all natural numbers >= 2.  The
> prime sieve definition I like most is this:
>
>     sieve (a:xs) = a:sieve [x | x<- xs, x `mod` a > 0]
>     primes = sieve [2..]
>
> Now you can just do
>
>         100 `take` primes
>
> to get first 100 primes.
>
> A stream such as primes will (and must) return the same Nth
> value every time so after computing a value it will be cached
> for later.  A stream in effect has storage proportional to
> the number of cdr (i.e. tail) operations done on it.  A
> channel on the other hand needs fixed minimum storage to
> affect one value transfer and it has no memory of what was
> already transferred.
>

The channel itself has no memory of what was transferred, but I can't
think of a good reason one couldn't also memoize the remainder
computation needed such that if the spawned process/thread were to
stay alive for multiple runs, one could avoid re-computing the result,
which is what Haskell is doing, except that it's also probably caching
the results of "take" applied to "primes" in your case :-)