From mboxrd@z Thu Jan  1 00:00:00 1970
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] Concurrency and message passing with Newsqueak 
In-reply-to: Your message of "Tue, 22 May 2007 11:32:35 PDT."
	<3e1162e60705221132p529b0ec6qf802e84374577cec@mail.gmail.com> 
From: Bakul Shah <bakul+plan9@bitblocks.com>
Date: Tue, 22 May 2007 16:34:40 -0700
Message-Id: <20070522233441.068EE5B3E@mail.bitblocks.com>
Topicbox-Message-UUID: 709c0cfc-ead2-11e9-9d60-3106f5b1d025

> > 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.