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Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes)

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Am 05.06.2013 22:43, schrieb Arved Sandstrom:
> On 06/05/2013 04:44 AM, Sebastian wrote:
[snip]
>>
> Me I wouldn't re-invent the wheel in this case. Implementing this type
> of laziness is not that straightforward.
>
> And why not use interop between JVM languages? This way you leverage the
> strengths. I usually have Java as the main language, but for some
> functionality in some apps I use Scala. Maintainability goes up, for
> starters, as does reliability and speed of development.
>
> It's all ultimately JVM bytecode.
>
> AHS
>

It's not everyone has a management who sees it this way. In our large
project (we're talking several hundred developers and a really
complicated build process), I don't see anyone advocating more
complexity by adding another compiler and language.

Anyway, it's turned out not to be too complicated in straight Java.
One can simply extend a straightforward implementation of the usual
cons-list with the following features, giving one a
FilterableConsList<T>:
a) accept a Function<T> to generate the elements in the tail
    in sequence
b) accept a Supplier<FilterableConsList<T>> to supply a tail
c) accept a Predicate<T> to filter out non-matching elements
    from the tail
and make the implementation class LazyConsList<T> lazy for the tail
(but eager for the head).

It's also easy to implement AbstractCollection by providing an iterator
and make the thing streamable by providing a spliterator of unknown
size.

With that, the driver for generating the first 100 primes becomes:

LazyConsList<Integer> s = sieve(LazyConsList.newList(2, x -> x + 1));
s.stream().limit(100).forEach(x->System.out.print(x + " "));

where the interesting sieve() method is defined like this:

private LazyConsList<Integer> sieve(FilterableConsList<Integer> s) {
   Integer p = s.head();
   return cons(p, () -> sieve(s.tail().filter(x -> x % p != 0)));
}

Now I think that looks pretty neat (and almost exactly like the
corresponding Scala code.)

-- Sebastian

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Java 8 Streams and Eratosthenes Sebastian <news@seyweiler.dyndns.org> - 2013-06-04 23:54 +0200
  Re: Java 8 Streams and Eratosthenes Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-06-04 19:35 -0300
    Re: Java 8 Streams and Eratosthenes Sebastian <news@seyweiler.dyndns.org> - 2013-06-05 09:03 +0200
      Re: Java 8 Streams and Eratosthenes Sebastian <news@seyweiler.dyndns.org> - 2013-06-05 09:44 +0200
        Re: Java 8 Streams and Eratosthenes Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-06-05 17:43 -0300
          Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-10 01:27 +0200
            Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-06-09 22:28 -0300
              Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-10 19:43 +0200
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) markspace <markspace@nospam.nospam> - 2013-06-10 13:18 -0700
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-11 23:27 +0200
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-12 10:32 +0200
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-12 14:37 +0200
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) markspace <markspace@nospam.nospam> - 2013-06-12 12:05 -0700
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-12 22:53 +0200
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) markspace <markspace@nospam.nospam> - 2013-06-15 15:18 -0700
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-16 22:52 +0200
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-06-16 21:56 -0300
                Re: Lazy primes with Java 8 (was Re: Java 8 Streams and Eratosthenes) Sebastian <news@seyweiler.dyndns.org> - 2013-06-18 23:18 +0200
  Re: Java 8 Streams and Eratosthenes markspace <markspace@nospam.nospam> - 2013-06-04 17:24 -0700
    Re: Java 8 Streams and Eratosthenes Sebastian <news@seyweiler.dyndns.org> - 2013-06-05 09:09 +0200
      Re: Java 8 Streams and Eratosthenes markspace <markspace@nospam.nospam> - 2013-06-05 09:45 -0700
        Re: Java 8 Streams and Eratosthenes Sebastian <news@seyweiler.dyndns.org> - 2013-06-05 21:32 +0200
    Re: Java 8 Streams and Eratosthenes Sebastian <news@seyweiler.dyndns.org> - 2013-06-05 09:47 +0200

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