python延迟执行

runrig suggested Mark Dominus出色的Higher Order Perl技术.使用HOP免费提供的样品代码Stream module,Eratosthenes的筛子是

#! /usr/bin/env perl

use strict;
use warnings;

use Stream qw/ filter head node promise show tail upfrom /;

use subs 'sieve';  # no parens on recursive calls
sub sieve {
  my($s) = @_;
  my $n = head $s;
  node $n,promise { sieve filter { $_[0] % $n != 0 } tail $s };
}

sub primes { sieve upfrom 2 }

show primes,10;

输出：

$./primes
2 3 5 7 11 13 17 19 23 29

Python

从gist by alexbowe借用代码,使用流的Python筛选

#! /usr/bin/env python

null_stream = (None,None)

def reduce(f,result,stream):
    if stream is null_stream: return result
    return reduce(f,f(result,head(stream)),tail(stream))

def take(N,stream):
    if N <= 0 or stream is null_stream: return null_stream
    return (head(stream),lambda: take(N-1,tail(stream)))

def filter(pred,stream):
    if stream is null_stream: return null_stream
    if pred(head(stream)):
        return (head(stream),lambda: filter(pred,tail(stream)))
    return filter(pred,tail(stream))

def integers_from(N): return (N,lambda: integers_from(N+1))
def head((H,_)): return H
def tail((_,T)): return T()
def to_array(stream): return reduce(lambda a,x: a + [x],[],stream)

def sieve(stream):
    if stream is null_stream: return null_stream
    h = head(stream)
    return (h,lambda: sieve(filter(lambda x: x%h != 0,tail(stream))))

def primes(): return sieve(integers_from(2))

print to_array(take(10,primes()))

输出：

$./prymes 
[2,3,5,7,11,13,17,19,23,29]

其他可能性

在某些语言中,流模式是不可见的.例如,延迟评估是Haskell的一个特性,因此您可以将质数定义为

primes = sieve [2 ..]
  where sieve (x:xs) =
          let remains = filter (not . isMultipleOf x) xs
          in x : sieve remains
        isMultipleOf a b = b `mod` a == 0