python delayed execution

Question

To implement streams as delayed lists in Lisp it's recommended to use Lisp macros.

(defmacro cons-stream (a b)
   (cons ,a (delay ,b)))

(defmacro delay (expr)
  `(memo-proc (lambda () ,expr)))

What would by Python and Perl way to do the same thing?

EDIT. Is it possible to use such a cool construct as streams

(define primes (sieve (integers-starting-from 2)))

in languages like Python and Perl

Answer 1

In Python, the closest structure would probably be a generator expression.

In Perl, there is not a native lazy list, but the language provides all of the primitives that are needed to build one. I have written a lazy list library called List::Gen which is available on CPAN.

use List::Gen '*';

my $primes = <2..>->filter(\&is_prime);  # where you provide &is_prime

say "@$primes[0..10]";  # lazily finds the first 11 primes

the <2..> bit could be written verbosely as range(2, 9**9**9)