In Re: [OT] How do I really learn javascript?, dragonchild remarks, "A lot of programmers have a program they always write to learn a new language." Upon reading this, I realized that I always write a prime number generator. Here's one I wrote in Perl and posted in Re: ulam's spiral too slow:

use strict;
use warnings;

my @primes = ( 2, 3 );
my $candidate = $primes[-1] + 2;
my $max_prime = shift || 100_000;

CANDIDATE:
while ( $candidate < $max_prime ) {
    my $sqrt_candidate = sqrt $candidate;
  PRIME:
    foreach my $prime ( @primes ) {
        last PRIME     if $prime > $sqrt_candidate;
        next CANDIDATE if 0 == $candidate % $prime;
    }
    push @primes, $candidate;
}
continue {
    $candidate += 2;
}
print join q{,}, @primes;
print "\n";
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I've written programs like this in Python, C, BASIC, and others. I wrote one in C when learning to use an arbitrary precision math library once. It's a fairly straight forward math problem, and I can tell if the output is correct without any test framework. It requires looping, and I can expand it to use subroutines when it's time to learn that.

I recall another programmer saying he always writes a hex dump program in a new language. Of course, there's also the ever-popular Hello world program.

I'd love to hear from the monks. What program do you write in a new language, and why?