I was asking this the other day, here's what I found out.

I wanted to know how to use Math::Pari to declare and then use a new function written in GP. After a brief exchange with the author, I discovered that the only problem was that Math::Pari hiccups on braces and spaces - and these are in any case not needed (at least for dealing with numbers), they are pure "syntactic sugar".

So I knocked up a local function:

use Math::Pari qw/ PARI /;
sub PARIdo {
    my $s = shift;
    $s =~ s/[{}\s]+//g;
    PARI($s);
}
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.. and was then able to use it to write nice clean GP function declarations:

PARIdo(q{
  mygcd(a, b) = {
    if (a > b, {
      mygcd(b, a);
    }, {
      if (a == 0, {
        b;
      }, {
        mygcd(b % a, a);
      });
    });
  }
});
print PARI("mygcd(12,8)");
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I don't yet have a good solution for passing arguments in. You can interpolate them:

my $v1 = PARI(12);
my $v2 = PARI(8);
print PARI(qq{mygcd($v1,$v2)});
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.. but it is distressingly inefficient to convert large numbers to string and back. I've done some experiments with setting up a PARIvar, but haven't worked out yet if it is possible to assign to such a variable from perl space.

I'd welcome any other tips and tricks for getting the most out of perl/PARI hybrid code.

Hugo