gistfile1.txt

sub leftrect(&f, $a, $b, $n) {
   my $h = ($b - $a) / $n;
   my $sum = [+] do f($_) for $a, *+$h ... $b-$h;    # gets prior def of &f somehow
#  my $sum = [+]   (f($_) for $a, *+$h ... $b-$h);   # works
   $h * $sum;
}
 
sub tryem(&f, $a, $b, $n, $exact) {
    say '              exact result: ', $exact;
    say '     rectangle method left: ', leftrect  &f, $a, $b, $n;
}
 
tryem { $_ ** 3 }, 0, 1, 100, 0.25;
 
tryem { 1 / $_ }, 1, 100, 100, log(100);