Higher-Order Parsing

`repeat`

Many rules are naturally expressed in terms of "repeated" items
For example, we might write

        # term → factor repeat( "*" factor )
        $term = $Factor - repeat(L("*") - $Factor);

It's not hard to express repeat with what we have already:

        # repeat($p) is:

        $p - repeat($p)  |  $nothing

But we can wrap this up as a function:

        sub repeat {
          my $p = shift;
          my $repeat_p;
          my $do_repeat_p = sub { $repeat_p->(@_) };  # proxy
          $repeat_p = alt(conc($p, $do_repeat_p), $nothing);
          return $repeat_p;
        }