Optional items

• Many rules are naturally expressed in terms of "optional" items

• Instead of:

     term → factor ("*" term | nothing)

• We might want to say something like:

     term → factor optional("*" term)

• We can define optional quite easily:

        sub optional {
          my $p = shift;
          return alt($p, $nothing);
        }

• Now this:

     $term = $Factor - (L("*") - $Term | $nothing);

• Becomes this:

     $term = $Factor - optional(L("*") - $Term);

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Copyright © 2007 M. J. Dominus