Higher-Order Parsing

Alternation

Atoms come in three varieties, not just one:

        atom → NUMBER | VAR | function "(" expression ")"

So we need the atom parser to try these three different things
It fails only if the upcoming tokens match none of them
Something like this:

        sub atom {
          my $in = shift;
          my ($result, $out);
          my $alt3 = conc(lookfor("FUNC"), 
                          lookfor("("), $Expression,  lookfor(")"),
                         );

          if (     ($result, $out) = lookfor("NUMBER")->($in)) {
            return ($result, $out);
          } elsif (($result, $out) = lookfor("VAR")->($in)) {
            return ($result, $out);
          } elsif (($result, $out) = $alt3->($in)) {
            return ($result, $out);
          } else {
            return;
          }
        }

But again, we'd have to write a lot of code that was very similar
So instead we'll write a function that assembles small parsers into big ones
Given parser functions A, B, etc.:

        alt(A, B, ...)

Will return a parser function that looks for A or for B, etc.