Higher-Order Parsing

Token parsers

The next simplest parser looks for a particular token:

        sub lookfor_PLUS {
          my $tokens = shift;
          my $tok = first($tokens);
          if ($tok->type eq "+") {
            return ("+", rest($tokens));
          } else {
            return;           # failure
          }
        }

        sub lookfor_NUMBER {
          my $tokens = shift;
          my $tok = first($tokens);
          if ($tok->type eq "NUMBER") {
            return ($tok->value, rest($tokens));
          } else {
            return;           # failure
          }
        }

Note that the "value" returned by lookfor_NUMBER is the value of the number token it finds