Higher-Order Parsing

Token parsers

In functional languages, we needn't write write 9 similar lookfor functions
Instead, we can have another function build them as required:

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

Now instead of lookfor_PLUS we just use lookfor("+")
Instead of lookfor_NUMBER we just use lookfor("NUMBER")