Higher-Order Parsing

トークンのパーサToken parsers

次に簡単なのは特定のトークンを探すパーサだ
The next simplest is a parser that looks for a particular token:

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

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

lookfor_NUMBERの返り値は見つかった数字トークンの値なので要注意
Note that the "value" returned by lookfor_NUMBER is the value of the number token it finds