パーサ$expressionはこうParsers

• Here's $expression:

     # expression → "(" expression ")" 
     #             | term ("+" expression | nothing)
     
     $expression = alt(conc(lookfor("(")),
                            $EXPRESSION,
                            lookfor(")"),
     
                       conc($TERM,
                            alt(conc(lookfor("+"), $EXPRESSION),
                                \&nothing));

• だからどうしたという感じもするけれど

  This doesn't look great, but:

1. してくれる作業の量を思うと、おどろくほど短くなっているし

   When you consider how much it's doing, it's amazingly brief, and

2. 演算子のオーバーロードを使うとこんな風にも書き直せる

   We can use operator overloading and rewrite it as:

      $expression = L("(") - $EXPRESSION - L(")")
                  | $TERM - (L("+") - $EXPRESSION | $nothing);

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Copyright © 2007 M. J. Dominus