関数型プログラミングの基礎 P２７７

リスト７．９１、７．９２のmatchiの定義を exp.call(pattern,pattern)から、exp(pattern)にかえてみた。 これでも動くようだが、call(pattern,pattern)を使う意味がいまいちわからない。
<script> /* #@range_begin(algebraic_type_maybe) */ var maybe = { match: (exp, pattern) => { return exp(pattern); }, just: (value) => { return (pattern) => { return pattern.just(value); }; }, nothing: (_) => { return (pattern) => { return pattern.nothing(_); }; } }; /* #@range_end(algebraic_type_maybe) */ var MAYBE = { /* #@range_begin(maybe_monad) */ /* unit:: T => MAYBE[T] */ unit: (value) => { return maybe.just(value); }, /* flatMap:: MAYBE[T] => FUN[T => MAYBE[U]] => MAYBE[U] */ flatMap: (instanceM) => { return (transform) => { return maybe.match(instanceM,{ /* 正常な値の場合は、transform関数を計算する */ just: (value) => { return transform(value); }, /* エラーの場合は、何もしない */ nothing: (_) => { return maybe.nothing(); } }); }; }, /* ヘルパー関数 */ getOrElse: (instanceM) => { return (alternate) => { return maybe.match(instanceM,{ just: (value) => { return value; }, nothing: (_) => { return alternate; } }); }; }, /* #@range_end(maybe_monad) */ }; var add = (maybeA,maybeB) => { return MAYBE.flatMap(maybeA)((a) => { return MAYBE.flatMap(maybeB)((b) => { return MAYBE.unit(a + b); }); }); }; var justOne = maybe.just(1); var justTwo = maybe.just(2); alert(MAYBE.getOrElse(add(justOne,justOne))(null) ); alert(MAYBE.getOrElse(add(justOne,maybe.nothing()))(null)); </script>