末尾呼び出し最適化

tailcall.js

// 参考
// 『Java による関数型プログラミング』7章

let TailRec = {
    done: function(result) {
        return {
            complete: true,
            result: result
        };
    },
    call: function(next) {
        return {
            complete: false,
            next: next
        }
    },
    calculate: function(recursive) {
        var iterator = recursive;
        while (true) {
            if (iterator.complete)
                return iterator.result;
            else
                iterator = iterator.next();
        }
    }
};

let factorial = function(n) {
    let fact = function(x, n) {
        if (n == 1)
            return TailRec.done(x);
        else 
            return TailRec.call(() => fact(x * n, n - 1));
    };
    return TailRec.calculate(fact(1, n));
};