計算機科学のブログ

Building Abstractions with Data - Symbolic Data - Example: Representing Sets - Sets as ordered lists - adjoin set

Structure and Interpretation of Computer Programs: JavaScript Edition(Harold Abelson(著)、Gerald Jay Sussman(著)、Julie Sussman(著)、The MIT Press)のChapter 2(Building Abstractions with Data)、2.3(Symbolic Data)、2.3.3(Example: Representing Sets)、Sets as ordered lists、Exercise 2.61の解答を求めてみる。

コード

function stringfy(x) {
    if (is_null(x)) {
        return "null";
    }
    if (is_pair(x)) {
        return "[" + stringfy(head(x)) + ", " + stringfy(tail(x)) + "]";
    }
    if (x === undefined) {
        return "undefined";
    }
    return x.toString();
}
function display(x) {
    return console.log(stringfy(x));
}
function pair(x, y) {
    return [x, y];
}
function head(z) {
    return z[0];
}
function tail(z) {
    return z[1];
}
function is_pair(x) {
    return Array.isArray(x);
}
function list(...args) {
    return args.length === 0 ?
        null :
        pair(args[0], list(...args.slice(1)));
}
function is_null(x) {
    return x === null;
}
function adjoin_set(x, set) {
    if (is_null(set)) {
        return pair(x, set);
    }
    const y = head(set);
    return x < y
        ? pair(x, set)
        : x === y
            ? set
            : pair(y, adjoin_set(x, tail(set)));
}
const set = list(1, 3, 6, 10);
function iter(n) {
    function f(m) {
        if (m <= n) {
            display(m);
            display(adjoin_set(m, set));
            f(m + 1);
        }
    }
    f(0);
}
display(set);
iter(11);

入出力結果(Terminal, Zsh)

% node answer2.61.js
[1, [3, [6, [10, null]]]]
0
[0, [1, [3, [6, [10, null]]]]]
1
[1, [3, [6, [10, null]]]]
2
[1, [2, [3, [6, [10, null]]]]]
3
[1, [3, [6, [10, null]]]]
4
[1, [3, [4, [6, [10, null]]]]]
5
[1, [3, [5, [6, [10, null]]]]]
6
[1, [3, [6, [10, null]]]]
7
[1, [3, [6, [7, [10, null]]]]]
8
[1, [3, [6, [8, [10, null]]]]]
9
[1, [3, [6, [9, [10, null]]]]]
10
[1, [3, [6, [10, null]]]]
11
[1, [3, [6, [10, [11, null]]]]]
%