Building Abstractions with Data - Hierarchical Data and the Closure Property - Example: A Picture Language - Frames - vector, constructor, selectors, addition, subtraction, scaling

Structure and Interpretation of Computer Programs: JavaScript Edition
楽天ブックス

開発環境

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)、2.2(Hierarchical Data and the Closure Property)、2.2.4(Example: A Picture Language)、Frames、Exercise 2.46の解答を求めてみる。

コード

function stringfy(x) {
    if (x === undefined) {
        return 'undefined';
    }
    if (is_null(x)) {
        return "null";
    }
    if (is_pair(x)) {
        return "[" + stringfy(head(x)) + ", " + stringfy(tail(x)) + "]";
    }
    return x.toString();
}
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 is_null(x) {
    return x === null;
}
function display(x) {
    return console.log(stringfy(x));
}
function make_vect(x, y) {
    return pair(x, y);
}
function xcor_vect(v) {
    return head(v);
}
function ycor_vect(v) {
    return tail(v);
}
function add_vect(v1, v2) {
    return make_vect(
        xcor_vect(v1) + xcor_vect(v2),
        ycor_vect(v1) + ycor_vect(v2)
    );
}
function sub_vect(v1, v2) {
    return make_vect(
        xcor_vect(v1) - xcor_vect(v2),
        ycor_vect(v1) - ycor_vect(v2)
    );
}
function scale_vect(s, v) {
    return make_vect(
        s * xcor_vect(v),
        s * ycor_vect(v)
    );
}

const v1 = make_vect(1, 5);
const v2 = make_vect(4, 2);
display(v1);
display(v2);
display(add_vect(v1, v2));
display(sub_vect(v1, v2));
display(scale_vect(10, v1));
display(scale_vect(10, v2));

入出力結果(Terminal, Zsh)

% node answer2.46.js
[1, 5]
[4, 2]
[5, 7]
[-3, 3]
[10, 50]
[40, 20]
%