Building Abstractions with Data - Introduction to Data Abstraction - Abstraction Barriers - segment, midpoint, constructor, selector

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)、2.1(Introduction to Data Abstraction)、2.1.2(Abstraction Barriers)、Exercise 2.2の解答を求めてみる。

コード

function display(x) {
    return console.log(x);
}
function stringfy(x) {
    return x.toString();
}
function average(x, y) {
    return (x + y) / 2;
}
function pair(x, y) {
    return [x, y]
}
function head(pair) {
    return pair[0];
}
function tail(pair) {
    return pair[1];
}
function make_segment(start_segment, end_segment) {
    return pair(start_segment, end_segment);
}
function start_segment(segment) {
    return head(segment);
}
function end_segment(segment) {
    return tail(segment);
}
function make_point(x, y) {
    return pair(x, y);
}
function x_point(point) {
    return head(point);
}
function y_point(point) {
    return tail(point);
}
function print_point(point) {
    return display(
        "(" + stringfy(x_point(point)) + ", " +
        stringfy(y_point(point)) + ")"
    );
}
function midpoint_segment(segment) {
    const start_seg = start_segment(segment);
    const end_seg = end_segment(segment);
    return make_point(
        average(
            x_point(start_seg),
            x_point(end_seg)
        ),
        average(
            y_point(start_seg),
            y_point(end_seg)
        )
    );
}
const start_point = make_point(-1, 2);
const end_point = make_point(1, -2);
const segment = make_segment(start_point, end_point);
const mid_point = midpoint_segment(segment);

print_point(mid_point);

print_point(
    midpoint_segment(
        make_segment(
            make_point(2, 3),
            make_point(10, -1)
        )
    )
);

入出力結果(Terminal, Zsh)

% node answer2.2.js 
(0, 0)
(6, 1)
%