計算機科学のブログ

Building Abstractions with Data - Hierarchical Data and the Closure Property - Sequences as Conventional Interfaces - Sequence Operations - reverse, fold right, fold left

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)、2.2.3(Sequences as Conventional Interfaces)、Sequence Operations、Exercise 2.39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)) + "]";
    }
    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 list(...args) {
    return args.length === 0 ?
        null :
        pair(args[0], list(...args.slice(1)));
}
function display(x) {
    return console.log(stringfy(x));
}
function accumulate(op, initial, sequence) {
    return is_null(sequence) ?
        initial :
        op(
            head(sequence),
            accumulate(op, initial, tail(sequence))
        );
}
function append(seq1, seq2) {
    return accumulate(pair, seq2, seq1)
}
function fold_right(op, initial, sequence) {
    function iter(rest) {
        return is_null(rest) ?
            initial :
            op(
                head(rest),
                iter(tail(rest))
            );
    }
    return iter(sequence);
}
function fold_left(op, initial, sequence) {
    function iter(result, rest) {
        return is_null(rest) ?
            result :
            iter(
                op(result, head(rest)),
                tail(rest)
            )
    }
    return iter(initial, sequence);
}
function reverse1(sequence) {
    return fold_right((x, y) => append(y, list(x)), null, sequence);
}
function reverse2(sequence) {
    return fold_left((x, y) => pair(y, x), null, sequence);
}

function iter(sequence) {
    if (!is_null(sequence)) {
        const s = head(sequence);
        display(s);
        display(reverse1(s));
        display(reverse2(s));
        iter(tail(sequence));
    }
}
iter(
    list(
        list(),
        list(1),
        list(1, 2),
        list(1, 2, 3, 4, 5)
    )
);

入出力結果(Terminal, Zsh)

% node answer2.39.js
null
null
null
[1, null]
[1, null]
[1, null]
[1, [2, null]]
[2, [1, null]]
[2, [1, null]]
[1, [2, [3, [4, [5, null]]]]]
[5, [4, [3, [2, [1, null]]]]]
[5, [4, [3, [2, [1, null]]]]]
%