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]]]]]
%