Building Abstractions with Data - Hierarchical Data and the Closure Property - Hierarchical Structures - Mapping over trees - square tree, abstraction
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)、2.2.2(Hierarchical Structures)、Mapping over trees、Exercise 2.31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 map(f, items) {
return is_null(items) ?
null :
pair(f(head(items)), map(f, tail(items)));
}
function display(x) {
return console.log(stringfy(x));
}
function square(x) {
return x * x;
}
function tree_map(f, tree) {
return map(
sub_tree =>
is_pair(sub_tree) ?
square_tree(sub_tree) :
f(sub_tree),
tree
);
}
function square_tree(tree) {
return tree_map(square, tree);
}
const tree = list(
1,
list(
2,
list(
3,
4
),
5),
list(
6,
7
)
);
display(tree);
display(square_tree(tree));
入出力結果(Terminal, Zsh)
% node answer2.31.js
[1, [[2, [[3, [4, null]], [5, null]]], [[6, [7, null]], null]]]
[1, [[4, [[9, [16, null]], [25, null]]], [[36, [49, null]], null]]]
%