A Shadow Puppet Walks Its Family Tree

The dragon bows beautifully. Then its wrist catches on a control rod, and the supposedly terrifying creature appears to check its watch.

Articulated shadow puppetry makes mechanical relationships visible: the body carries an upper arm, the upper arm carries a forearm, and the forearm carries a hand. Before worrying about patter or dramatic timing, I want a rehearsal checklist that follows those connections outward.

That is a job for recursive tree traversal, a staple of the recursive programming tradition that took hold in the 1960s. Each part gets the same instruction: announce yourself, then give that instruction to each of your children.

let children: [String: [String]] = [
    "body": ["head", "upper arm"],
    "upper arm": ["forearm"],
    "forearm": ["hand"]
]
func rehearse(_ part: String, depth: Int = 0) {
    print(String(repeating: "  ", count: depth) + part)
    for child in children[part] ?? [] {
        rehearse(child, depth: depth + 1)
    }
}
rehearse("body")

The result reads like a small mechanical family portrait:

body
  head
  upper arm
    forearm
      hand

Visiting the parent before its children makes this a preorder traversal. The indentation records depth; the function calls remember where to resume after each branch. When the head finishes, execution returns to the body and continues with the upper arm.

Perl delivers the same stage directions with a hash of array references:

use strict;
use warnings;
my %children = (
    body => ['head', 'upper arm'],
    'upper arm' => ['forearm'], forearm => ['hand']
);
sub rehearse {
    my ($part, $depth) = @_;
    print(('  ' x $depth) . "$part\n");
    rehearse($_, $depth + 1) for @{$children{$part} // []};
}
rehearse('body', 0);

Both versions assume a tree: every part has one parent, and nothing leads back to an ancestor. A linkage that loops back would need explicit protection against revisiting parts. For a tree with n parts, the walk takes O(n) time, with active calls proportional to its deepest branch.

This checklist cannot judge a convincing bow. It can make sure I inspect every joint before the audience arrives. The dragon’s dignity still depends on my wrist.