I Know What I Need, But I Don't Know How To Build It
Pick the thing you need. Then derive it: meaning, state, operations, rules, examples, representation, pseudocode — and only then the code, one rung at a time.
“I know I need a shopping cart. I have no idea how to implement a shopping cart.” The gap is not a missing tutorial; it is a missing method. Every stage below asks you to write before it shows you anything, because the code is the last step of a derivation, not the first thing to search for.
Which concept?
Grouped by exercise level; the level is how far the derivation goes, not how hard the code is.
How much help?
Every stage's content is shown right after your attempt box.
Playlist — twelve stages
I know I need a playlist. I have no idea how to implement a playlist.
Define the concept
In one sentence, without any code: what is it? Then answer for yourself — does it have identity, who owns it, how long does it exist, should it survive a reload or a login?
Playlist = An ordered list of tracks that a listener plays through from a current position.
- Does a playlist have identity? Yes. Two playlists with the same tracks in the same order are still two playlists — "Running" and "Focus" can be identical today and diverge tomorrow. It has a name and, once saved, an id.
- Who owns it? The listener who made it. In V0 there is one listener and ownership is implicit; a shared playlist (V4) is the first time "who may change it?" becomes a rule.
- How long does it exist? Until it is deleted. Unlike a cart it has no natural end — nothing "checks out" a playlist — so persistence is the expected case, not the exception.
- Should it survive reload? A listener who built a fifty-track list and lost it on reload would leave. V1 in memory says no on purpose, so that the behaviour is right before storage is added.
- Should the current position survive reload? It depends on the product: a music app resumes where you were; a shared party playlist restarts. The position is state of the *listening*, not of the list, which is why it is challenged below.
The catalog is one derivation, not the only one (impl §60). A different set of examples yields different rules; a different first requirement yields a different V1; a map instead of an array is not wrong, only an answer to a different question. Compare your derivation with the reference for the differences, then decide which ones were rules and which were style.
Take it further
The flagship exercise: the cart typed by you, with the reference hidden until you ask.
Make It Smaller Until You Know How to Build It →The principle behind the ladder, and the primitive chain it produces.
The Implementation Loop →The loop the twelve stages follow, and why the code is last.