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.
Booking — twelve stages
I know I need bookings — a meeting room, a seat, a time slot. I have no idea how to implement a booking system.
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?
Booking = A claim on one resource for one interval of time, made by someone, which excludes every other claim on that resource during that interval.
- Does a booking have identity? Yes. Two bookings of the same room by the same person for the same hour on different days are two bookings; cancelling one must not cancel the other. It needs an id from the first version, because cancel has to name one.
- Who owns it? The person who made it — the owner is who may cancel it. The resource is owned by whoever manages rooms or seats; the booking only points at it.
- How long does it exist? From the moment it is made until it is cancelled — or, in history-keeping versions, forever with a cancelled flag. Whether the interval has passed does not delete it; past bookings are the record of what happened.
- Should it survive reload? Yes, from the first real version. A booking that disappears on restart lets the room be booked twice, which is the one thing the concept exists to prevent.
- Should it survive login? It is keyed by the owner and the resource, so it is visible wherever the owner logs in; anonymous bookings need a contact instead of an owner, which is a modification.
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.