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.
Scheduler — twelve stages
I know I need to run tasks at a given time. I have no idea how to implement a scheduler.
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?
Scheduler = A set of tasks, each with a time at which it should run, that answers "which tasks are due now?" and hands them over exactly when their time has come.
- Does a scheduled task have identity? Yes. Two tasks scheduled for the same time are two tasks; each can be cancelled on its own, so each needs an id the caller keeps.
- Who owns it? The caller that scheduled it — a user, a service, another task. The owner matters the moment cancel needs authorisation: only whoever scheduled it may cancel it.
- How long does it exist? From schedule until it runs or is cancelled. What happens after — a record that it ran, a retry — is a different concept (a job log) and is not stored here in V0.
- Should it survive reload? In V0 no: a restart forgets every pending task. Every real scheduler says yes, because "the reminder was lost when we deployed" is the first bug reported — that is V2.
- Should it survive login? Tasks belong to the system, not to a session. A task scheduled by a user runs whether or not they are logged in; their login only decides who may cancel it.
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.