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 need X→What is X?→What must it remember?→What can happen to it?→What must always hold?→Examples→Represent the state→Which structure?→Each operation→Pseudocode→Implement one→Test with examples→Edge cases→Integrate

“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.

Say what you need

beginner

intermediate

advanced

How much help?

Difficulty dial

Every stage's content is shown right after your attempt box.

Pick a concept to begin. Nothing is shown until you have written something at each stage.

The principle

If you do not know how to build the thing, make the thing smaller until you reach something you do know how to build. Go one primitive lower →

SIMPLIFIED

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

Shopping Cart From Nothing →

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.