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.
Search Index — twelve stages
I know I need full-text search over documents. I have no idea how to implement a search index.
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?
Search Index = A structure, derived from a set of documents, that answers "which documents contain these words?" without reading every document.
- Does an index have identity? Weakly. Two indexes built from the same documents are interchangeable, because the index is derived; the documents have identity, the index has a version — "built from which documents, when".
- Who owns it? The service that owns the documents. Nobody edits an index by hand; it is written by the same code path that writes documents, or rebuilt from them.
- How long does it exist? As long as the documents do, but it can be thrown away at any moment and rebuilt — that is the property that makes it an index and not a database.
- Should it survive reload? V1 in memory says no and rebuilds on start; V3 says yes because rebuilding a large corpus on every restart is the cost that forces persistence.
- Should it survive login? Not a question the index answers. Which documents a user may see is a permission filter applied to the result ids, owned by authorisation, not by the index.
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.