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.
Comments — twelve stages
I know I need comments on a post. I have no idea how to implement comments.
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?
Comments = A message a person attaches to a post, or to another comment, that others can read in order.
- Does a comment have identity? Yes, strongly. Two comments with the same author and body are two comments: written at different moments, possibly replying to different things. Every comment needs an id from the first version, because replies point at it.
- Who owns it? Its author — the person who wrote it, who is the only one allowed to edit it. The post owns the thread; a moderator (V4) may act on any comment without owning it.
- How long does it exist? Until it is deleted — and even then, if it has replies, something has to remain so the replies still have a parent. Lifetime is the question this concept is really about.
- Should it survive reload? Always. A comment is written to be read by others later; a comment that exists only in one browser is a draft. V1 keeps it in memory only so that the behaviour can be right before storage exists.
- Should it survive login? The comment belongs to the post, not the session; logging out changes who may edit, not what exists.
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.