9 · Dependencies & unlocks

Real subjects have an order: you can’t grasp borrowing before ownership, or a derived rule before its axioms. alix lets a deck declare what it builds on, and uses that both to sequence your study and to gate decks until you’re ready.

Declaring prerequisites: requires:

A deck names its prerequisites with a requires: list in its frontmatter (repeatable):

---
requires:
  - rust-ownership
  - deck-9w2c7x4k1m8q3z5t0v6b2n4d8f
---

## What does the borrow checker prevent?
Aliasing a value while it's mutably borrowed.

Each entry is a filename or a deck id. A filename (rust-ownership, with or without the .md) resolves next to the requiring deck or in your decks directory. A deck id (deck- followed by a 26-character token) resolves to the deck that carries that id wherever it lives, so the edge survives the prerequisite being renamed. An entry counts as an id only when it is exactly deck- plus a 26-character canonical token and nothing else; any other value, including a deck-… name written with the .md extension or a ./-prefixed path, is read as a filename. Prefer the id form for a rename-proof edge, the filename form for readability. Directives aren’t card content, so adding or changing them never touches card progress. A missing prerequisite or a dependency cycle is treated as non-blocking. A broken edge never hides a deck.

Dependencies don’t change what you review

requires: is about order and gating, not session contents. When you review (or browse) a deck, the session holds exactly that deck’s cards; prerequisites are never pulled in, so the reveal/review you study under is always the deck’s own. What dependencies shape is the picker’s dependency tree (foundations shown first) and, for a deck with a source:, the exam gate below.

Unlocks

The same requires: graph drives unlocks, with no extra syntax, and the gate is the exam, not drilling. You can review any deck at any time, in any order; what requires: controls is exam order: a deck with a source: can’t sit its exam until each of its sourced prerequisites has passed its own exam, and passing a foundation’s exam unlocks the exams that build on it. A prerequisite with no source: at all has no exam to pass, so it never gates: its edge is just a suggested order in the tree. (alix doctor warns when an exam-grounded deck requires one without exam grounding, since that edge can’t gate an exam; add a source: to the prerequisite to make it real. It also flags a dangling requires:, one naming a deck that does not exist, so a renamed or deleted prerequisite is caught rather than silently dropping the edge, and distinguishes an id-mode value that points nowhere from a card id pasted by mistake or a file that merely shares a required id’s name; see the doctor reference.) (A trace masters by passing its exam (retracing the path from memory) so it gates and unlocks like any exam-grounded deck.)

In the picker a deck whose exam is locked shows a 🔒, but it stays drillable: only the exam waits on the prerequisites.

This is what turns a folder of decks into a curriculum: order the material by requires:, and alix gates each step’s exam on passing the last. It’s the backbone of the AI exam’s notion of mastery (a later chapter) and of how alix workspace generate lays out a generated learning plan.