# ADR 0006: Element Schema Is Declarative, Rich Content Is Typst

## Status

Accepted — for the decisions below. Several sub-decisions are deliberately left
open; see *Open Questions / Deferred*.

Builds on ADR-0005, which fixed that an element instance = a kind tag + data
conforming to that kind's schema, and that a kind's minimal contract to the
framework is its **data schema only**. This ADR fixes **how that schema is
expressed** and **what the schema's "rich content" leaves actually are**.

## Context

ADR-0005 left the schema representation and the kind-definition mechanism as the
largest OPEN items — and they are the foundation the rule-based checker stands
on (the checker validates "does this data conform to its kind's schema?"). The
domain is formula-heavy and content-structured: a worked example has a problem
statement, options, a solution; a theorem has a statement, a proof, prerequisite
lemmas. These fields are not bare strings — they are *rich content*.

Two facts about Typst were verified against its source
(`/Users/sjfhsjfh/Typst/typst`) and drive the decisions below:

- A `.typ` file evaluates to a `Module` that carries **both** a `scope` of
  `#let` exports **and** a body `Content` (the rendered top-level markup).
  `Content` retains a `Span` that resolves back to a `FileId` + byte range, so
  click-to-jump from rendered content to source is possible.
- `Content` has no `Deserialize`; and Typst's `Source::new` **requires** a
  `FileId`. Source evaluated with a detached span loses its file identity:
  `span.id()` becomes `None` (click-to-jump breaks) and a relative
  `import "../x"` fails with *"cannot access file system from here"*. Relative
  imports and click-to-jump only work when the source is a real file (with a
  `VirtualPath`) in the compiler's `World`.

## Decision

### The data schema is declarative — a JSON Schema

A kind declares its data shape as a **declarative JSON Schema** (field names,
types, constraints), not as a program and not as a bespoke schema DSL. The
checker reads the schema and validates instance data structurally against it.
Whether stdlib kinds author their schema by hand-written JSON or via a host-
language builder is an implementation detail, not part of this contract.

### Field types: built-in scalars plus a `content` leaf type

Schema field types are the built-in scalars (string / number / boolean / enum /
list / object, as in ordinary JSON Schema) **plus one extension type:
`content`**. JSON Schema has no native notion of "typst content", so this is a
deliberate **extension** of JSON Schema (a custom type / `format`). Any
implementation must honor the same `content` extension rather than inventing its
own — this is the seam between the schema layer (A) and the rich-content layer.

### A `content` value is a Typst source; its meaning is the module's body content

The value of a `content` field is a **Typst source**. Its denotation is the
**body content** of that source evaluated as a module (Typst's `Module.content`)
— not its `#let` exports. The canonical stored form is the **source text**, not
a serialized `Content`: `Content` cannot be deserialized and its spans are only
meaningful with the source present, so content is re-evaluated from source when
needed, never round-tripped as data.

### Rich content is never free-floating — it has a virtual path

Because detached Typst source loses click-to-jump and cannot do relative
imports, a `content` value is **not** an anonymous text blob. It is a first-class
file in the compiler's `World`, carrying a **virtual path**. This is what makes
its spans resolve (click-to-jump) and its relative imports anchor. Concretely,
the engineering file constitutes a **virtual file system with internal path
structure**; each rich-content field sits at a path within it.

(This ADR commits only to *"there is a path structure"*. Its concrete on-disk
form — that the engineering file is a real directory tree — is fixed separately
in ADR-0007. ADR-0006 does not assume the path structure is a disk directory.)

### Import boundary: within the engineering file, plus packages

A relative `import` inside a rich-content source may resolve only to other paths
**within the same engineering file's path structure** (reusing definitions local
to this lesson), plus Typst **`@package`** ecosystem imports. It may **not**
reach into other engineering files. Cross-lesson reuse, if any, goes through a
separate mechanism (question bank / reuse — still OPEN, see ADR-0005 Deferred),
not through raw cross-file imports.

## Consequences

- The checker can introspect a rendered `Content` (its element type, fields,
  downcasts) to validate rich content beyond "is it present", and can use spans
  to point diagnostics back at the offending source location.
- Canonical state is source text; `Content` is a transient product of
  evaluation. Storage, diffing, and agent edits all operate on source, not on
  serialized content.
- Extending JSON Schema with `content` means conformance has a shared extension
  point: implementations align on one `content` semantics instead of each
  inventing a rich-text encoding.
- Rich content requiring a virtual path means the engineering file is inherently
  a path-structured container, not a bag of anonymous snippets — which is what
  ADR-0007 then grounds on disk.
- The import boundary keeps an engineering file self-contained: it depends only
  on itself and on published packages, never on a sibling lesson's internals.

## Open Questions / Deferred

- **Content element constraints (Y)** — whether a `content` field may be further
  constrained to admit only certain Typst element kinds (e.g. "inline only, no
  figure", "exactly one final expression"). Deferred: we will know which
  constraints are needed only once real stdlib kinds exist. Until then a
  `content` field means "a block of content", inner element shape unconstrained.
- **stdlib kind set** — the first batch of framework-provided kinds and their
  individual schemas (each is later, per-kind work).
- **JSON Schema dialect** — which draft, and exactly which keywords are
  permitted in a kind schema (a subset/profile is likely, to keep the checker
  tractable). Not pinned here.
- **On-disk form** — fixed in ADR-0007 (directory tree); layout conventions
  (manifest, element ordering) deferred beyond that.
