# ADR 0037: Batch & Combined Export

## Status

Accepted. **Extends/refines ADR-0009 and ADR-0011** (export target = a build
producing a typed artifact) by adding **two** export dimensions that today have
no home: (1) building **multiple targets of one lesson** in one batch, and (2)
**combining multiple lessons into one** artifact — a 讲义合集 / course bundle.
It does **not** redefine the SingleFile vs FileTree artifact distinction
(ADR-0011) or the single-`typstCompile`-step MVP; it adds the *collection*
semantics on top.

## Context

Today `cph build --target T` builds exactly one target `T` of one engineering
file into one artifact. Two real needs fall outside that:

1. **A lesson's multiple versions.** A lesson already declares several targets
   (student handout, teacher plan, slides, script). Producing all of them is
   today N separate `cph build` invocations with no shared invocation, ordering,
   or failure summary. Teachers preparing a lesson want "build the whole lesson
   in all needed forms" as one action.

2. **Combining lessons into one deliverable.** ADR-0005 deferred "course =
   arrangement of lessons" — a course/unit is *not* an engineering file; it is
   an arrangement of lessons "modeled elsewhere". The elsewhere is empty. A real
   deliverable is a **讲义合集 / course bundle** — several lessons ordered into
   one document (e.g. "期中复习合集", a term bundle, a topic compilation). This
   spans multiple engineering files and currently has no model and no CLI path.

Both are product-plain features (the 老师端 exports; a bundle is what a teacher
hands a class), not architectural speculation.

## Decision

### The existing single-lesson single-target build is the atomic unit

ADR-0009/0011's model — a target is a build over one lesson producing one
`Artifact` via ordered `Step`s — is unchanged and remains the *unit*. Nothing
below replaces it; the new semantics are **aggregations over that unit**.

### Dimension 1 — Batch: build a set of targets of one lesson

`cph build` on a lesson gains the ability to produce **several targets in one
invocation**, as one batched operation:

- The lesson root `manifest.toml` `[targets.*]` already enumerates the declared
  targets and their order (ADR-0008/0011). Building "all declared targets" is the
  default batch: each declared target builds to its own artifact
  (`build/<target>.{pdf,md}`), in declaration order.
- A batched build is **non-transactional and independent per target**: each
  target is a separate build with its own artifact, own diagnostics, own
  exit/result. One target failing (e.g. teacher plan PDF) does not block the rest
  (student PDF), matching the per-target independent-failure stance of
  ADR-0009's "missing render for a used kind ⇒ warning, non-blocking".
- The batch emits a **summary**: a per-target ledger (ok/failed + its artifact
  or error), and a **non-zero aggregate exit if any target failed** to produce
  its artifact. A target that fails to produce its artifact is a real defect
  (this repo's fail-fast stance — don't paper over bugs), distinct from
  ADR-0009's "missing render rule ⇒ warning" (a policy-level skip, non-error).
  "Don't block the rest" still holds: every target is attempted, but a single
  failure makes the aggregate non-zero so CI/observability catch it. This is the
  CLI's job; it is the natural "build the whole lesson" affordance.

### Dimension 2 — Combined: arrange multiple lessons into one artifact

A **course bundle** is a new, lightweight, second kind of engineering-file-adjacent
unit: an **ordered arrangement of lessons** (ADR-0005's deferred "course =
arrangement of lessons" finally given a concrete export home).

#### Bundle carrier: a directory containing `bundle.toml`

A bundle is a **directory containing `bundle.toml`**. A directory gives the
bundle a stable root for relative lesson paths and a home for build output
(echoing ADR-0007's "engineering file = directory"). The `bundle.toml` carries:

- `[info]` — the bundle's own title/author (of the 合集);
- `[targets.*]` — the bundle's build configuration, reusing ADR-0011's build
  mechanism;
- an ordered `lessons` array — each entry: a lesson path (relative to the
  bundle root, pointing at each engineering-file root, each a self-contained
  directory tree per ADR-0007) plus optional per-lesson per-target overrides.

A bundle target produces an **ordered concatenation/assembly of the lessons'
artifacts or content** into one `Artifact`, reusing the ADR-0011 artifact ADT:

- `SingleFile` — a combined document (讲义合集): the lessons' content assembled
  in order into one compiled document, with the existing cross-reference /
  `@label` machinery working because it is one compiled document (the same
  reason ADR-0011 gives for why SingleFile concatenation works at all).
- `FileTree` — each lesson to its own file plus a generated index (ADR-0011's
  third-party-archive case, now with a first-class multi-lesson trigger).

A bundle target's steps are the **ordered typed steps** of ADR-0011, but the
"map" now operates at the level of whole lessons rather than a single lesson's
parts: a step like `assembleLessons` (ordered inclusion of each lesson's
content/artifact) plus the existing `typstCompile`/`shell` steps for assembly
and any post-processing. Concretely the framework provides a
`typstCompile`-style step that pulls each listed lesson's content in order into
one document (mirroring how a single lesson's template pulls its parts).

#### Renumbering in a `SingleFile` bundle: template-resident, default reset per lesson

Numbering is presentation, which ADR-0011 already owns to the template file (not
the manifest), so the **bundle target's template decides** whether auto-counters
reset at lesson boundaries; the framework ships a helper to reset counters at a
lesson boundary. The **recommended default resets auto-counters at each lesson
boundary**: lessons are authored self-contained, so an internal "例题3" means
that lesson's 例题3; cross-lesson continuation would silently break author
references. A genuine "全书 continuous numbering" is an explicit template
override. `@label` cross-references stay global (resolved by label name,
independent of counters); only auto-increment counters reset.

#### Bundles do not nest (MVP)

A bundle references lessons only, not other bundles, until a real need appears —
mirrors the tree-nesting simplicity and keeps the first bundle target minimal.

### Invariant: lessons stay self-contained; combination is export-time only

Opening the door to combining lessons must **not** open the door to cross-lesson
imports inside a lesson's own content (ADR-0006's import boundary: within one
engineering file plus `@package`, never into a sibling lesson). A bundle is
allowed to *assemble already-authored lessons at export time* — reading their
content for the combined artifact — but no lesson's rich content may `import`
another lesson's internals as part of *its own* authoring. Combination is a
**projection over self-contained lessons**, exactly as a render target is a
projection over a lesson. This keeps each engineering file independently
checkable, buildable, and movable, and avoids reintroducing cross-file coupling
ADR-0006 explicitly rejected.

### CLI surface

- `cph build <lesson>` → all declared targets (the batch default).
- `cph build <lesson> --target student --target teacher` → the named subset
  (the multi-version batch), in the given order.
- `cph bundle <bundle-path> --target <name>` → build one combined bundle target
  (`SingleFile` merged doc or `FileTree`), giving the multi-lesson merge.
  (`cph build` on a bundle root is the batch-of-bundle-targets equivalent.)

## Consequences

- **One lesson, many versions** is one command with a per-target ledger — the
  natural 老师端 "导出全部版本" action, and any single failure surfaces as a
  non-zero aggregate.
- **Course = arrangement of lessons** gets a concrete, export-focused home (the
  bundle), discharging the ADR-0005 deferred item without inventing a full
  course-authoring model.
- The **artifact/distinction and build-step machinery (ADR-0011) is reused** — a
  bundle target is just a build whose inputs are whole lessons, not a new
  parallel export engine.
- **Self-containment stays** (ADR-0006/0007): each lesson remains independently
  checkable and buildable; the bundle only reads them for assembly. A lesson and
  a bundle can version/evolve independently.
- The teacher surface can offer "export all versions" (batch) and "compile into
  a 合集" (combined) as two concrete, productisible actions.

## Open Questions / Deferred

- **Bundle-of-bundles / nesting** — no nesting in MVP; re-open only when a real
  need appears.
- **Dedup/caching across targets and lessons** — none in MVP, consistent with
  ADR-0011's "no caching in MVP".
- **Exact counter-reset semantics in a `SingleFile` bundle** — the default
  (reset per lesson) is decided; the precise mechanism (which counters, how the
  template override is expressed) is settled with the first bundle template
  implementation.