# Composition objects — a content-addressed, data-driven UI model

Everything the system stores is an **object**: typed, content-addressed (`:cid`), in one graph.
"Post" was the blog's word; the unit is an object. A *document* is an object whose **body** is a
composition over other objects' CIDs. This is the cards-as-objects decision, generalised.

## One mechanism: ordered, labelled forks

An object forks into children via **labelled, ordered edges** (the relations engine + `order` on
the edge value + an optional `when`). There is no separate "composition system" — relations *are*
the forks. The **label** says what a fork means:
- **structural** (`contains`) → ordered, part of identity, rendered;
- **cross-cutting** (`tagged`, `related`, `author`) → loose links, not structural.

Multiple relations from an object *are* its fork. No "multiple DAGs per object" — fork immediately;
differently-labelled forks (`body` vs `aside`) give named slots. **Join** = a child CID referenced
by two forks — free, because content-addressed. The whole structure is a **Merkle DAG** (git trees
/ IPFS / artdag): `:cid` = hash over `fields + contains-forks (child-CID + order + when)`.

## The body is a tiny UI language (the render-fold is its interpreter)

A body is a composition node. Four combinators + leaves + references:

| node | meaning | strategy |
|------|---------|----------|
| `(seq …)` | **sequence** | render all (block), in order |
| `(row …)` / `(grid …)` | **layout** (par) | render all, side-by-side |
| `(alt (when P n) … (else n))` | **conditional** (or) | render the FIRST child whose `when` holds |
| `(each src tmpl)` | **iteration** (loop) | eval `src` → items; render `tmpl` per item (item bound) |
| `(ref CID)` | transclude | fetch object by CID, render its body |
| `(card TYPE fields)` | leaf | render via the card-type's `:template` (host/blog--instantiate) |
| `(tmpl NAME)` | **recursion** | a named template, may reference itself |

`seq/row` = render-**all** passing children; `alt` = render-**first** passing child. So **and/or/choice
all come from one axis (`when` on forks) × the container's all/first strategy** — `Alt` isn't a new
node kind, it's "first" instead of "all".

## The two fundamentals we designed IN

1. **Recursion** — `(tmpl NAME)` may reference itself; `(each (children) (tmpl NAME))` renders trees
   (comment threads, nested nav, the `/meta` type hierarchy itself). Terminates naturally when a
   query runs dry; a **depth guard** in the context backstops it.
2. **The context is an environment, not a flat dict.** `when` reads it; `each` *extends* it
   (`:item`). Make it extensible + reactive-ready and the two non-composition axes plug in with NO
   new combinators:
   - **Behaviour / interactivity** (Slice 9 lifecycles/effects) — a button references a behaviour;
   - **Reactivity / local state** (the reactive runtime) — `alt(when local-state=active-tab)` is a
     tabset, `alt(when accordion-open)` an accordion; a *live* `each` re-renders on data change.
   The static render-fold becomes a live, interactive UI purely by making the context live.

## The unifying property

**The object's CID is its *definition* (the query, the template, every `when`-variant). The
*rendering* is the *execution* (which items, which branch, which context).** The object is the
program; the render is the run. One immutable content-addressed object encodes its whole
responsive/personalised/variant space; rendering picks the path. Render-fold and the Slice-9
behaviour interpreter are the **same shape** — interpreters over content-addressed objects + the
decidable-core predicate set + the graph. The system converges on: objects + small interpreters.

## Beyond content — composition is universal; a fold per domain

The render-fold isn't "the content renderer" — it's **fold #1**. The composition DAG is a
**universal algebra** (`seq/par/alt/each` over content-addressed objects); *content* is just one
*interpretation*. Same structure, a different **fold** per domain — what changes is what the
combinators and leaves *mean*:

| domain | the fold | `seq` | `par` | `alt`+`when` | `each` | substrate |
|--------|----------|-------|-------|-----------|--------|-----------|
| **content** | render → HTML | block order | layout/columns | choose variant | map items | `compose.sx` (done) |
| **behaviour** | execute → effects | steps in order | concurrent | branch (if/cond) | for-each | `[[project_flow_on_sx]]` |
| **query** | eval → results | join/chain | union | conditional | iterate/quantify | `[[project_relations_on_sx]]` (Datalog) |
| **pipeline** | reduce → data | dataflow stages | parallel ops | choose path | fan-out | `[[project_artdag_on_sx]]` (content-addressed DAG) |
| **types** | extent → set | — | ∧ intersection | — | ∨ union | the type algebra (`make-and!`/`make-or!`) |

So **"relations just a fork" generalises**: a `contains` fork folded by *render* is a document; a
`then` fork folded by *execute* is a workflow step; a `depends-on` fork folded by *eval* is a
dependency graph. **The relation kind + the fold = the domain.** This isn't aspirational — the
repo's `X-on-sx` loops ALREADY ARE these folds (flow = execute, Datalog = eval, artdag = a
content-addressed composition DAG); we just hadn't seen them as one shape. The composition DAG is
the **convergence point** the whole fleet has been circling.

The payoff is concrete: **build the composition machinery ONCE** (forks + ordered edges + the four
combinators + a fold framework) → reuse for every domain by writing one interpreter. **The block
editor edits *any* composition** — author a workflow like a document, same structure, one editor.
The whole system collapses to four ideas: **content-addressed objects + a composition algebra +
per-domain folds + the decidable-core predicates (`when`).** The render-fold's shape (walk the
composition, dispatch combinators, recurse, read the context) is the *template* for every other fold.

## What lives elsewhere (not composition primitives)

Transclusion = a `ref` leaf. Sort/filter/limit/group = the *source query* language (Datalog).
`each` reconciliation keys = the item's CID (free). Empty / missing-CID = render-fold robustness
(the per-block guard). Async/streaming, events, local state = the behaviour + reactive axes.

## Build roadmap

1. **Keystone (this):** `lib/host/compose.sx` — the render-fold interpreter over seq/row/alt/each/
   ref/card/tmpl, with the context-as-environment, `when` predicates, and recursion + depth guard.
   Self-contained proof: render one composed object two ways (auth on/off) + a recursive tree.
2. Wire it to objects: a document's `:body` is a composition node; `contains` forks carry order;
   `host/blog-render` dispatches to the render-fold when `:body` is present (else the legacy
   `sx_content` path). Card leaves render via the existing card-type `:template`.
3. `each` source = a graph query (`(query is-a Event)` → `host/blog-instances-of`) — data-driven.
4. Live context: route auth/device/locale into the context; reactive values later.
5. The typed importer decomposes Ghost Lexical into card objects + a `contains` body (cards-as-
   objects), instead of one `sx_content` string.
6. The block editor edits the body (insert/reorder/`alt`/`each`) — the metamodel editor for content.
7. **Prove universality with a second fold.** Write a tiny `execute`-fold over the *same*
   `seq/alt/each` structure that *runs* a workflow (leaves = effects; `seq` = steps in order, `alt`
   = branch, `each` = for-each) — the way the recursive tree proved recursion, this proves the
   composition algebra is domain-agnostic. Then the *behaviour* model (Slice 9) is "an `execute`-fold
   over a composition object", not a separate system.
8. **Factor out the shared machinery** once two folds exist: the fork model (ordered, labelled,
   `when`), the combinator dispatch, the context-environment, and recursion become a reusable
   `compose` core; each domain (`render`, `execute`, `eval`, …) supplies only its leaf + combinator
   semantics. The block editor + the metamodel UI then generalise to *every* fold — one composition
   editor authors documents, workflows, queries, and pipelines alike.