n0 XML durable addressing

Status: Accepted direction for a Version 4 proving implementation.

This RFD defines the identity and typed-value boundary that must exist before declarative animation can be specified. It does not define animation syntax, time, timelines, keyframes, interpolation, easing, composition, playback, or scheduling.

The decision is:

1. authored identity, component occurrence, property identity, compiled runtime identity, and effective frame values are five different facts;
2. n0 XML Version 4 gives every authored render node and every use a durable owner-local id;
3. a materialized node address combines its authored owner and member with an ordered path of durable use occurrences;
4. a property address adds one key from a closed typed property registry;
5. a runtime may compile that durable address to a generation-checked handle;
6. an immutable sparse value map may override registered base values for one evaluation without mutating authored content; and
7. an empty map is exactly the existing static scene.

These decisions also serve inspection, durable diagnostics, previews, future cross-session operations under a stable canonical source-identity contract, and other evaluated-state systems. Animation is one future producer of effective values, not the owner of identity or the property registry.

Why one id is not the whole answer

A component member can be authored once and materialized many times. The same member label under uses first-card and second-card must therefore retain one definition-local identity while producing two distinct occurrences. The materialized engine slot for either occurrence can also be deleted and reused. Finally, opacity and width on that occurrence have different types and invalidate different stages.

Collapsing those facts into one string or integer makes at least one lifetime dishonest. The format instead keeps the following tiers separate:

Source spans, sibling indexes, element paths, human-readable name values, arena slots, hashes of property strings, and generated draw commands belong to none of the durable tiers.

Prior-art constraints

The separation is a synthesis of browser and game-engine practice rather than a new all-purpose identifier:

• Blink keeps a live target element and a typed property/attribute identity, then writes sampled SVG values into a separate animated slot before style, layout, and paint. The Chromium SVG animation study records that pipeline.
• Unity's GenericBindingUtility compiles hierarchy/type/property descriptors into BoundProperty handles whose runtime identity is an index plus version. Transient float, integer, and object-reference values are supplied separately to SetValues. The versioned index accelerates a binding; it does not make the authored hierarchy path durable.
• Unreal's FMovieSceneObjectBindingID combines persistent binding identity with sequence-instance context, while FMovieScenePropertyBinding keeps property identity separate.

The useful common shape is durable authored target, instance context, typed property, replaceable runtime handle, and transient value storage. None of these systems justifies serializing a runtime slot or using a hierarchy string as the only identity.

Scope

This RFD specifies:

• the Version 4 id grammar and uniqueness scopes;
• component-root, member, use-occurrence, nested-use, and identity preservation through slot projection;
• the abstract node and property address forms;
• the contract for a closed typed property registry;
• nullability and sparse effective-value semantics;
• compilation to replaceable generation-checked runtime targets;
• strict validation and canonical-writing requirements; and
• the static-equivalence law.

It does not specify:

• a textual selector or URI syntax for addresses;
• paint-layer, stroke-layer, gradient-stop, text-run, or lens-operation member identity;
• mutation, collaboration, or merge protocols;
• archive encoding;
• animation or event semantics; or
• incremental evaluation and compositor optimization.

The abstract address is intentionally decided before a compact spelling. A later syntax can serialize the structure without forcing every processor to parse an opaque path string.

Version 4 source contract

Version 4 inherits the complete Version 3 language: source modules, components, typed scalar parameters, and named slot projection. Its only new authored vocabulary is durable id on render elements and use.

<grida version="4">
  <component id="profile-card" width="320" height="96">
    <rect id="avatar" width="64" height="64"/>
    <text id="display-name">Ada River</text>
    <slot name="actions"/>
  </component>

  <container id="feed" layout="flex" direction="column">
    <use id="first-card" href="#profile-card">
      <text id="first-action" slot="actions">Follow</text>
    </use>
    <use id="second-card" href="#profile-card">
      <text id="second-action" slot="actions">Message</text>
    </use>
  </container>
</grida>

Required IDs

In Version 4:

• every authored render element has exactly one id;
• every use has exactly one id;
• id is a literal and cannot be supplied by a component parameter;
• id uses the existing lowercase-kebab identifier grammar;
• name remains an optional human label and is never an identity fallback;
• tspan, prop, arg, slot, paints, strokes, and gradient stops do not gain id from this RFD; and
• the generated host viewport node has no authored identity.

A component is a definition construct rather than an authored render element. Its existing id remains the module-local export symbol that qualifies the component owner; it is not copied or reused as a render-node ID. The root member is the tagged member variant Root; descendants use the tagged variant Node(authored-id). The tag keeps Root distinct from a legal descendant id="root".

Versions 0 through 3 retain their exact grammar. They neither require nor accept render-node or use-occurrence id, and processors must not synthesize durable addresses for them.

Uniqueness scope

One lexical owner owns one identity namespace:

• the scene tree in one source unit is one owner; and
• each component definition is a separate owner.

Within an owner, render-element IDs and use IDs share one namespace and must be unique across the complete lexical subtree, including caller-authored slot assignments. Component export IDs remain in the source unit's component-export namespace and may coincide with a member ID without ambiguity.

Sharing the render/use namespace guarantees unambiguous owner-local lookup and prevents two differently typed authored sites from claiming the same token. Refactoring Node(card) into Use(card) still changes identity and must be an explicit edit. Allowing IDs to repeat in different components keeps definition-local names concise and is safe because the owner is part of every address.

Version 4 source units link only Version 4 component sources. A processor must not claim a complete Version 4 address space while expanding an older definition whose members and uses have no durable IDs. Older callers likewise cannot link a newer Version 4 definition.

This deliberately tightens the earlier monotone link pattern:

Versions 1 through 3 remain unchanged. Version 4 trades backward linking for the stronger invariant that every authored node in its materialized closure has a durable address; partial identity would make target validity depend on which dependency happened to be older.

Stability and editing

An ID survives parsing, canonical writing, reformatting, sibling insertion, reordering, reparenting within the same owner, and unrelated property edits. Changing an ID changes identity. A tool may rename and retarget references in one transaction, but a reader never guesses that two different IDs denote the same object.

That is authored-member stability, not unconditional occurrence-address stability. Reparenting through a different component use or slot projection changes the ordered occurrence path. Changing canonical source identity, renaming a component export, changing a member/use ID, or changing an enclosing use path likewise changes the corresponding address.

Moving a subtree within one owner preserves its IDs. Copying a subtree into an owner where any copied ID already exists must mint non-conflicting IDs before the copy becomes valid. The source language does not prescribe an editor's ID generation algorithm; it only requires the committed result to satisfy the grammar and uniqueness rules.

Address model

Authored owner

An authored owner is one of:

scene-owner     = (canonical source identity, scene)
component-owner = (canonical source identity, component export id)

Canonical source identity is supplied by the source-resolution environment, as already required by the module RFD. A relative href, source bytes, or working-directory path is not substituted for it.

Authored member

An authored member is a tagged variant:

Root
Node(authored render-element id)

The pair (owner, member) identifies authored definition intent but not one materialized component instance.

Use occurrence

A use occurrence is:

(caller owner, authored use id)

The referenced component, source span, href, and name remain valuable provenance, but none replaces the caller-owned use identity.

Materialized node address

A materialized node address is:

(authored owner, authored member, ordered use-occurrence path)

The path is outermost first. An ordinary scene node has an empty path. A member inside one component use has one segment. A member inside nested uses has one segment per expansion boundary.

For the example above, let S be the entry's canonical source identity. The two component roots and avatars differ only by their typed occurrence path:

(ComponentOwner(S, profile-card), Root,         [Use(SceneOwner(S), first-card)])
(ComponentOwner(S, profile-card), Node(avatar), [Use(SceneOwner(S), first-card)])
(ComponentOwner(S, profile-card), Root,         [Use(SceneOwner(S), second-card)])
(ComponentOwner(S, profile-card), Node(avatar), [Use(SceneOwner(S), second-card)])

The scene root's id="feed" is its member ID; it is not a use-path segment.

Caller-authored slot content retains its caller owner and member. Its path still includes the receiving use and every enclosing use through which that caller content was materialized. Projection changes placement and painter context; it does not transfer authorship to the callee.

One logical materialized node occurrence has one canonical node address. Distinct canonical addresses must never resolve to the same logical occurrence except where a future RFD explicitly defines an aliasing construct; Version 4 defines none. An implementation may still share immutable backing data for repeated component definitions, as long as each occurrence retains a distinct address and observable instance behavior.

Typed property addresses

A property address is exactly:

(materialized node address, registered property key)

Property keys come from one closed, versioned registry rather than arbitrary field paths. Each registry entry defines:

• its canonical semantic key;
• its exact value type, including any property-specific optional state;
• the node kinds on which it applies;
• how to read the authored base value;
• validation of an effective value;
• a conservative set of affected stages: measure, layout, transform, bounds, paint, and resource; and
• deterministic equality.

The initial registry is exactly the following. Keys are semantic property identities, not necessarily XML attribute spellings. active, for example, is the positive runtime value projected from the inverse hidden source attribute; layout, fills, and strokes are whole model values assembled from their compact or structured source syntax before this boundary.

Impact legend: M = measure, L = layout, T = transform, B = bounds, P = paint, and R = resource.

Number is not one universal numeric domain. Rotation is any finite number; growth is finite and non-negative; opacity and corner smoothing are finite in [0, 1]. Fixed sizes and present optional constraints are finite and non-negative. A present aspect ratio contains two finite positive numbers. Layout, corner, paint, gradient-stop, image-resource, and stroke values retain the validation rules of their static model and renderer contracts. Validation checks the complete effective node state, not only each replacement in isolation: for example, smoothing plus elliptical corner radii and per-side strokes plus smoothing must be rejected regardless of which member came from authored state. An aggregate paint or stroke that the renderer cannot materialize is invalid rather than silently omitted. Applicability and domain validation happen before evaluation.

That shared authored-state fence includes finite, mathematically invertible gradient transforms and, for a linear gradient, a binary32 unit-space endpoint distance greater than 2^-15. Authored source, canonical writing, and a replacement Paints value therefore cannot disagree about malformed gradient intent or a ramp that would be replaced with a degenerate color.

Backend representability is a later, resolved-value question. A conforming evaluator composes each visible gradient transform with its actual resolved paint box using the backend's numeric representation, verifies that the resulting local matrix is invertible, and proves that its selected gradient implementation can construct the shader before publishing or drawing the frame. A very small or large authored transform may fail for one box and succeed for another; the paint box can worsen or rescue its conditioning. Failure must identify the owning node and whether the paint came from a fill, stroke, or text run, and must occur before any frame pixels are emitted. This evaluation- time failure does not make the mathematically valid authored aggregate invalid.

The registry does not, by itself, make a property animatable. A later animation specification must select a subset and define interpolation, composition, boundary, and resource rules for every selected key.

Source spelling and semantic property identity are related but not required to be byte-equal. For example, compact fill="#fff" and an expanded fill element both project the same ordered fills property. The source-level hidden spelling projects a visibility value rather than creating a second runtime concept. A processor must not create separate effective-value keys for compact and expanded syntax.

Structural facts are outside this registry: node kind, parent, child order, component reference, use path, slot assignment, text-run partition, and path command topology cannot be changed by an effective-value entry.

The first registry is deliberately node-level. Whole ordered fills and strokes are aggregate typed values because the current scene model owns those values at the node. Replacing either aggregate may change its internal list length; addressing one paint, stroke, stop, run, or lens operation remains invalid until that subobject has a durable identity contract.

Nullability and absence

There is no universal overlay null. Two states must not collapse:

1. No effective-value entry — read the authored base value.
2. One exact registered value — use that validated typed value for this evaluation.

When the model property itself is optional, optionality belongs inside its registered type—for example, OptionalNumber(None) versus OptionalNumber(Some(24)). That typed None is a real property value and is still distinct from an absent map entry. It is never encoded as an empty string, zero, false, NaN, a generic null token, or a magic enum member.

Source omission remains governed by the static property grammar. It may mean a default, inheritance, automatic sizing, or an absent optional value depending on that property. It is not automatically equivalent to effective-map absence.

Materialization and compilation

Materialization produces an ordinary scene plus a checked bidirectional relationship between canonical node addresses and live scene nodes. It must reject an address collision rather than select one node by traversal order.

A runtime may compile a property address to the equivalent of:

(runtime scene incarnation, arena slot, slot generation, property key)

That compiled target is a replaceable acceleration artifact. It is valid only for the materialization generation against which it was checked. Deletion, slot reuse, rematerialization, identity edits, or relevant component-link changes require validation or recompilation. A stale target must fail closed; it must never affect a new node that happens to reuse the same slot.

Slot generation alone is insufficient: two freshly materialized scenes can both contain slot 7 at generation zero. An implementation must therefore include a non-authored runtime-scene/incarnation scope or make applying a compiled target to any scene other than the exact borrowed scene unrepresentable. That scope remains a runtime lifetime guard and is not a sixth authored identity tier.

Cloning or replacing a scene likewise requires rebinding unless the runtime can prove both values share one non-divergent arena identity. Slot generations must never wrap and revive an ancient target; exhaustion fails explicitly or retires the slot permanently.

Neither the compiled target nor a hash derived from it may be written as the authored address. Conversely, a renderer does not resolve source strings or walk component definitions for every frame; it consumes compiled targets.

Immutable effective values

An effective-value input is an immutable sparse map:

compiled property target -> typed property value

Before evaluation it validates that every target is live and generation correct, every key applies to the target node, and every value has the declared type and domain. Duplicate targets are errors unless a later producer defines composition before constructing the map.

The reference evaluation is:

materialized ordinary scene + effective values + declared environment
  -> measure and layout
  -> transforms and bounds
  -> draw list
  -> raster and spatial queries

Every downstream consumer observes the same effective values. Layout-affecting values are available before measurement and layout. Paint-affecting values are not sampled independently by the painter. Queries use the resolved result of the same evaluation; query-relevant traversal and clip state must be captured there rather than supplied later through another independently pairable base or effective-value view. Damage includes draw-list changes so paint-only value changes cannot produce empty damage.

The declared resource environment is a separate immutable input. A logical image reference can retain the same identifier while its decoded bytes or readiness changes, so equality of resolved geometry and draw-list commands is complete only under the same resource-environment snapshot. The resolved result, ordered draw list, and environment identity form one immutable frame product; complete visual comparison accepts two such products rather than separately pairable parts. A snapshot change must invalidate retained raster state and produce conservative damage for resource-backed items, or for the complete scene when affected items cannot be identified. Evaluation still performs no resource I/O. Rastering a retained frame product against a different resource-environment incarnation or revision must fail before drawing; a raw unchecked draw-list replay cannot claim complete-frame semantics.

Evaluation never mutates the authored scene, writes effective values into source, emits document operations, performs source or resource I/O, or reads a clock.

Static-equivalence law

The empty map is the permanent static oracle:

effective-values = empty
  => authored base values at every registered property
  => identical resolved geometry and content
  => identical draw list
  => identical queries
  => identical pixels

This law is exact, not approximate. The existing static entry point may remain as a thin call using the empty map. It must not become a separate semantic pipeline.

Validation and diagnostics

A strict Version 4 processor rejects at least:

• a render element or use without id;
• an empty, malformed, bound, or duplicate id;
• a Version 4 source linking a component source of another version;
• a materialized address collision or a materialized node without one address;
• an unknown property key;
• a property key inapplicable to the target kind;
• a value of the wrong registered type;
• a generic null token instead of the key's exact registered value type;
• a non-finite or out-of-domain numeric value;
• an aggregate paint, stroke, corner, or layout value outside the renderer's declared capability;
• a valid replacement that forms an invalid complete node state with the remaining authored values;
• a stale runtime scene or slot generation;
• two effective entries for the same compiled target; and
• an attempt to address an unregistered subobject or structural field.

Diagnostics identify the authored source and owner, member ID, complete use path, property key when present, and the immediate violated rule. Useful forms include:

• Version 4 <rect> requires a lowercase-kebab id;
• duplicate id "avatar" in component "profile-card"; first declared on <ellipse>;
• member "label" through scene use "second-card" has no registered property "font-family";
• property "max-width" expects OptionalNumber, received Number;
• property "fills" contains an image fit that this renderer cannot materialize;
• corner-smoothing cannot be nonzero with elliptical corner radii;
• compiled target for use "first-card", member "avatar" belongs to a replaced scene; and
• compiled target for use "first-card", member "avatar" is stale: expected generation 2, found 3.

Source spans may enrich those messages. They do not become address fields.

Canonical writing

A canonical Version 4 writer:

• emits version="4";
• emits every required render and use ID unchanged at the semantic level;
• never substitutes name, source position, or a generated path for a missing ID;
• preserves component export ID separately from member IDs;
• writes the existing canonical static property forms;
• never serializes occurrence paths, runtime handles, generations, effective values, or resolved output as duplicate authored state; and
• refuses to write an invalid identity graph rather than silently renaming it.

An editor may generate or repair IDs as an explicit authored edit before calling the canonical writer. The writer itself is not an identity policy.

Current model compatibility

The current scene contract distinguishes a stable string user-node ID from an ephemeral numeric runtime node ID, which is the correct lifetime split for an ordinary flat scene. Its lookup namespace is scene-global, however, and one string does not preserve Version 4's definition-local member plus ordered use path. Repeating one component member would either collide or require a flattened generated string whose segments and escaping become an accidental second address grammar.

Version 4 therefore keeps its structured address in the linked/materialized program above the ordinary scene. A runtime that requires one flat user-node key may lower each logical occurrence to a collision-free opaque key, but it must preserve the structured source address for inspection, diagnostics, recompilation, and source writing. The opaque key is not the canonical authored address and must not be reconstructed by splitting a concatenated string.

This is a declared incompatibility with representing live component occurrences through the current flat ID field alone. The RFD does not change the scene paint, ordered-paint, stroke, layout, or runtime-slot models, and it does not require the ordinary materialized scene to become component-aware.

Considered alternatives

Conformance

A conforming Version 4 reader and materializer:

1. MUST enforce required owner-local render and use IDs and the exact version-link rule.
2. MUST preserve owner/member identity and ordered occurrence paths across component expansion and slot projection.
3. MUST expose a one-to-one canonical address for every authored materialized node and reject collisions.
4. MUST NOT treat provenance, source spans, names, runtime slots, or generations as authored identity.
5. MUST validate typed property targets through one closed registry.
6. MUST distinguish sparse absence from an optional state inside an exact registered value type and MUST NOT define a universal overlay null.
7. MUST reject stale compiled targets and invalid typed values before evaluation.
8. MUST keep authored state immutable and make an empty effective-value input observationally identical to static evaluation.
9. MUST feed one effective evaluation to layout, draw-list construction, query, damage, and rendering.
10. MUST treat resource-environment revision as frame identity for cache and damage even when logical resource identifiers are unchanged.
11. MUST NOT claim that registration makes a property animatable.

Deferred work

Native animation grammar is explicitly deferred by the n0 XML animation RFD. Any future proposal must state its time model, source placement, target-reference spelling, keyframes, interpolation, easing, composition, event posture, static processing, resource behavior, and playback boundary.

Durable identity for paint layers, strokes, gradient stops, text runs, path or vector members, and lens-operation parameters is also deferred. Until each domain has stable member identity, a cross-target property address may name only the registered node-level value. A later RFD may add structured member segments without changing the five-tier separation accepted here.