Let a variable declare struct fields

The value type was already there: StructValue, textOf, sameValue,
writeTyped/mergeStruct and the struct port type came with the first
part, and Get/Set variable carry any-ports, so a bundle already flowed
through the graph unchanged. What was missing was the declaration.

PropertyDef gains kind 'struct' with a list of fields, VARIABLE_KINDS a
fifth kind, variableDef a fifth parameter. A declared field is a
primitive - a list field would need a nested editor and therefore the
path in the slot address, the same step "a list of structs" is waiting
for. coerceValue now returns PropertyValue rather than FieldValue, the
first value type that is not a field; none of its five callers break.

In the inspector a struct variable is a block, not a tab. The list tab
exists because hundreds of rows fit in no inspector row; a struct is the
opposite case - few fields that fit under each other. It is the port
editor Pack already has, one level down and with a value column: type,
key and default value in one place. A placed object sees the same fields
as plain value rows: overriding yes, declaring no.

Adding or removing a field touches the declaration, the default, the
behavior's value and every instance override in one command, so a stored
value can never keep a shape its declaration no longer promises. The row
handlers read the bundle at the moment of the change rather than closing
over a snapshot - a field writes the whole bundle, so with a snapshot
the second edited row revives the first one's old value. defSignature
carries the fields, otherwise a new row appears only at the next
rebuild.

Fixes a fixture fault this uncovered: step 10 pushed its text object
into the scene without instanceEdits, which overrideSites walks with
Object.entries. AddStructFieldCommand is the first caller to reach it
from that state; the step was green alone and failed three checks in the
full suite.

Moves the dynamic-ports note to done - all three of its parts are built.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/commands/naming.ts

@@ -18,3 +18,8 @@ export function nextAssetName(project: Project, base: string): string {
 export function nextVariableName(asset: BehaviorAsset, base = 'var'): string {
   return firstFree(new Set(asset.variables.map((v) => v.key)), (i) => `${base}${i}`, 1);
 }
+
+/** "field1", "field2", … – free within the one struct, which is the only place the name has to be unique. */
+export function nextFieldName(fields: readonly { key: string }[], base = 'field'): string {
+  return firstFree(new Set(fields.map((f) => f.key)), (i) => `${base}${i}`, 1);
+}

src/core/commands/variableCommands.ts

@@ -1,7 +1,21 @@
-import type { PropertyValue } from '../model/Properties';
+import {
+  type FieldValue,
+  isStruct,
+  type PropertyBag,
+  type PropertyValue,
+  type StructValue,
+} from '../model/Properties';
 import { overrideSites } from '../model/instance';
 import type { Project } from '../model/Project';
-import { isCommonProp, type PropertyDef, type VariableKind, variableDef } from '../properties/PropertySchema';
+import {
+  emptyField,
+  isCommonProp,
+  type PropertyDef,
+  type StructDef,
+  type StructFieldDef,
+  type VariableKind,
+  variableDef,
+} from '../properties/PropertySchema';
 import { type Removed, removeFirst, restoreAt } from '../util/list';
 import { type Command, CommandError } from './Command';
 import { requireBehavior } from './require';
@@ -123,8 +137,179 @@ export class RenameVariableCommand implements Command {
 }
 
 const INVALID_NAME = 'Variable names must start with a letter and contain only letters, digits or _';
+const INVALID_FIELD = 'Field names must start with a letter and contain only letters, digits or _';
 
 /** Identifier-like, and not a common property key – a variable "enabled" would shadow the switch. */
 export function isValidVariableName(name: string): boolean {
-  return /^[A-Za-z_][A-Za-z0-9_]*$/.test(name) && !isCommonProp(name);
+  return isValidFieldName(name) && !isCommonProp(name);
+}
+
+/** A field lives inside its struct, so the reserved property keys do not apply to it. */
+export function isValidFieldName(name: string): boolean {
+  return /^[A-Za-z_][A-Za-z0-9_]*$/.test(name);
+}
+
+function requireStruct(project: Project, assetId: string, key: string): StructDef {
+  const def = requireBehavior(project, assetId).variables.find((v) => v.key === key);
+  if (!def || def.kind !== 'struct') throw new CommandError(`Variable "${key}" is not a struct`);
+  return def;
+}
+
+/**
+ * Every value of one struct variable: the declared default, the behavior's own, and each instance
+ * override. A field is added to or taken from all of them at once, so no value keeps a shape its
+ * declaration no longer promises. The `id` is what pairs a site up again when the change is undone.
+ */
+interface StructSite {
+  id: string;
+  read(): StructValue;
+  write(value: StructValue): void;
+}
+
+function structSites(project: Project, assetId: string, key: string): StructSite[] {
+  const asset = requireBehavior(project, assetId);
+  const def = asset.variables.find((v) => v.key === key);
+  const sites: StructSite[] = [];
+  if (def?.kind === 'struct')
+    sites.push({
+      id: 'declared',
+      read: () => def.default,
+      write: (value) => {
+        def.default = value;
+      },
+    });
+  sites.push(bagSite('asset', asset.props, key));
+  for (const site of overrideSites(project, (id) => id === assetId))
+    // An override the object does not have stays absent: writing one here would invent a deviation.
+    if (isStruct(site.bag[key])) sites.push(bagSite(`${site.object.id}/${site.path}`, site.bag, key));
+  return sites;
+}
+
+function bagSite(id: string, bag: PropertyBag, key: string): StructSite {
+  return {
+    id,
+    read: () => {
+      const value = bag[key];
+      return isStruct(value) ? value : {};
+    },
+    write: (value) => {
+      bag[key] = value;
+    },
+  };
+}
+
+function withoutField(value: StructValue, key: string): StructValue {
+  const next: Record<string, FieldValue> = { ...value };
+  delete next[key];
+  return next;
+}
+
+/** Rebuilt rather than reassigned, so the field keeps its place in the value as well. */
+function renameField(value: StructValue, from: string, to: string): StructValue {
+  const next: Record<string, FieldValue> = {};
+  for (const [key, item] of Object.entries(value)) next[key === from ? to : key] = item;
+  return next;
+}
+
+export class AddStructFieldCommand implements Command {
+  readonly label: string;
+
+  constructor(
+    private readonly assetId: string,
+    private readonly key: string,
+    private readonly field: StructFieldDef,
+  ) {
+    this.label = `Add field ${field.key}`;
+  }
+
+  execute(project: Project): void {
+    const def = requireStruct(project, this.assetId, this.key);
+    if (!isValidFieldName(this.field.key)) throw new CommandError(INVALID_FIELD);
+    if (def.fields.some((f) => f.key === this.field.key))
+      throw new CommandError(`Field "${this.field.key}" already exists`);
+    def.fields = [...def.fields, { ...this.field }];
+    for (const site of structSites(project, this.assetId, this.key))
+      site.write({ ...site.read(), [this.field.key]: emptyField(this.field.kind) });
+  }
+
+  undo(project: Project): void {
+    const def = requireStruct(project, this.assetId, this.key);
+    def.fields = def.fields.filter((f) => f.key !== this.field.key);
+    for (const site of structSites(project, this.assetId, this.key))
+      site.write(withoutField(site.read(), this.field.key));
+  }
+}
+
+/** Takes the field's value out of every struct that carries it, and gives each one back on undo. */
+export class RemoveStructFieldCommand implements Command {
+  readonly label: string;
+  private removed?: { field: Removed<StructFieldDef>; values: Map<string, FieldValue> };
+
+  constructor(
+    private readonly assetId: string,
+    private readonly key: string,
+    private readonly field: string,
+  ) {
+    this.label = `Remove field ${field}`;
+  }
+
+  execute(project: Project): void {
+    const def = requireStruct(project, this.assetId, this.key);
+    const fields = [...def.fields];
+    const field = removeFirst(fields, (f) => f.key === this.field);
+    if (!field) throw new CommandError(`Field "${this.field}" not found`);
+    const values = new Map<string, FieldValue>();
+    for (const site of structSites(project, this.assetId, this.key)) {
+      const value = site.read();
+      const item = value[this.field];
+      if (item !== undefined) values.set(site.id, item);
+      site.write(withoutField(value, this.field));
+    }
+    def.fields = fields;
+    this.removed = { field, values };
+  }
+
+  undo(project: Project): void {
+    if (!this.removed) return;
+    const def = requireStruct(project, this.assetId, this.key);
+    const fields = [...def.fields];
+    restoreAt(fields, [this.removed.field]);
+    def.fields = fields;
+    for (const site of structSites(project, this.assetId, this.key)) {
+      const value = this.removed.values.get(site.id);
+      if (value !== undefined) site.write({ ...site.read(), [this.field]: value });
+    }
+  }
+}
+
+export class RenameStructFieldCommand implements Command {
+  readonly label: string;
+
+  constructor(
+    private readonly assetId: string,
+    private readonly key: string,
+    private readonly from: string,
+    private readonly to: string,
+  ) {
+    this.label = `Rename field ${from}`;
+  }
+
+  execute(project: Project): void {
+    this.rename(project, this.from, this.to);
+  }
+
+  undo(project: Project): void {
+    this.rename(project, this.to, this.from);
+  }
+
+  private rename(project: Project, from: string, to: string): void {
+    const def = requireStruct(project, this.assetId, this.key);
+    if (from === to) return;
+    if (!isValidFieldName(to)) throw new CommandError(INVALID_FIELD);
+    if (!def.fields.some((f) => f.key === from)) throw new CommandError(`Field "${from}" not found`);
+    if (def.fields.some((f) => f.key === to)) throw new CommandError(`Field "${to}" already exists`);
+    def.fields = def.fields.map((f) => (f.key === from ? { ...f, key: to } : f));
+    for (const site of structSites(project, this.assetId, this.key))
+      site.write(renameField(site.read(), from, to));
+  }
 }

src/core/properties/PropertySchema.ts

@@ -3,9 +3,11 @@ import { type AssetType, isPlaceableType } from '../model/AssetTypes';
 import {
   type FieldValue,
   isList,
+  isStruct,
   type ListValue,
   type PropertyBag,
   type PropertyValue,
+  type StructValue,
   textOf,
 } from '../model/Properties';
 
@@ -47,14 +49,27 @@ export type PropertyDef =
    * list, so one `list` port fits them all. There is no `object` item kind: a list is stored in the
    * project file, and an object is a live handle that no reload survives.
    */
-  | { key: string; label: string; kind: 'list'; default: ListValue; items: ListItemKind };
+  | { key: string; label: string; kind: 'list'; default: ListValue; items: ListItemKind }
+  /**
+   * `fields` is the declaration; the value carries exactly those keys, which is what lets a struct
+   * keep its shape while single fields are written (`mergeStruct`).
+   */
+  | { key: string; label: string; kind: 'struct'; default: StructValue; fields: readonly StructFieldDef[] };
 
 export const LIST_ITEM_KINDS = ['number', 'text', 'boolean', 'color'] as const;
 export type ListItemKind = (typeof LIST_ITEM_KINDS)[number];
 
+/** A field holds what a list item holds: a field that were itself a struct is what `FieldValue` rules out. */
+export interface StructFieldDef {
+  key: string;
+  kind: ListItemKind;
+}
+
+export type StructDef = Extract<PropertyDef, { kind: 'struct' }>;
+
 export type PropertySchema = readonly PropertyDef[];
 
-export const VARIABLE_KINDS = ['number', 'boolean', 'text', 'list'] as const;
+export const VARIABLE_KINDS = ['number', 'boolean', 'text', 'list', 'struct'] as const;
 export type VariableKind = (typeof VARIABLE_KINDS)[number];
 
 /**
@@ -176,6 +191,7 @@ export function variableDef(
   kind: VariableKind,
   defaultValue?: PropertyValue,
   items?: ListItemKind,
+  fields?: readonly StructFieldDef[],
 ): PropertyDef {
   switch (kind) {
     case 'number':
@@ -192,11 +208,40 @@ export function variableDef(
         default: isList(defaultValue) ? defaultValue : [],
         items: items ?? 'number',
       };
+    case 'struct': {
+      const declared = fields ?? [];
+      return { key, label: key, kind, fields: declared, default: structOf(declared, defaultValue) };
+    }
   }
 }
 
+/** The empty value of a kind, so a field that is not there yet still has one. */
+const EMPTY_FIELD: Record<ListItemKind, number | string | boolean> = {
+  number: 0,
+  text: '',
+  boolean: false,
+  color: 0,
+};
+
+export function emptyField(kind: ListItemKind): number | string | boolean {
+  return EMPTY_FIELD[kind];
+}
+
+/**
+ * A struct over exactly the declared fields: a declared field missing from `from` starts empty
+ * rather than absent, so a struct value always has the shape its declaration promises.
+ */
+export function structOf(fields: readonly StructFieldDef[], from: PropertyValue | undefined): StructValue {
+  const source = isStruct(from) ? from : {};
+  const value: Record<string, FieldValue> = {};
+  for (const field of fields)
+    value[field.key] =
+      field.key in source ? coerceListItem(source[field.key]!, field.kind) : emptyField(field.kind);
+  return value;
+}
+
 /** Clamps / coerces a raw value so it satisfies the definition. */
-export function coerceValue(def: PropertyDef, value: PropertyValue): FieldValue {
+export function coerceValue(def: PropertyDef, value: PropertyValue): PropertyValue {
   switch (def.kind) {
     case 'number': {
       let n = typeof value === 'number' ? value : Number(value);
@@ -222,6 +267,8 @@ export function coerceValue(def: PropertyDef, value: PropertyValue): FieldValue
     }
     case 'list':
       return isList(value) ? value.map((item) => coerceListItem(item, def.items)) : def.default;
+    case 'struct':
+      return structOf(def.fields, value);
   }
 }
 

src/core/store/serialize.ts

@@ -27,6 +27,7 @@ import {
   type PropertyDef,
   type PropertySchema,
   schemaFor,
+  type StructFieldDef,
   VARIABLE_KINDS,
   variableDef,
   type VariableKind,
@@ -176,13 +177,28 @@ function normalizeVariables(raw: unknown): PropertyDef[] {
     const items = (LIST_ITEM_KINDS as readonly string[]).includes(String(v.items))
       ? (v.items as ListItemKind)
       : undefined;
-    const def = variableDef(key, kind, undefined, items);
+    const def = variableDef(key, kind, undefined, items, normalizeStructFields(v.fields));
     def.default = coerceValue(def, (v.default ?? def.default) as PropertyValue);
     defs.push(def);
   }
   return defs;
 }
 
+/** An unreadable field is dropped rather than defaulted: an invented key would claim a value. */
+function normalizeStructFields(raw: unknown): StructFieldDef[] {
+  const fields: StructFieldDef[] = [];
+  const seen = new Set<string>();
+  for (const item of Array.isArray(raw) ? raw : []) {
+    const f = asRecord(item);
+    const key = typeof f.key === 'string' ? f.key : '';
+    if (!/^[A-Za-z_][A-Za-z0-9_]*$/.test(key) || seen.has(key)) continue;
+    if (!(LIST_ITEM_KINDS as readonly string[]).includes(String(f.kind))) continue;
+    seen.add(key);
+    fields.push({ key, kind: f.kind as ListItemKind });
+  }
+  return fields;
+}
+
 function normalizePorts(raw: unknown): DeclaredPort[] {
   const ports: DeclaredPort[] = [];
   const seen = new Set<string>();