A composition's `itemModel` patterns are never compiled, so every `$each` field is empty

Found while probing palette as a composition (SPEC-165).

A composition's frontmatter accepts itemModel, emitTag and emitAttributes on a content field (FIELD_KEYS, packages/runes/src/lib/composition.ts). This is the same item extraction cast uses in TypeScript (plugins/business/src/tags/cast.ts). An each over the field then binds $each.<name> for every emitted attribute.

But in YAML a pattern is a string. parseHeadingExtract compiles its patterns into a RegExp, while parseFields passes itemModel through untouched. The resolver applies a pattern only when it is a RegExp (field.pattern instanceof RegExp, packages/runes/src/lib/resolver.ts:788), so a string pattern is skipped with no error.

claude/v041-plan-runes-keep-content View source

Relationships

ID:SPEC-145Status:accepted

Composed runes

ID:SPEC-165Status:draft

A color primitive, inline compositions, and the design runes

Oct 11, 10:20 AM5896395
Created (confirmed)by bjornolofandersson

Steps to Reproduce

Define a composition with a list field:

content:
  type: sequence
  fields:
    rows:
      match: list
      itemModel:
        fields:
          - { name: name, match: text, pattern: "^\\s*([^:]+?)\\s*:" }
          - { name: value, match: text, pattern: remainder }
      emitTag: span
      emitAttributes: { name: $name, value: $value }

Its template iterates the field and prints {% $each.name %} = {% $each.value %}.

Use it on - Primary: #2563EB.

Expected

Primary = #2563EB.

Actual

= for every item. The each runs once per item, so the list is read, but every regex field is empty. There is no error or warning. Patching parseFields to compile string patterns, as parseHeadingExtract does, gives the expected output.

Fix

In parseFields, compile every itemModel field's pattern that is not remainder, at every nesting level (ItemFieldDefinition.itemModel nests).
Report an invalid regex as content-model-invalid, naming the field, as parseHeadingExtract does.
Add a test that a YAML itemModel binds $each fields.

Notes

Patterns consume the text they match, in order. In the example value must be remainder, because a second pattern expecting the colon finds it already gone. The composed-runes guide should say so when it documents itemModel.
An each must place its item with a bare {% slot /%} (SPEC-145 D11), even when the template only reads $each.*. The emitted item then renders nothing. That is harmless, but it is noise for this use. Whether a $each-only slot may omit it is a question for SPEC-165, not part of this fix.