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Type-System Depth Backlog

Status: Planned · Date: 2026-06-13 · Backlog contract for RFC §10 "Type-system depth".

The gap

IR v2 carries scalar typed values (string, int, double, bool) and the contract layer enforces scalar type/bounds/requiredness. Dogfooding exposed the next real pressure points:

  • F1 lists: Ontology fields such as implements, appliesTo, states, relation oneOf, and compatibility lists currently travel as comma-joined strings.
  • F2 unions: cardinality.max is semantically int | "*", but contracts cannot express that.

Those are real gaps, but adding a full configuration-language type system would violate the core invariant: Hypercode is an addressable topology plus resolved properties, not a replacement for CUE/Pkl/Dhall/KCL.

Design constraints

  • Keep the core smaller than YAML: no arbitrary maps in .hcs v1.
  • Preserve deterministic IR hashing and JSON Schema compatibility.
  • Keep selector contracts monotonic: richer types must still only narrow.
  • Preserve consumer ownership of target schemas. Hypercode may type a value; it must not learn DomainOntologyPackage, Terraform, or prompt-specific schemas.

Candidate sequence

  1. HC-125 list scalars. Add a minimal list literal or sanctioned list scalar convention, with IR v2/v3 migration rules and contract syntax such as list<string>.
  2. HC-126 union or pattern constraints. Cover int | "*" either with explicit unions (int | string) or a narrower scalar pattern form for sentinel values. This must include monotonicity rules.
  3. HC-127 nested property naming convention. Decide whether flattened keys such as card_min stay a consumer convention or get a core spelling convention for target-path provenance.

Acceptance criteria for any type-depth change

  • RFC and EBNF/Hypercode_Syntax.md syntax updates.
  • Reader/parser tests for good and bad literals.
  • Contract monotonicity tests and HC2104 value-enforcement tests.
  • IR schema update and migration note.
  • A dogfooding example that removes at least one F1/F2 workaround without making .hcs visibly more complex.

Release posture

This is not a 0.6.0 blocker. The current scalar model is honest and usable; the type-depth work should wait until at least two consumers need the same richer shape, so the extension is common infrastructure rather than Ontology-specific schema leakage.