Feature f008 — Write tier: computed updates (`|=`)

Status: Draft (stub — gated on f001 M2) Epic: Fidelity write tier (f006–f008) Owner: yqr maintainers Related: yqr-f006 (write tier v1 — the assignment/-i core this extends), yqr.f001 M2 (arithmetic + builtins — the right-hand evaluator this needs), yqr-m002 §4/§6.2 (write-tier seam)

Stub. This feature cannot be built until the expression evaluator exists, so it is a roadmap marker only. It is scoped here to keep the epic’s shape complete; detail is deferred until f001 M2 lands.

1. Scope

jq’s update-assignment operator: p |= f, which sets the node at path p to the result of running filter f on its current value — e.g.:

  • .spec.replicas |= (. + 1) (increment)
  • .image |= sub("nginx:.*"; "nginx:1.28") (string transform)
  • .metadata.labels.env |= ascii_upcase

Unlike f006’s = (whose RHS is a literal or a copied path), |= computes the new value from the old one, so it requires the expression evaluator — arithmetic, comparisons, and the string/collection builtins delivered by f001 M2.

2. Dependency

Hard-gated on f001 M2 (arithmetic/builtins). Until then, f006 rejects |= with a “not yet supported” message. Once M2 exists, |= reuses:

  • f006’s path → span → set_value write path and its re-parse guard, and
  • the M2 evaluator to compute the RHS value from the selected node.

3. Structural-integrity contract

Identical to f006 §7 — the computed value is written via the guarded set_value, so only the target node’s bytes change or the edit is refused.

4. Acceptance criteria (outline)

  • .n |= (. + 1) rewrites the scalar, byte-exact elsewhere.
  • A |= whose RHS filter errors leaves the document unchanged.
  • Style-matched quoting via set_value for computed string results.

(Criteria firm up alongside the f001 M2 evaluator.)