A record or struct field cannot have an open-record type with an anonymous tail

GAP-TYPES-012

An anonymous open record is rejected as a field type, so a field cannot hold 'any record with at least these fields'.

Lifecycle

accepted

Owner

language

Scope

reference/spec/types.md#open-rows

Origin

metel-core#1478 review of anonymous open-record tails

Review

revisit together with GAP-TYPES-010 if existential rows are ever added

Affects · linked records

spec.types.generics.open-rows.legality-5language spec rule

RFC-0181rfc

gapKnown gaps ↗

Gap

record W { d: { x: i64, .. } } is rejected with T0032. The field's type would be existential, so two values of one nominal type could hide different rows, which does not fit a nominal type. The same applies to the anonymous splice (record W { s: Session<..> }).

Impact

A field cannot hold an arbitrary wider record. The use is covered by a row parameter on the record, record W<row R> { d: { x: i64, ..R } }, at the cost of making W generic over the row.

Affects

• spec.types.generics.open-rows.legality-5
• RFC-0181

Resolution

Accepted by RFC-0181. This would follow from an existential row (GAP-TYPES-010), not stand alone.