A written function type cannot say it accepts any record with at least these fields

GAP-TYPES-011

An open record with an anonymous tail is rejected inside a function type, and a named row fixes the row for the whole signature, so no written type means 'works for every wider record'.

Lifecycle

accepted

Owner

language

Scope

reference/spec/types.md#open-rows

Origin

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

Review

revisit if a real function must apply one callback to two differently shaped records it holds

Affects · linked records

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

RFC-0181rfc

gapKnown gaps ↗

Gap

|{ x: i64, .. }| -> i64 is rejected with T0032. Read as a fresh row of the enclosing signature it would let the caller choose the row, so the callee could not call the function on records of its own; read as "accepts every wider record" it is a rank-2 type, which the checker does not model. A named row, |{ x: i64, ..R }| -> i64, is supported and fixes R for the signature.

A closure with an open-row parameter already generalizes, so a value of the rank-2 kind exists (one closure accepts records of different shapes); only its written type does not.

Impact

A callback over records of one shape is written with a shared R and works. A function that must apply one callback to differently shaped records it holds itself cannot say so: it takes two callbacks, or is generic over two rows.

Affects

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

Resolution

Accepted by RFC-0181. A rank-2 function type needs higher-rank checking for a body that calls a parameter of that type, which Metel has no other use for. Reopen when a real function needs it.