RFC-0121 open rows were incomplete

LIMIT-TYPES-001

Resolved: row extension, reusable row positions, decomposition, and row-conditional implementations are implemented.

Lifecycle

resolved

Owner

metel-frontend

Scope

architecture/spec/type-inference.md#type-inference

Origin

RFC-0121 entering 3-integrated with no implementation yet

Affects · linked records

spec.types.generics.open-rows.legality-1language spec rule

spec.types.generics.open-rows.legality-2language spec rule

spec.types.generics.open-rows.legality-3language spec rule

spec.types.generics.row-conditional-impls.legality-1language spec rule

spec.types.generics.row-conditional-impls.legality-2language spec rule

spec.types.generics.row-conditional-impls.legality-3language spec rule

limitationKnown limitations ↗

Limitation

RFC-0121 is implemented in installments (metel-core#1306), and what is left is the part the Language Spec still states as design.

Implemented: row parameters (record R used as ..R; the separate row kind was removed by RFC-0186); ..R in a record type's tail, a struct's residual-projection tail, generic-argument position and reusable nested/return/field type positions; a where R = { label: Type, ..Rest } decomposition, which bounds R and derives Rest at each call; by-value width subtyping when the row's fields are concretely known at the narrowing site; and row-conditional impls (extend<record R: { .. }> Session<..R>, inherent and aspect) on a nominal target, and aspect impls on a record target (extend<record R: { .. }> { ..R }: A, extend { x: f64 }: A; anonymous and nominal records), with row-versus-row coherence and runtime dispatch.

• A record-target impl with an aspect that takes type parameters, implemented more than once for one record target, resolves as it does for a nominal type; residual dispatch after a partial move is implemented and covered by the RFC-0121 fixture suite.

Impact

At the time, row-polymorphic code over a nominal type worked, including typestate (authenticate / send_data as separate impls), and a local aspect can be implemented for every record of a given shape. The standard library provides the blanket Display and Copy impls for records. Row extension now also permits the tail before newly-added fields, such as { ..R, auth: String }.

Affects

• spec.types.generics.open-rows.legality-1
• spec.types.generics.open-rows.legality-2
• spec.types.generics.open-rows.legality-3
• spec.types.generics.row-conditional-impls.legality-1
• spec.types.generics.row-conditional-impls.legality-2
• spec.types.generics.row-conditional-impls.legality-3

Resolution

Resolved by metel-core#1301, #1306, and #1384. The grammar and reusable-type implementation now accept both { fields, ..R } and { ..R, fields } forms.