.. is not supported in an anonymous record pattern

LIMIT-TYPE-INFERENCE-003

A pattern ending in .. is rejected for a concrete anonymous record; name every field.

Lifecycle

known

Owner

metel-frontend

Scope

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

Origin

metel-core#1218 limitation analysis; typechecker/inference/patterns.rs

Affects · linked records

arch.type-inference.requirement-1architecture requirement

metel-frontend/src/pipeline/type_checking/inference/patterns.rscode path

infer_pattern metel-frontend/src/pipeline/type_checking/inference/patterns.rscode

limitationKnown limitations ↗

Limitation

A pattern that ends in .. is accepted by the grammar (RecordRest) and by the parser, but the typechecker rejects it when the scrutinee is a concrete anonymous record. Reproduced against current develop:

fun main() {
    let r := { x = 1, y = 2 };
    match (r) {
        { x, .. } => { assert(x == 1); },
    }
}

[T0001] type error: `..` is not yet supported in an anonymous record pattern -- name every field, or match a named struct instead

The row-bounded generic case (fun f<record T: { x: f64, .. }>(p: T)) is handled separately and works.

Impact

Visible to Metel programmers: partial matching of an anonymous record needs every field named. The Language Spec specifies .. for struct patterns and for row-bounded records but is silent on a concrete anonymous record, so the spec text should say which way this goes (a follow-up); until then this is recorded as an implementation limitation.

Affects

• arch.type-inference.requirement-1
• metel-frontend/src/pipeline/type_checking/inference/patterns.rs

• metel-frontend/src/pipeline/type_checking/inference/patterns.rs::infer_pattern

Resolution

None yet. If the spec decides .. is not allowed there, this record is re-filed as a GAP-* (ADR-0057) and superseded.