Record Display is a runtime built-in, not Metel code

LIMIT-TYPES-003

Display for a record of Display fields is a built-in in the evaluator, a stand-in for RFC-0174's comptime for; no user aspect, and no Eq/Hash, can walk a record's fields; Clone is likewise a built-in.

Lifecycle

planned

Owner

metel-interpreter

Scope

architecture/spec/evaluation.md#evaluation

Origin

metel-core#1338: implementing Display for records after RFC-0123's where all R: Display landed

Affects · linked records

spec.types.generics.field-wise-row-constraints.legality-1language spec rule

spec.types.anonymous-records.legality-1language spec rule

record_to_string metel-interpreter/src/evaluator/record_display.rscode

limitationKnown limitations ↗

Limitation

std::core declares extend<row R> { ..R }: Display where all R: Display, so every record whose fields are all Display is Display: anonymous and nominal records, nested records, and records with fields whose types have a Display impl of their own. The impl's body is not Metel code. Nothing in the language can iterate a row's fields, so the method is a native whose host function (metel-interpreter/src/evaluator/record_display.rs, intercepted by label in call_runtime_callable because it needs the runtime to call each field's own to_string) formats the receiver at runtime, fields in label order:

let p := { x = 1, y = 2.5, s = "hi" };
println("${p}");        // { s: hi, x: 1, y: 2.5 }

The condition is checked statically (where all R: Display; a field without Display is T0012 naming the field). Only the body is built in. Consequences:

• The format and the aspect are fixed in the evaluator. There is no way to write a different blanket impl, or the same impl for another aspect.
• No other aspect gets this for free. Eq, Hash and Default for records, and any user aspect (a serializer, a schema generator), cannot walk a record's fields. Copy needs no body: std::core provides it as an ordinary bodyless impl, so it is not affected by this limitation (metel-core#1337).
• A nominal record's own Display still wins, by brand-first dispatch (spec.types.generics.row-conditional-impls.legality-2).

Impact

Visible to Metel programmers: records print and compare through Display where they could not before, but a user cannot write the same kind of field-walking impl for their own aspect. Contributors: the built-in is a special case that every further derived aspect would have to repeat in Rust.

Affects

• spec.types.generics.field-wise-row-constraints.legality-1
• spec.types.anonymous-records.legality-1

• metel-interpreter/src/evaluator/record_display.rs::record_to_string

Resolution

Planned: RFC-0174 (1-under-review, design settlement metel-core#1340) specifies a comptime for loop over a row's fields with computed access self.[field.name], checked once against a symbolic field. When it is implemented the std::core impl is rewritten in Metel with that loop and record_display.rs, the label interception and the StdCoreRecordToString native key are deleted. Until then this record stays open; the implementation of Display itself is metel-core#1338.