Row-typed records

ral records are open-row-polymorphic maps with scoped-label typing (Leijen 2005). [host: String, port: Int] has type [host: String, port: Int | ρ], where the tail variable ρ stands for unknown further fields:

• Field selection unifies the target with [label: α | ρ] and returns α.
• Label mismatch permutes labels past each other into a shared fresh tail (the Rémy 1989 rewrite).
• Open rows compose. A block { |opts| ... } accepts [host: String, port: Int | ρ] and adds or overrides fields without knowing the tail, so record-passing composes.

Duplicate labels are permitted; selection takes the first. Several consequences follow:

• A spread prepends and thus shadows. [...$base, port: 9090] over $base : [host: String | ρ] infers [port: Int, host: String | ρ].
• No restriction operator (Pre / Abs) is needed. The rewrite already preserves the relative order of same-label entries, so shadowing is coherent.
• Records and tags draw on two label alphabets — backtick-prefixed for variants and tag-keyed records, bare for ordinary records — and the two never unify.
• Structural equality of closed records is order-insensitive: [a: 1, b: 2] == [b: 2, a: 1].

Scoped labels and spread shadowing are how ral expresses optionality and defaults at the level of data rather than argument lists — see optionality-via-variants and fixed-arity. Rows type data only, never effects — that refusal is argued against the literature in rows-and-handlers.

Realised in type-inference (row unification by the Rémy rewrite).

Cite: RATIONALE §“Record types and scoped labels”; docs/SPEC.md §2, §6.