A mapped scalar's host type is named per language

Status: accepted, 2026-10-07; amended 2026-10-07, the Kotlin entry's shape. Serves The compiler decides and Two runtimes, one compiler. Built 2026-10-07. Reopen if Relay's customScalarTypes takes a shape of its own for more than one language.

Context#

config.rs documents customScalarTypes as "the Swift type a custom scalar reads as". The lowering puts that Swift type on the plan IR: TypePlan::Named.mapped in pipeline/plan.rs is "the Swift type a custom scalar reads as", filled in pipeline/lower.rs from the configuration. The IR is meant to be the seam no language owns: pipeline/plan.rs opens by saying nothing after it sees a Relay type, and a Kotlin emitter reads the same IR.

The operation's onError has the same leak. OperationPlan.error_behavior is a string filled from OnError::swift_case, the case of Baton.ErrorBehavior that names the value.

Decision#

• customScalarTypes keeps Relay's key. A value is either a string, the Swift type as today, or an object by language, for example {"swift": "Foundation.Decimal", "kotlin": {"type": "java.math.BigDecimal", "converter": "baton.scalars.Decimals"}}.
• A language's entry has the shape that language needs. Swift's is the type, which conforms to MappedScalar and so parses and renders itself. Kotlin's is the type and the converter: a Kotlin type the runtime does not own cannot implement an interface, so an object implementing ScalarConverter<T> parses and renders it, and the compiler must be told the type, since nothing about a converter object names it.
• The plan carries the scalar's schema name, and each language's writer resolves the host type from the configuration.
• OperationPlan.error_behavior carries the onError value, not a Swift case name.
• The goldens prove each step: the Swift written does not change by a byte.

Evidence#

• The Kotlin emitter's first milestone, 2026-10-07: a constructor parameter of an input object or an operation needs its type, and a converter object named alone gives the compiler nothing to print for it; the entry became the type and the converter together.

• The compiler as built, by reading: the doc comment of custom_scalar_types in compiler/src/config.rs; mapped on TypePlan::Named in compiler/src/pipeline/plan.rs, and its doc comment; the lowering that fills it from self.config.custom_scalar_types in compiler/src/pipeline/lower.rs; error_behavior: Option<String> on the operation's plan, filled by behavior.swift_case() in the same file; OnError::swift_case in config.rs.

• The seam as stated: the module comment of pipeline/plan.rs.

Not chosen#

• A second key per language: Relay has one key, and a web project already has it.
• The Swift type on the IR, with the Kotlin writer translating it: a Swift name is not a fact of the schema, and the translation would be a table from one language's types to another's.