add-dto-mapper

name	add-dto-mapper
description	Add a mapper that converts a remote DTO into a data-layer model. Use when asked to "map a dto", "add a mapper", "convert <X>Dto to a model", or whenever a data source's DTO needs turning into the data layer's own model type.

Add a DTO → model mapper

The mapper is the boundary that decouples the data layer's models from the wire format. Past the mapper, nothing knows about DTOs — so a JSON/field change on a DTO ripples no further than its mapper. Scope this skill to one concept's mapper.

Where things live

Thing	Module	Package	Notes
DTO (wire format)	:datasources:remote	...remote.dto	@Serializable, mirrors the JSON
Data model	:data:core	...data.model	plain data class, no serialization annotations
Mapper	:data:core	...data.mappers	internal extension fun Dto.toData()

:data:core depends on :datasources:remote, so the mapper can see the DTO; the DTO never depends on the model (the dependency points one way, inward).

Steps

1. DTO (already exists, or add it)

In :datasources:remote, ...remote.dto — see the add-remote-datasource skill:

@Serializable
data class CharacterDto(val id: Int, val name: String)

2. Data model

:data:core, ...data.model/Character.kt — the data layer's own type, free of any wire concerns:

data class Character(val id: Int, val name: String)

3. Mapper

:data:core, ...data.mappers/CharacterMapper.kt — one extension function per concept. Always name it toData() — every DTO → model mapper follows this one convention, so the call site reads the same everywhere (dto.toData()):

internal fun CharacterDto.toData() = Character(
    id = id,
    name = name,
)

Keep mapping logic total and pure: provide defaults for nullable/absent DTO fields here rather than leaking nullability into the model.

4. Use it at the result boundary

In the repository, feed the mapper into toDataResult (see add-repository):

remoteDatasource.getCharacter(id).toDataResult { it.toData() }

Verify

./gradlew :data:core:compileDebugSources --console=plain

(Set JAVA_HOME to a JDK 21 — e.g. an SDKMAN *-jbr — if the shell can't find Java.)

Checklist

• DTO is in ...remote.dto and @Serializable.
• Data model is a plain data class in ...data.model.
• Mapper is an internal extension fun named toData() in ...data.mappers, one per concept.
• Mapping is total/pure — no nullable leakage into the model.
• :data:core:compileDebugSources succeeds.