Serialization

Arrow Core types, such as Either or NonEmptyList, often appear as part of serializable types. We've decided to keep the Core library as slim as possible; in particular we didn't want to depend on particular serialization libraries. As a result, you need a bit of additional ceremony compared to using built-in types, which we discuss in this section.

kotlinx.serialization

If you're using kotlinx.serialization, you need to depend on the arrow-core-serialization with the same version of your arrow-core. Declare your serializable types as usual. However, when one of the fields mentions a type from Arrow Core,

@Serializable
data class Book(val title: String, val authors: NonEmptyList<String>)

you need to "import" the serializer into the file. The easiest way is to include a UseSerializers annotation at the very top.

@file:UseSerializers(
  EitherSerializer::class,
  IorSerializer::class,
  OptionSerializer::class,
  NonEmptyListSerializer::class,
  NonEmptySetSerializer::class
)

The list above mentions all the serializers, but you only need to add those which are used in your fields. Don't worry too much: if you miss one, the kotlinx.serialization plug-in gives you an error.

Additionally, you can use arrow.core.serialization.ArrowModule to register run-time contextual serialization support of the Arrow Core types.

val format = Json { serializersModule = ArrowModule }

or by merging with another serializers module

val format = Json { serializersModule = myModule + ArrowModule }

This will allow for use of the Arrow Core types as the value to be serialized without specifying the serializer explicitly:

val payload = format.encodeToString(nonEmptyListOf("hello", "world"))

note

Using @UseSerializers to provide static compile-time serialization of fields containing Arrow Core types is usually preferable to annotating fields with @Contextual and relying on run-time resolution, wherever possible.

Additional reading

Marshalling Arrow Types in Ktor by Garth Gilmour includes additional information about using Arrow Core types in a Ktor project.

Jackson

If you're using Jackson for serialization, this module adds support for Arrow types. You just need to call an additional method when creating the mapper.

val mapper = ObjectMapper()
    .registerKotlinModule()
    .registerArrowModule()   // <- this is the one