Custom types¶
To support a custom type, you’ll need to provide an implicit Codec
for that type.
This can be done by writing a codec from scratch, mapping over an existing codec, or automatically deriving one. Which of these approaches can be taken, depends on the context in which the codec will be used.
Providing an implicit codec¶
To create a custom codec, you can either directly implement the Codec
trait, which requires to provide the following
information:
encode
andrawDecode
methods- codec meta-data (
CodecMeta
) consisting of:- schema of the type (for documentation)
- validator for the type
- media type (
text/plain
,application/json
etc.) - type of the raw value, to which data is serialised (
String
,Int
etc.)
This might be quite a lot of work, that’s why it’s usually easier to map over an existing codec. To do that, you’ll need to provide two mappings:
- an
encode
method which encodes the custom type into the base type - a
decode
method which decodes the base type into the custom type, optionally reporting decode errors (the return type is aDecodeResult
)
For example, to support a custom id type:
def decode(s: String): DecodeResult[MyId] = MyId.parse(s) match {
case Success(v) => DecodeResult.Value(v)
case Failure(f) => DecodeResult.Error(s, f)
}
def encode(id: MyId): String = id.toString
implicit val myIdCodec: Codec[MyId, TextPlain, _] = Codec.stringPlainCodecUtf8
.mapDecode(decode)(encode)
Note that inputs/outputs can also be mapped over. However, this kind of mapping is always an isomorphism, doesn’t allow any validation or reporting decode errors. Hence, it should be used only for grouping inputs or outputs from a tuple into a custom type.
Automatically deriving codecs¶
In some cases, codecs can be automatically derived:
Automatic codec derivation usually requires other implicits, such as:
- json encoders/decoders from the json library
- codecs for individual form fields
- schema of the custom type, through the
SchemaFor[T]
implicit
For case classes types, SchemaFor[_]
values are derived automatically using Magnolia, given
that schemas are defined for all of the case class’s fields. It is possible to configure the automatic derivation to use
snake-case, kebab-case or a custom field naming policy, by providing an implicit tapir.generic.Configuration
value:
implicit val customConfiguration: Configuration =
Configuration.default.withSnakeCaseMemberNames
Alternatively, SchemaFor
values can be defined by hand, either for whole case classes, or only for some of its fields.
For example, here we state that the schema for MyCustomType
is a String
:
implicit val schemaForMyCustomType: SchemaFor[MyCustomType] = SchemaFor(Schema.SString)
Next¶
Read on about validation.