How can I use implicit conversions for two types that resolve to the same base type?

Question

type JSON = String
type YAML = String

implicit def json2yaml(in:JSON):YAML = "some conversion"

val js:JSON = "some json"
val yml: YAML = js

The code above assigns "some json" to yml. The implicit conversion is not used. Is that because both YAML and JSON resolve to String? Is there a way to make encourage the code to pick up the conversion?

Answer 1

Note that relationship between type and its alias is that of equivalence

If 𝑡 is defined by a type alias type 𝑡 = 𝑇, then 𝑡 is equivalent to 𝑇.

which means 𝑡 and 𝑇 are interchangeable in all contexts. When we define alias

type JSON = String

then String is not a "base" type of JSON, instead it is JSON itself. Thus

implicit def json2yaml(in: JSON): YAML = "some conversion"

is equivalent to

implicit def json2yaml(in: String): String = "some conversion"

which is why implicit conversion does not happen.

Instead of implicit conversion (which tends to be discouraged) consider extension method approach like so

case class Json(v: String)
case class Yaml(v: String)

implicit class JsonToJaml(json: Json) {
  def asYaml: Yaml = Yaml("some conversion which uses ${json.v}")
}

val js: Json = Json("some json")
val yml: Yaml = js.asYaml