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scalashapeless

Tagged Type vs class extends AnyVal

To introduce more type safety, we can either use tagged type provided by shapeless or create a class which extends AnyVal. What are the differences and advantage/disadvantage to use one over the other?

Example:

trait CountryCodeTag
type CountryCode = String @@ CountryCodeTag

class CountryCode(code: String) extends AnyVal
Solution

  • type CountryCode = String @@ CountryCodeTag

    + String @@ CountryCodeTag is a subtype of String, i.e. all methods from String can be used directly: countryCode.toUpperCase.

    − String @@ CountryCodeTag can be accidentally used where some String is expected, i.e. it's less type-safe.

    − Creating new values is a little awkward: "a".asInstanceOf[String @@ CountryCodeTag] or val tagger = new Tagger[CountryCodeTag]; tagger("a").

    − Dependence on Shapeless (although this can be done manually).

    class CountryCode(code: String) extends AnyVal

    + It's more type-safe.

    − Methods from String are available with some extra efforts:

    class CountryCode(val code: String) extends AnyVal
new CountryCode(countryCode.code.toUpperCase)

    or

    class CountryCode(val code: String) extends AnyVal 
object CountryCode {
  def unapply(...) = ...
}
countryCode match { case CountryCode(code) => new CountryCode(code.toUpperCase) }

    or

    case class CountryCode(code: String) extends AnyVal
countryCode.copy(code = countryCode.code.toUpperCase)

    + Creating new values is a little more natural: new CountryCode("a").

    + No extra dependencies (it's plain Scala).