Defining Methods on Records

Extension Methods

You can define methods on records by defining extension methods in a usual manner.

object Extension {
  extension (p: % { val name: String; val age: Int }) {
    def firstName: String = p.name.split(" ").head
  }
}

import Extension.firstName

val person = %(name = "tarao fuguta", age = 3)
// person: % {
//   val name: String
//   val age: Int
// } = %(name = tarao fuguta, age = 3)

person.firstName
// res0: String = "tarao"

As you see, you need to explicitly import the method in this case.

Extension Methods with Tags

Records can be tagged and it enables extension methods to be found automatically.

import com.github.tarao.record4s.Tag

trait Person
object Person {
  extension (p: % { val name: String; val age: Int } & Tag[Person]) {
    def firstName: String = p.name.split(" ").head
  }
}

val person0 = %(name = "tarao fuguta", age = 3)
// person0: % {
//   val name: String
//   val age: Int
// } = %(name = tarao fuguta, age = 3)

val person = person0.tag[Person]
// person: % {
//   val name: String
//   val age: Int
// } & Tag[Person] = %(name = tarao fuguta, age = 3)

person.firstName
// res2: String = "tarao"

The mechanism is that the type of person is an intersection type with Tag[Person], which adds extension methods in object Person to the target of implicit search at the method invocation on person. Without the tag, the method invocation statically fails.

person0.firstName
// error: 
// value firstName is not a member of com.github.tarao.record4s.%{val name: String; val age: Int}
// error: 
// Line is indented too far to the left, or a `}` is missing
// error: 
// Line is indented too far to the left, or a `}` is missing

Methods on Generic Record Types

Extension methods can be generic ⸺ i.e., you can specify the least fields that are necessary for the methods.

trait Person
object Person {
  extension [R <: % { val name: String }](p: R & Tag[Person]) {
    def firstName: String = p.name.split(" ").head
  }
}

val person = %(name = "tarao fuguta", age = 3).tag[Person]
// person: % {
//   val name: String
//   val age: Int
// } & Tag[Person] = %(name = tarao fuguta, age = 3)
person.firstName
// res5: String = "tarao"

val personWithEmail = person + (email = "tarao@example.com")
// personWithEmail: % {
//   val name: String
//   val age: Int
//   val email: String
// } & Tag[Person] = %(name = tarao fuguta, age = 3, email = tarao@example.com)
personWithEmail.firstName
// res6: String = "tarao"