builder.sc

//> using dep "com.github.tarao::record4s:0.11.2"

import com.github.tarao.record4s.%
import com.github.tarao.record4s.typing.Record.Append
import scala.util.NotGiven as !

type NameField = %{ val name: String }
type AgeField = %{ val age: Int }
type OsField = %{ val os: String }

type ShouldNotHave[R, Field] = ![R <:< Field]

extension [R <: %](p: R) {
  def withName[RR <: %](name: String)(using
    R ShouldNotHave NameField,
    Append.Aux[R, NameField, RR]
  ): RR = p + name
  def withAge[RR <: %](age: Int)(using
    R ShouldNotHave AgeField,
    Append.Aux[R, AgeField, RR]
  ): RR = p + age
  def withOs[RR <: %](os: String)(using
    R ShouldNotHave OsField,
    Append.Aux[R, OsField, RR]
  ): RR = p + os
}

val e = %()

val fulfilled = e.withName("windymelt").withAge(30).withOs("openSUSE")
fulfilled.name
fulfilled.age
fulfilled.os

pretty-syntax.sc

//> using dep "com.github.tarao::record4s:0.11.2"

import com.github.tarao.record4s.%
import com.github.tarao.record4s.typing.Record.Concat
import scala.util.NotGiven as !

type NameField = %{ val name: String }
type AgeField = %{ val age: Int }
type OsField = %{ val os: String }

type ShouldNotHave[R, Field] = ![R <:< Field]

type ++[R1, R2] = Concat[R1, R2]

type :=[Out0 <: %, A] =
  A match {
    case Concat[r1, r2] =>
      Concat[r1, r2] { type Out = Out0 }
  }

extension [R <: %](p: R) {
  def withName[RR <: %](name: String)(using
    R ShouldNotHave NameField,
    RR := R ++ NameField,
  ): RR = p + name
  def withAge[RR <: %](age: Int)(using
    R ShouldNotHave AgeField,
    RR := R ++ AgeField,
  ): RR = p + age
  def withOs[RR <: %](os: String)(using
    R ShouldNotHave OsField,
    RR := R ++ OsField,
  ): RR = p + os
}

val e = %()

val fulfilled = e.withName("windymelt").withAge(30).withOs("openSUSE")
fulfilled.name
fulfilled.age
fulfilled.os

https://twitter.com/windymelt/status/1753437923868033233