Example code to go along with the guardrail internals talk on LanguageAbstraction from https://www.youtube.com/playlist?list=PL5ecYVRYIkSGtui_rZkXNXCRIWlTFdaGr

TypeBag.scala

import java.math.BigInteger
import cats._, cats.implicits._

trait TypeBag {
  type Number
  type String
}

trait HighLevel extends TypeBag {
  type Number = BigInteger
  type String = java.lang.String
}

trait LowLevel extends TypeBag {
  type Number = Byte
  type String = Array[Byte]
}

abstract class Interface[TB <: TypeBag, F[_]] {
  def getInput(): F[TB#String]
  def parseString(input: TB#String): F[List[TB#Number]]
  def pickNumber(nums: List[TB#Number]): F[Option[TB#Number]]
}

implicit object HighLevelInterface extends Interface[HighLevel, cats.Id] {
  def getInput(): String = scala.io.StdIn.readLine("Input: ")
  def parseString(input: String): List[BigInteger] = input.split(',').map(x => new BigInteger(x.trim)).toList
  def pickNumber(nums: List[BigInteger]): Option[BigInteger] = nums.headOption
}

implicit object LowLevelInterface extends Interface[LowLevel, cats.Id] {
  def getInput(): Array[Byte] = {
    println("Enter bytes, terminated by -1")
    @scala.annotation.tailrec
    def consumeAll(acc: Array[Byte]): Array[Byte] = {
      val next = scala.io.StdIn.readByte()
      if (next == -1) acc
      else consumeAll(acc :+ next)
    }
    consumeAll(new Array(0))
  }
  def parseString(input: Array[Byte]): List[Byte] = input.toList
  def pickNumber(nums: List[Byte]): Option[Byte] = nums.headOption
}

object Work {
  def doWork[TB <: TypeBag, F[_]: Monad](implicit I: Interface[TB, F]): F[Option[TB#Number]] = {
    for {
      input <- I.getInput()
      parsed <- I.parseString(input)
      picked <- I.pickNumber(parsed)
    } yield picked
  }
}