Generate Default Case Class instances with HLists

DefaultFor.scala

package com.trizzle

import java.util.UUID

import shapeless.{::, Generic, HList, HNil, Lazy}

trait DefaultFor[T] {
  val default: T
}

trait LowPriorityDefaults {

  def defaultFor[T](t: T) = new DefaultFor[T] {
    override val default = t
  }

  implicit val defaultForString = defaultFor("")
  implicit val defaultForInt = defaultFor(0)
  implicit val defaultForLong = defaultFor(0L)
  implicit val defaultForUUID = defaultFor(UUID.randomUUID())
  implicit val defaultForDouble = defaultFor(0d)
  implicit val defaultForBool = defaultFor(false)
  implicit def defaultForOption[T] = defaultFor[Option[T]](None)
}

object DefaultFor extends LowPriorityDefaults {

  def apply[T](implicit cp: Lazy[DefaultFor[T]]) = cp.value.default

  implicit def hnilDefault = new DefaultFor[HNil] {
    override val default = HNil
  }

  implicit def hconsDefault[H, T <: HList](implicit headDef: Lazy[DefaultFor[H]], taildDef: Lazy[DefaultFor[T]]) = new DefaultFor[H :: T] {
    override val default = {
      headDef.value.default :: taildDef.value.default
    }
  }

  implicit def classAuxDefault[T,R](implicit gen: Generic.Aux[T,R],  conv: DefaultFor[R]): DefaultFor[T] = new DefaultFor[T] {
    override val default = {
      gen.from(conv.default)
    }
  }
}

Usage.scala

package com.trizzle



case class Person(id: Long, firstName: String, lastName: String, middleName: Option[String])

object TestDefaults {
   val person =  DefaultFor[Person] //Person(0,"","",None)
}

object TestOverrides {
   
   //Provides alternative implementation for DefaultFor[Option[T]]
   //for all [T] that have a DefaultFor in implicit scope 
   implicit def defaultForSome[T](implicit df: DefaultFor[T]): DefaultFor[Option[T]] = new DefaultFor[Option[T]] {
      override val default = Some(df.default) 
   }
  val person =  DefaultFor[Person] //Person(0,"","",Some("")) 
}