martin-kolinek/scala-deriving

Simple macro which allows for easy creation of type class instances based on existing instances.

This is inspired by newtype ... deriving ... syntax from haskell.

Example:

trait IsCat[T] {
    def meow(cat:T):String
    def mate(cat1:T, cat2:T):T
}

class Cat(val name:String) {
}

//this imitates the newtype
case class CatInABox(cat:Cat) {
} 

implicit val catIsCat = new IsCat[Cat] {
    def meow(cat:Cat) = "meow"
    def mate(cat1:Cat, cat2:Cat) = new Cat(s"${cat1.name} junior")
}

//here we create a derived instance based on existing instance
implicit val catInABoxIsCat = deriving[CatInABox, IsCat].equiv(_.cat, CatInABox)

val cat = CatInABox(new Cat("cat"))

implicitly[IsCat[CatInABox]].meow(cat)