I recently implemented a utility function for Shapeless:
trait IndexOf[X, L <: HList] extends DepFn0 with Serializable { type Out <: Nat }
object IndexOf {
def apply[X, L <: HList](implicit indexOf: IndexOf[X, L]): Aux[X, L, indexOf.Out] = indexOf
type Aux[X, L <: HList, Out0] = IndexOf[X, L] { type Out = Out0 }
implicit def indexOfHead[X, T <: HList]: Aux[X, X :: T, _0] =
new IndexOf[X, X :: T] {
type Out = _0
def apply() = Nat._0
}
implicit def indexOfOthers[X, H, T <: HList, I <: Nat]
(implicit indexOf: IndexOf.Aux[X, T, I]): Aux[X, H :: T, Succ[I]] =
new IndexOf[X, H :: T] {
type Out = Succ[I]
def apply() = Succ[I]
}
}
This is so verbose. I would definitely like to have something like pattern matching in Scala type definition, like something below:
type IndexOf[X, L <: HList] = (X, L) match {
case (X, X :: T) ==> _0
case (X, H :: T) ==> Succ[IndexOf[X, T]]
}
(=> cannot be used because it itself is a type. Here I used ==> as a placeholder)
Another example, the Pred on Nats. It’d be very nice to have
type Pred[N <: Nat] = N match {
case Succ[P] ==> P
}
And this, after compilation, became all these implicit defs (each case clause is compiled into one implicit def). So this is just a syntactic sugar over the current implicit defs.