How can I use Shapeless to create a function abstracting over arity

Question

Let\'s consider a specific example. I have lots of functions that take a variable number of arguments, and return a Seq[T]. Say:

def nonNeg(start: I         


        

Answer 1

For 2 and more (in code below to 4) parameters you can use implicit parameters feature, for resolve result type by input parameter type

sealed trait FuncRes[F] {
  type Param
  type Result
  def func : F => Param => Result
}

class Func[T, R](fn : T => R) {
  trait FR[F, P] extends FuncRes[F] { type Param = P; type Result = R }

  implicit def func2[T1,T2] = new FR[(T1,T2) => T, (T1,T2)] {
    def func = f => p => fn(f.tupled(p))
  }

  implicit def func3[T1,T2,T3] = new FR[(T1,T2,T3) => T, (T1,T2,T3)] {
    def func = f => p => fn(f.tupled(p))
  }

  implicit def func4[T1,T2,T3,T4] = new FR[(T1,T2,T3,T4) => T, (T1,T2,T3,T4)] {
    def func = f => p => fn(f.tupled(p))
  }

  def makeFunc[F](f : F)(implicit ev : FuncRes[F]): ev.Param => ev.Result = 
    ev.func(f)
}

and after your def javaNonNeg = makeJava(nonNeg) function will look like:

object asJavaFunc extends Func((_ : Seq[Int]).asJava)  
import asJavaFunc._

def javaNonNeq = makeFunc(nonNeg _)  

And of course it has some disadvantages, but generally it satisfy your needs.