In scala shapeless library, is it possible to write a generic arity function when the arity > 22 (presumably using one of shapeless macros)?

Question

The following code is a typical demo of one of shapeless' use case:

  def getHList[P <: Product, F, L <: HList](p: P)(implicit gen: Generic.Aux[P, L]): L = {
    gen.to(p)
  }

    val v = getHList(1, 2, 3, 4, 5, 6, 7, 8, 9)

This gives the proper result, unfortunately, it relies on scala's tuple syntactic suger, and doesn't work when the number of argument > 22:

    val v = getHList(1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0)

(this generates an error that looks this the follow)

[Error] /xxx/HListSuite.scala:41: 29 more arguments than can be applied to method getHList: (p: P)(implicit gen: shapeless.Generic.Aux[P,L])L
one error found

FAILURE: Build failed with an exception.

I wonder if there is a macro or another scala feature I can use to break this limitation, any advice?

I'm using scala 2.12.8 but can upgrade to 2.13 at any time.

Answer 1

• If your goal is to generate an HList longer than 22 then there are plenty of ways

  type _23 = Succ[_22]
  val _23: _23 = new _23
  
  1 :: 2 :: 3 :: 4 :: 5 :: 6 :: 7 :: 8 :: 9 :: 10 :: 11 :: 12 :: 13 :: 14 :: 15 :: 16 :: 17 :: 18 :: 19 :: 20 :: 21 :: 22 :: 23 :: HNil
  import shapeless.syntax.std.traversable._
  import shapeless.ops.hlist.Fill
  (1 to 23).toHList[the.`Fill[_23, Int]`.Out]
  (1 to 23).toSizedHList(_23)
  implicitly[_1 *--* _23].apply //takes long
  

  Be careful, some of these calculations take long.

• Also you can define Product23, Tuple23

  getHList(Tuple23(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23))
  

  although syntax sugar just with brackets will not work. The name of case class isn't significant, it can be MyClass instead of Tuple23.

• In Dotty there is TupleXXL.

• If your goal is to write a method like getHList you can try to make it curried

  //libraryDependencies += "com.github.dmytromitin" %% "auxify-macros" % "0.6", scalacOptions += "-Ymacro-annotations" (in 2.13)
  import com.github.dmytromitin.auxify.macros.{aux, instance}
  
  def curriedGetHList[N <: Nat] = new PartiallyAppliedCurriedGetHList[N]
  
  class PartiallyAppliedCurriedGetHList[N <: Nat] {
    def apply[A](a: A)(implicit cghl: CurriedGetHList[N, A]): cghl.Out = cghl(a)
  }
  
  @aux @instance
  trait CurriedGetHList[N <: Nat, A] {
    type Out
    def apply(a: A): Out
  }
  object CurriedGetHList {
    implicit def mkCurriedGetHList[N <: Nat, A]
    (implicit
     helper: CurriedGetHListHelper[N, A, A :: HNil]
    ): Aux[N, A, helper.Out] = instance(a => helper(a :: HNil))
  }
  
  @aux @instance
  trait CurriedGetHListHelper[N <: Nat, A, L <: HList] {
    type Out
    def apply(acc: L): Out
  }
  object CurriedGetHListHelper {
    implicit def one[A, L <: HList]
    (implicit reverse: Reverse[L]): Aux[_1, A, L, reverse.Out] = instance(acc => reverse(acc))
  
    implicit def succ[N <: Nat, A, L <: HList]
    (implicit helper: Lazy[CurriedGetHListHelper[N, A, A :: L]]): Aux[Succ[N], A, L, A => helper.value.Out] =
      instance(acc => a => helper.value(a :: acc))
  }
  
  curriedGetHList[_10](1).apply(2)(3)(4)(5)(6)(7)(8)(9)(10)
  

  or

  def curriedGetHList[N <: Nat] = new HListBuilder[N, HNil](HNil)
  
  class HListBuilder[N <: Nat, L <: HList](l: L) {
    def apply[A](a: A)(implicit bhl: BuildHList[N, L, A]): bhl.Out = bhl(l, a)
  }
  
  @aux @instance
  trait BuildHList[N <: Nat, L <: HList, A] {
    type Out
    def apply(l: L, a: A): Out
  }
  trait LowPriorityBuildHList {
    implicit def succ[N <: Nat, L <: HList, A]: BuildHList.Aux[Succ[N], L, A, HListBuilder[N, A :: L]] =
      BuildHList.instance((l, a) => new HListBuilder(a :: l))
  }
  object BuildHList extends LowPriorityBuildHList {
    implicit def one[L <: HList, A](implicit reverse: Reverse[A :: L]): Aux[_1, L, A, reverse.Out] =
      instance((l, a) => (a :: l).reverse)
  }
  
  curriedGetHList[_23](1).apply(2).apply(3).apply(4).apply(5).apply(6).apply(7).apply(8).apply(9).apply(10)
    .apply(11).apply(12).apply(13).apply(14).apply(15).apply(16).apply(17).apply(18).apply(19).apply(20)
    .apply(21).apply(22).apply(23)
  

• Or you can divide long tuple into tuple of tuples and use deep Generic (1 2 3 4).

• One more option is to write a whitebox macro

  import scala.language.experimental.macros
  import scala.reflect.macros.whitebox
  
  def getHList(xs: Any*): HList = macro getHListImpl
  def getHListImpl(c: whitebox.Context)(xs: c.Tree*): c.Tree = {
    import c.universe._
    xs.foldRight[Tree](q"_root_.shapeless.HNil: _root_.shapeless.HNil")((h, t) => q"_root_.shapeless.::($h, $t)")
  }
  
  getHList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)
  

  Since the macro is whitebox its return type will be proper, Int :: Int :: ... :: HNil.

• Maybe the easiest is to use shapeless.ProductArgs

  def getHList = new ProductArgs {
    def applyProduct[L <: HList](l: L): L = l
  }
  
  getHList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)
  

  Actually ProductArgs are implemented in Shapeless via scala.Dynamic and whitebox macro.