Implicit parameter precedence

Question

I was trying to convert generic type to HList:

trait ToHList[T] { type Out <: HList def apply(value: T): Out } trait LowPriorityToHList { implicit def default[T]: ToHList.Aux[T, T :: HNil] = new ToHList[T] { override type Out = T :: HNil override def apply(value: T): T :: HNil = value :: HNil } } object ToHList extends LowPriorityToHList { type Aux[T, Out0] = ToHList[T] { type Out = Out0 } def apply[T](implicit toHList: ToHList[T]): Aux[T, toHList.Out] = toHList implicit def toHList[T, Repr <: HList, N <: Nat](implicit gen: Generic.Aux[T, Repr], len: Length.Aux[Repr, N], lt: LT[Nat._0, N]): ToHList.Aux[T, Repr] = new ToHList[T] { override type Out = Repr override def apply(value: T): Repr = gen.to(value) } } object Main extends App { println(ToHList.apply[Int].apply(1)) // expected 1 :: HNil println(ToHList.apply[(Int, Int)].apply((1, 2))) // expected 1 :: 2 :: HNil }

I intended to give priority to ToHList.toHList over ToHList.default but this code cause following compilation error:

[error] ToHList.scala:39:24: ambiguous implicit values: [error] both method toHList in object ToHList of type [T, Repr <: shapeless.HList, N <: shapeless.Nat](implicit gen: shapeless.Generic.Aux[T,Repr], implicit len: shapeless.ops.hlist.Length.Aux[Repr,N], implicit lt: shapeless.ops.nat.LT[shapeless.Nat._0,N])ToHList.Aux[T,Repr] [error] and method default in trait LowPriorityToHList of type [T]=> ToHList.Aux[T,T :: shapeless.HNil] [error] match expected type ToHList[(Int, Int)] [error] println(ToHList.apply[(Int, Int)].apply((1, 2))) // expected 1 :: 2 :: HNil

I want to give priority to ToHList.toHList over ToHList.default. How can I fix this error?

Dmytro Mitin · Accepted Answer · 2021-06-19 18:37:31Z

If both toHList and default are applicable then they have the same priority, so they make ambiguity. Indeed, although default is defined in a low-priority super-trait but it's more specific than toHList. See details in Why is this implicit ambiguity behaviour happening?

So there is no reason to put default into a low-priority super-trait, this will not make desired impact. But if you put toHList and default into the same object, default will win as more specific. And from expected 1 :: 2 :: HNil it seems you want vice versa toHList to win. You can use shapeless.LowPriority

object ToHList { type Aux[T, Out0] = ToHList[T] { type Out = Out0 } def apply[T](implicit toHList: ToHList[T]): Aux[T, toHList.Out] = toHList implicit def toHList[T, Repr <: HList, N <: Nat](implicit gen: Generic.Aux[T, Repr], len: Length.Aux[Repr, N], lt: LT[Nat._0, N] ): ToHList.Aux[T, Repr] = new ToHList[T] { override type Out = Repr override def apply(value: T): Repr = gen.to(value) } implicit def default[T](implicit lowPriority: LowPriority ): ToHList.Aux[T, T :: HNil] = new ToHList[T] { override type Out = T :: HNil override def apply(value: T): T :: HNil = value :: HNil } }

Alternatively in this specific case you can use shapeless.Refute, shapeless.OrElse

implicit def default[T](implicit orElse: OrElse[Refute[Generic[T]], Generic.Aux[T, HNil]] ): ToHList.Aux[T, T :: HNil] = new ToHList[T] { override type Out = T :: HNil override def apply(value: T): T :: HNil = value :: HNil }

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