问题
Here is a simple example:
trait Base {
type Out
def v: Out
}
object Base {
type Aux[T] = Base { type Out = T }
class ForH() extends Base {
type Out = HNil
override def v: Out = HNil
}
object ForH extends ForH
}
class TypeClass[B]
trait TypeClassLevel1 {
def summon[B](b: B)(implicit ev: TypeClass[B]): TypeClass[B] = ev
}
object TypeClass extends TypeClassLevel1 {
implicit def t1: TypeClass[Base.Aux[HNil]] = new TypeClass[Base.Aux[HNil]]
implicit def t2: TypeClass[Int] = new TypeClass[Int]
}
it("No Aux") {
val v = 2
TypeClass.summon(v) // works
}
it("Aux") {
val v = new Base.ForH()
TypeClass.summon(v) // oops
TypeClass.summon(Base.ForH) // oops
val v2 = new Base.ForH(): Base.Aux[HNil]
TypeClass.summon(v2) // works!
}
The object Base/ForH clearly has a stable path, this eliminate the possibility of the compiler not being able to resolve type ForH.Out
.
What bothers me is not how incapable the compiler is to figure out the fact that ForH <:< Aux[HNil]
, but how easy it is to patch it up, just by a simple type upcast (last 2 lines). IMHO both features (type lambda & type classes) are important aspect of functional programming, why they can't work together at the same time?
If you are familiar with the compiler design I'll have an extra question: what does it take to improve the type class search algorithm to make it happen? Thanks a lot for your opinion.
UPDATE 1: a specific fix has been proposed but I have another problem trying to generalise it, please see In scala, how to make type class working for Aux pattern? - Part 2 for detail
回答1:
So the compiler is able to infer ForH <:< Aux[HNil]
, but (tbh I don't know exactly why) when resolving an implicit when the return type uses a type lambda, it gets confused if you don't use a type bound.
Anyway that's probably not a great explanation, but at least I can make your code compile. Just change t1
to:
implicit def t1[T <: Base.Aux[HNil] ]: TypeClass[T] = new TypeClass[T]
this works for me in scastie using Scala 2.13.4.
来源:https://stackoverflow.com/questions/65838535/in-scala-how-to-make-type-class-working-for-aux-pattern