How to make aux type compile? - scala

Question

I have the following example with aux type, that does not compile:
trait Foo[A] {
type B
def value: B
}
object Foo {
type Aux[A0, B0] = Foo[A0] { type B = B0}
implicit def footInt = new Foo[Int] {
override type B = String
override def value = "Hey!"
}
implicit def fooString = new Foo[String] {
override type B = Boolean
override def value = true
}
}
import cats._
import cats.implicits._
object App {
def main(args: Array[String]): Unit = {
val x: Id[String] = "Hello"
println(foo(1))
println(foo("Hello"))
println(fooAux(1)(x))
}
def foo[T](t: T)(implicit f: Foo[T]): f.B = f.value
def fooAux[T, R] (t: T) (implicit f: Foo.Aux[T, R], id: Id[R]) : R = f.value
}
the compile complains:
Error:(15, 22) not enough arguments for method fooAux: (implicit f: com.sweetsoft.Foo.Aux[Int,R], implicit id: cats.Id[R])R.
Unspecified value parameter id.
println(fooAux(1)(x))
Error:(21, 55) parameter value id in method fooAux is never used
def fooAux[T, R] (t: T) (implicit f: Foo.Aux[T, R], id: Id[R]) : R = f.value
Error:(4, 23) Unused import
import cats.implicits._
What am I doing wrong?

Answer 1

It compiles with minor modifications:
trait Foo[A] {
type B
def value: B
}
object Foo {
type Aux[A0, B0] = Foo[A0] { type B = B0 }
implicit val footInt = new Foo[Int] {
override type B = String
override def value = "Hey!"
}
implicit def fooString = new Foo[String] {
override type B = Boolean
override def value = true
}
}
object App {
type Id[X] = X
def main(args: Array[String]): Unit = {
implicit val x: Id[String] = "Hello"
println(foo(1))
println(foo("Hello"))
println(fooAux(1))
}
def foo[T](t: T)(implicit f: Foo[T]): f.B = f.value
def fooAux[T, R] (t: T) (implicit f: Foo.Aux[T, R], id: Id[R]): R = id
}
The first error is pretty clear: the second argument list of fooAux needs two arguments, but you pass just a single x.
The second error message is also quite mundane: you don't ever use id.
The rest works almost as-is as soon as you provide an implicit String (I assume that you wanted to make x implicit).