Scala inference on higher kinded types versus evidences

Question

I am trying to provide some typeclasses implementations which involves higher kinded types, as in the following code:

trait Blah[A,B] { def apply(b:B):List[A] } object Blah { implicit def blah1[A,B<:List[A]]:Blah[A,B] = new Blah[A,B]{ def apply(b:B):List[A] = b } implicit def blah2[A,B](implicit ev: B<:<List[A]):Blah[A,B] = new Blah[A,B]{ def apply(b:B):List[A] = b } } object Stuff { def method[A,B](b:B)(implicit blah:Blah[A,B]):List[A] = { blah(b) } method(List(2,3)) }

If I remove the method Blah.blah2 , the compiler fails because it can't resolve an implicit. Why does the second form (using an implicit evidence) works instead, and when it should be used?

sarveshseri · Accepted Answer · 2015-02-27 19:03:50Z

Well... its because it is not able to infer correct types as the first implicit blah1 expects a relation between A and B.

scala> :pa // Entering paste mode (ctrl-D to finish) trait Blah[A,B] { def apply(b:B):List[A] } object Blah { implicit def blah1[A,B<:List[A]]:Blah[A,B] = new Blah[A,B] { def apply(b:B):List[A] = b } } def method[A,B](b:B)(implicit blah:Blah[A,B]):List[A] = { blah(b) } // Exiting paste mode, now interpreting. defined trait Blah defined object Blah method: [A, B](b: B)(implicit blah: Blah[A,B])List[A] scala> method( List( 4, 5) ) <console>:37: error: could not find implicit value for parameter blah: Blah[A,List[Int]] method( List( 4, 5) ) ^

See... its looking for implicit of type Blah[ A, List[ Int ] ]

We just need to help it with types and it will work,

scala> method[ Int, List[ Int] ]( List( 3, 4, 5 ) ) res1: List[Int] = List(3, 4, 5)

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