Scala cats and traverse syntax for Either - doesn't compile

Question

I am trying to use traverse (or sequence which is pretty much the same for my task) from cats library https://typelevel.org/cats/typeclasses/traverse.html . I want to traverse a List[A] with function A => Either[L,R] to get Either[L,List[R]] as a result.

Consider the following tiny example (I use scala-2.12.6, cats-core-1.3.1, sbt-1.1.2):

import cats.implicits._

def isOdd(i: Int): Either[String, Int] = 
  if (i % 2 != 0) Right(i) else Left("EVEN")

val odd: Either[String, List[Int]] = (1 to 10).toList.traverse(isOdd)

It doesn't compile, it gives:

no type parameters for method traverse: (f: Int => G[B])(implicit evidence$1: cats.Applicative[G])G[List[B]] exist so that it can be applied to arguments (Int => Either[String,Int])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : Int => Either[String,Int]
[error]  required: Int => ?G[?B]
[error]     val odd: Either[String, List[Int]] = (1 to 10).toList.traverse(isOdd)

type mismatch;
[error]  found   : Int => Either[String,Int]
[error]  required: Int => G[B]
[error]     val odd: Either[String, List[Int]] = (1 to 10).toList.traverse(isOdd)
[error]                                                                   ^

could not find implicit value for evidence parameter of type cats.Applicative[G]
[error]     val odd: Either[String, List[Int]] = (1 to 10).toList.traverse(isOdd)
[error]                                                                   ^

Answer 1

The Partial Unification compiler flag is required. In scala 2.12

add scalacOptions += "-Ypartial-unification" in build.sbt

Thanks Thomas