Implicits resolution with Scala-cats

Question

Example is taken from Tagless Final in Mastering Functional Programming:

trait Capabilities[F[_]] {
  def resource(name: String): F[String]
  def notify(target: String, text: String): F[Unit]
}

import cats.Monad

def income[F[_]](implicit M: Monad[F], C: Capabilities[F]): F[Unit] =
  for {
    contents <- C.resource("sales.csv")
    total = contents
      .split("\n").toList.tail  // Collection of lines, drop the CSV header
      .map { _.split(",").toList match  // List[Double] - prices of each of the entries
    { case name :: price :: Nil => price.toDouble }
    }
      .sum
    _ <- C.notify("admin@shop.com", s"Total income made today: $total")
  } yield ()

In order to compile is, I must include:

import cats.implicits._

Without this, I get an error:

Error:(21, 27) value map is not a member of type parameter F[String] contents <- C.resource("sales.csv")

Two questions:

1. Type of F is not known in advance. But there are Monad and Capabilities define implicitly for F. Why Scala compiler cannot identify it without implicits import from the cats.
2. Usually I prefer to find a certain type and do not import all stuff from the cats. For instance, import only cats.instances.list._, to be more precise. What exactly is used by Scala compiler from cats.implicits._, in order to be able to compile this code? And more important, what algorithm you use, to find it?
3. I've also found, if I add -Xprint-args option, code is compiled successfully even without cats.implicits._ importing. Can you please explain, how it affects it?

Answer 1

1. What Scala compiler cannot identify without implicits import are just syntaxes. Instances are resolved properly without imports (because of implicit parameters).

2. Out of cats.implicits._ you actually use only

   import cats.syntax.functor._
   import cats.syntax.flatMap._
   

3. https://github.com/scala/scala/pull/5909

   The option behaves like -Xshow-phases and stops the compiler after printing.

---

Without syntaxes imported you can write

def income[F[_]](implicit M: Monad[F], C: Capabilities[F]): F[Unit] =
  M.flatMap(C.resource("sales.csv"))(contents => {
    val total = contents
      .split("\n").toList.tail // Collection of lines, drop the CSV header
      .map {
        _.split(",").toList match // List[Double] - prices of each of the entries
          { case name :: price :: Nil => price.toDouble }
      }
      .sum
    M.map(C.notify("admin@shop.com", s"Total income made today: $total"))(_ => ())
  })