Why are there two parameter groups in cats Bifunctor[F[_, _]].bimap function?

Question

I stumbled upon this problem when trying to implement a Bifunctior type class for maps (Bifunctor[Map[_, _]]).

Bifunctor is defined like this in cats:

/**
   * The quintessential method of the Bifunctor trait, it applies a
   * function to each "side" of the bifunctor.
   *
   * Example:
   * {{{
   * scala> import cats.implicits._
   *
   * scala> val x: (List[String], Int) = (List("foo", "bar"), 3)
   * scala> x.bimap(_.headOption, _.toLong + 1)
   * res0: (Option[String], Long) = (Some(foo),4)
   * }}}
   */
  def bimap[A, B, C, D](fab: F[A, B])(f: A => C, g: B => D): F[C, D]

As the comment states, this function can be called using two functions (in one parameter group) as its input like this: x.bimap(_.headOption, _.toLong + 1). This tells me that this is clearly not the bimap function being called since this one has two parameter groups ((fab: F[A, B])(f: A => C, g: B => D)). I have been wondering if there is some kind of implicit type conversion that I am not aware of happening here. How does it work? What do I need to implement to get a Bifunctor type class for maps?

Answer 1

An instance of type class Bifunctor for Map can be defined as follows

implicit val mapBifunctor: Bifunctor[Map] = new Bifunctor[Map] {
  override def bimap[A, B, C, D](fab: Map[A, B])(f: A => C, g: B => D): Map[C, D] = 
    fab.map { case (k, v) => f(k) -> g(v) }
}

In x.bimap(_.headOption, _.toLong + 1) implicits are resolved twice:

• firstly, instance Bifunctor[Tuple2] is found (import cats.instances.tuple._ or import cats.instances.all._ or import cats.implicits._),

• secondly, extension method is resolved (import cats.syntax.bifunctor._ or import cats.syntax.all._ or import cats.implicits._)

Thus x.bimap(_.headOption, _.toLong + 1) is transformed to

implicitly[Bifunctor[Tuple2]].bimap(x)(_.headOption, _.toLong + 1)

or

Bifunctor[Tuple2].bimap(x)(_.headOption, _.toLong + 1)

or

toBifunctorOps(x)(catsStdBitraverseForTuple2).bimap(_.headOption, _.toLong + 1)