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Usage examples

This is all good, but in practice, I rarely put functions inside of contexts. I find that the function map2 in Apply is more useful. It is defined in Apply by using product and map. The product defines itself, using ap and map:

trait Apply[F[_]] extends Functor[F] … {
  (...)
  def map2[A, B, Z](fa: F[A], fb: F[B])(f: (A, B) => Z): F[Z] =
    map(product(fa, fb))(f.tupled)
    override def product[A, B](fa: F[A], fb: F[B]): F[(A, B)] =
    ap(map(fa)(a => (b: B) => (a, b)))(fb)

The map2 object allows for applying a function to two values inside of an F context. This can be used to combine two values inside F, as follows:

def parseIntO(s: String): Option[Int] = Either.catchNonFatal(s.toInt).toOption
parseIntO("6").map2(parseIntO("2"))(_ ...