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| import cats._ | |
| import cats.implicits._ | |
| // F[A] can fold A if F is a Foldable and A is a Monoid | |
| def folding[F[_]: Foldable, A: Monoid](fa: F[A]): A = | |
| implicitly[Foldable[F]].fold(fa) | |
| // Option[A] is a Monoid if A is a Semigroup | |
| def foldingOption[A: Semigroup](a: A): Option[A] = | |
| folding(List(Option(a))) // List is Foldable | |
| foldingOption(34) // Some(34) | |
| // traversing is folding, but instead of semigroup combining, e.g. 1 |+| 2 = 3, | |
| // we perform applicative combining, e.g. G(1) |+| G(List(2, 3, 4)) = G(List(1, 2, 3, 4)) | |
| // sequencing is traversing where each element is already an effect G (that is, it's | |
| // traversing with identity) | |
| def traversing[F[_]: Traverse, G[_]: Applicative, A, B](fa: F[A])(f: A => G[B]): G[F[B]] = | |
| implicitly[Traverse[F]].traverse(fa)(f) | |
| def sequencing[F[_]: Traverse, G[_]: Applicative, A](fa: F[A]): G[F[A]] = | |
| implicitly[Traverse[F]].traverse(fa)(a => implicitly[Applicative[G]].pure(a)) | |
| traversing(List(35))((i: Int) => Option(i + 100)) // Some(List(135)) | |
| def traverseList[G[_]: Applicative, A, B](fa: List[A])(f: A => G[B]): G[List[B]] = | |
| fa.foldRight(Applicative[G].pure(List.empty[B]))( | |
| (a, acc) => Applicative[G].map2(f(a), acc)(_ :: _) | |
| ) | |
| traverseList(List(36, 37))((i: Int) => Option(i + 100)) // Some(List(136, 137)) | |
| def traverseOption[G[_]: Applicative, A, B](fa: Option[A])(f: A => G[B]): G[Option[B]] = | |
| fa.fold(Applicative[G].pure(Option.empty[B]))( | |
| a => Applicative[G].map(f(a))(Option(_)) | |
| ) | |
| traverseOption(Option(36))((i: Int) => List(i + 100)) // List(Some(136)) |
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