Companion object

We’ve been using Cats’ syntax and fetch.syntax throughout the examples since it’s more concise and general than the methods in the Fetch companion object. However, you can use the methods in the companion object directly.

Note that using cats syntax gives you a plethora of combinators, much richer that what the companion object provides.


Plain values can be lifted to the Fetch monad with Fetch#pure:

def fetchPure[F[_]: Concurrent]: Fetch[F, Int] = Fetch.pure(42)[IO](fetchPure).unsafeRunSync() shouldBe res0


Errors can also be lifted to the Fetch monad via Fetch#error.

def fetchFail[F[_]: Concurrent]: Fetch[F, Int] =
  Fetch.error(new Exception("Something went terribly wrong"))[IO](fetchFail).attempt.unsafeRunSync().isLeft shouldBe res0


Fetch is built using Cats’ data types and typeclasses and thus works out of the box with cats syntax. Using Cats’ syntax, we can make fetch declarations more concise, without the need to use the combinators in the Fetch companion object.

Fetch provides its own instance of Applicative[Fetch]. Whenever we use applicative operations on more than one Fetch, we know that the fetches are independent meaning we can perform optimizations such as batching and concurrent requests.

If we were to use the default Applicative[Fetch] operations, which are implemented in terms of flatMap, we wouldn’t have information about the independency of multiple fetches.


The tuple apply syntax allows us to combine multiple independent fetches, even when they are from different types, and apply a pure function to their results. We can use it as a more powerful alternative to the product method:

def fetchThree[F[_]: Concurrent]: Fetch[F, (Post, User, Post)] =
  (getPost(1), getUser(2), getPost(2)).tupled[IO](fetchThree).unsafeRunSync()._2 shouldBe res0

More interestingly, we can use it to apply a pure function to the results of various fetches.

def fetchFriends[F[_]: Concurrent]: Fetch[F, String] = (getUser(1), getUser(2)).mapN {
  (one, other) =>
    s"${one.username} is friends with ${other.username}"
}[IO](fetchFriends).unsafeRunSync() shouldBe res0