To start with we are going to learn what an effect is, and the very basics of how IO works: how we can construct an IO, and how we can combine multiple IOs together.

We need to understand what an effect is and hence what problem that Cats Effect is solving. There is already an excellent blog post that does just that, so go and read it now.

Now you understand what problem Cats Effect is trying to solve, and what it means for IO to be an effect type, let's start using IO. Complete the challenge in code/src/main/scala/introduction/01-basics.scala.

Most exercises will have solutions, just like the one below. It's a good idea to read them even if you successfully solved the exercise, as they'll talk about the lessons you should learn from the exercise, not just the solution to the exercise itself.

The distinction between the description of an effect and the act of carrying out that description is made clear by defining run as a val. It can be a val because an IO is just a description. It does nothing until it is run.

  1. The different constructors get to the difference between effectful and pure programs. It doesn't matter if we run a pure program multiple time or only once: it always produces the same result. Hence IO.pure can evaluate its argument once (at the time of construction) and cache that value. This is not the case for effectful programs, which use the apply constructor. We can illustrate the difference by using, say, IO.pure(println("Hello")) and IO(println("Hello")).

  2. IO.realTime.flatMap(IO.println)

  3. If you replace 1 and 2 with effects (e.g. println) you can see in what order they are run. It's always the same: left-to-right.

  4. It's incorrect. This is a common error. IO is a description, so writing val _ = io does nothing.

  5. This will do

    def log[A](io: IO[A]): IO[A] = {
       .flatMap(_ => io)
       .flatMap(a => IO.println("Stopping").map(_ => a))

    A more stylish implementation would use a for comprehension.

  6. This is an exercise in reading type signatures. a *> b creates an IO that runs a, discards its result, and then runs b. We can write log as

    def log[A](io: IO[A]): IO[A] = {
     IO.println("Starting") *> io <* IO.println("Stopping")

    Whether we should do this or not depends on the context in which we're writing code. The *> and <* operators are a bit obscure, so they make our code less accessible to others. Use with caution.