A convenience trait for defining applications which are entirely within Resource. This is implemented as a relatively straightforward wrapper around IOApp and thus inherits most of its functionality and semantics.

This trait should generally be used for any application which would otherwise trivially end with cats.effect.kernel.Resource!.use (or one of its variants). For example:

object HttpExample extends IOApp {
  def run(args: List[String]) = {
    val program = for {
      config <- Resource.eval(loadConfig(args.head))
      postgres <- Postgres[IO](config.jdbcUri)
      endpoints <- ExampleEndpoints[IO](config, postgres)
      _ <- HttpServer[IO](config.host, config.port, endpoints)
    } yield ()

    program.useForever.as(ExitCode.Success)
  }
}

This example assumes some underlying libraries like Skunk and Http4s, but otherwise it represents a relatively typical example of what the main class for a realistic Cats Effect application might look like. Notably, the whole thing is enclosed in Resource, which is used at the very end. This kind of pattern is so common that ResourceApp defines a special trait which represents it. We can rewrite the above example:

object HttpExample extends ResourceApp.Forever {
  def run(args: List[String]) =
    for {
      config <- Resource.eval(loadConfig(args.head))
      db <- Postgres[IO](config.jdbcUri)
      endpoints <- ExampleEndpoints[IO](config, db)
      _ <- HttpServer[IO](config.host, config.port, endpoints)
    } yield ()
}

These two programs are equivalent.