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ResourceApp.scala
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ResourceApp.scala
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/*
* Copyright 2020-2021 Typelevel
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package cats.effect
import cats.syntax.all._
/**
* 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
* [[https://tpolecat.github.io/skunk/ Skunk]] and [[https://http4s.org 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
* `use`d 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.
*
* @see
* [[run]]
* @see
* [[ResourceApp.Simple]]
* @see
* [[ResourceApp.Forever]]
*/
trait ResourceApp { self =>
/**
* @see
* [[IOApp.run]]
*/
def run(args: List[String]): Resource[IO, ExitCode]
final def main(args: Array[String]): Unit = {
val ioApp = new IOApp {
override def run(args: List[String]): IO[ExitCode] =
self.run(args).use(IO.pure(_))
}
ioApp.main(args)
}
}
object ResourceApp {
/**
* A [[ResourceApp]] which takes no process arguments and always produces [[ExitCode.Success]]
* except when an exception is raised.
*
* @see
* [[IOApp.Simple]]
*/
trait Simple extends ResourceApp {
/**
* @see
* [[cats.effect.IOApp.Simple!.run:cats\.effect\.IO[Unit]*]]
*/
def run: Resource[IO, Unit]
final def run(args: List[String]): Resource[IO, ExitCode] = run.as(ExitCode.Success)
}
/**
* A [[ResourceApp]] which runs until externally interrupted (with `SIGINT`), at which point
* all finalizers will be run and the application will shut down upon completion. This is an
* extremely common pattern in practical Cats Effect applications and is particularly
* applicable to network servers.
*
* @see
* [[cats.effect.kernel.Resource!.useForever]]
*/
trait Forever { self =>
/**
* Identical to [[ResourceApp.run]] except that it delegates to
* [[cats.effect.kernel.Resource!.useForever]] instead of
* [[cats.effect.kernel.Resource!.use]].
*
* @see
* [[ResourceApp.run]]
*/
def run(args: List[String]): Resource[IO, Unit]
final def main(args: Array[String]): Unit = {
val ioApp = new IOApp {
override def run(args: List[String]): IO[ExitCode] =
self.run(args).useForever
}
ioApp.main(args)
}
}
}