More scaladoc

build.sbt

@@ -312,9 +312,7 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       ProblemFilters.exclude[MissingClassProblem]("cats.effect.SyncIO$Delay"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.apply"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.copy"),
-      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.this")
-    )
-  )
+      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.this")))
   .jvmSettings(
     javacOptions ++= Seq("-source", "1.8", "-target", "1.8")
   )

core/js/src/main/scala/cats/effect/IOApp.scala

@@ -20,13 +20,169 @@ import scala.concurrent.CancellationException
 import scala.concurrent.duration._
 import scala.scalajs.js
 
+/**
+ * The primary entry point to a Cats Effect application. Extend this
+ * trait rather than defining your own `main` method. This avoids the
+ * need to run [[IO.unsafeRunSync]] (or similar) on your own.
+ *
+ * `IOApp` takes care of the messy details of properly setting up
+ * (and tearing down) the [[unsafe.IORuntime]] needed to run the [[IO]]
+ * which represents your application. All of the associated thread
+ * pools (if relevant) will be configured with the assumption that
+ * your application is fully contained within the `IO` produced by
+ * the [[run]] method. Note that the exact details of how the runtime
+ * will be configured are very platform-specific. Part of the point
+ * of `IOApp` is to insulate users from the details of the underlying
+ * runtime (whether JVM or JavaScript).
+ *
+ * {{{
+ *   object MyApplication extends IOApp {
+ *     def run(args: List[String]) =
+ *       for {
+ *         _ <- IO.print("Enter your name: ")
+ *         name <- IO.readln
+ *         _ <- IO.println("Hello, " + name)
+ *       } yield ExitCode.Success
+ *   }
+ * }}}
+ *
+ * In the above example, `MyApplication` will be a runnable class with
+ * a `main` method, visible to Sbt, IntelliJ, or plain-old `java`. When
+ * run externally, it will print, read, and print in the obvious way,
+ * producing a final process exit code of 0. Any exceptions thrown within
+ * the `IO` will be printed to standard error and the exit code will be
+ * set to 1. In the event that the main [[Fiber]] (represented by the `IO`
+ * returned by `run`) is canceled, the runtime will produce an exit code of 1.
+ *
+ * Note that exit codes are an implementation-specific feature of the
+ * underlying runtime, as are process arguments. Naturally, all JVMs
+ * support these functions, as does NodeJS, but some JavaScript execution
+ * environments will be unable to replicate these features (or they simply
+ * may not make sense). In such cases, exit codes may be ignored and/or
+ * argument lists may be empty.
+ *
+ * Note that in the case of the above example, we would actually be
+ * better off using [[IOApp.Simple]] rather than `IOApp` directly, since
+ * we are neither using `args` nor are we explicitly producing a custom
+ * [[ExitCode]]:
+ *
+ * {{{
+ *   object MyApplication extends IOApp.Simple {
+ *     val run =
+ *       for {
+ *         _ <- IO.print("Enter your name: ")
+ *         name <- IO.readln
+ *         _ <- IO.println(s"Hello, " + name)
+ *       } yield ()
+ *   }
+ * }}}
+ *
+ * It is valid to define `val run` rather than `def run` because `IO`'s
+ * evaluation is lazy: it will only run when the `main` method is
+ * invoked by the runtime.
+ *
+ * In the event that the process receives an interrupt signal (`SIGINT`) due
+ * to Ctrl-C (or any other mechanism), it will immediately `cancel` the main
+ * fiber. Assuming this fiber is not within an `uncancelable` region, this
+ * will result in interrupting any current activities and immediately invoking
+ * any finalizers (see: [[IO.onCancel]] and [[IO.bracket]]). The process will
+ * not shut down until the finalizers have completed. For example:
+ *
+ * {{{
+ *   object InterruptExample extends IOApp.Simple {
+ *     val run =
+ *       IO.bracket(startServer)(
+ *         _ => IO.never)(
+ *         server => IO.println("shutting down") *> server.close)
+ *   }
+ * }}}
+ *
+ * If we assume the `startServer` function has type `IO[Server]` (or similar),
+ * this kind of pattern is very common. When this process receives a `SIGINT`,
+ * it will immediately print "shutting down" and run the `server.close` effect.
+ *
+ * One consequence of this design is it is possible to build applications which
+ * will ignore process interrupts. For example, if `server.close` runs forever,
+ * the process will ignore interrupts and will need to be cleaned up using
+ * `SIGKILL` (i.e. `kill -9`). This same phenomenon can be demonstrated by using
+ * [[IO.uncancelable]] to suppress all interruption within the application
+ * itself:
+ *
+ * {{{
+ *   object Zombie extends IOApp.Simple {
+ *     val run = IO.never.uncancelable
+ *   }
+ * }}}
+ *
+ * The above process will run forever and ignore all interrupts. The only way
+ * it will shut down is if it receives `SIGKILL`.
+ *
+ * It is possible (though not necessary) to override various platform-specific
+ * runtime configuration options, such as `computeWorkerThreadCount` (which only
+ * exists on the JVM). Please note that the default configurations have been
+ * extensively benchmarked and are optimal (or close to it) in most conventional
+ * scenarios.
+ *
+ * However, with that said, there really is no substitute to benchmarking your
+ * own application. Every application and scenario is unique, and you will
+ * always get the absolute best results by performing your own tuning rather
+ * than trusting someone else's defaults. `IOApp`'s defaults are very ''good'',
+ * but they are not perfect in all cases. One common example of this is
+ * applications which maintain network or file I/O worker threads which are
+ * under heavy load in steady-state operations. In such a performance profile,
+ * it is usually better to reduce the number of compute worker threads to
+ * "make room" for the I/O workers, such that they all sum to the number of
+ * physical threads exposed by the kernel.
+ *
+ * @see [[IO]]
+ * @see [[run]]
+ * @see [[ResourceApp]]
+ * @see [[IOApp.Simple]]
+ */
 trait IOApp {
 
   private[this] var _runtime: unsafe.IORuntime = null
 
+  /**
+   * The runtime which will be used by `IOApp` to evaluate the
+   * [[IO]] produced by the `run` method. This may be overridden
+   * by `IOApp` implementations which have extremely specialized
+   * needs, but this is highly unlikely to ever be truly needed.
+   * As an example, if an application wishes to make use of an
+   * alternative compute thread pool (such as `Executors.fixedThreadPool`),
+   * it is almost always better to leverage [[IO.evalOn]] on the value
+   * produced by the `run` method, rather than directly overriding
+   * `runtime`.
+   *
+   * In other words, this method is made available to users, but its
+   * use is strongly discouraged in favor of other, more precise
+   * solutions to specific use-cases.
+   *
+   * This value is guaranteed to be equal to [[unsafe.IORuntime.global]].
+   */
   protected def runtime: unsafe.IORuntime = _runtime
+
+  /**
+   * The configuration used to initialize the [[runtime]] which will
+   * evaluate the [[IO]] produced by `run`. It is very unlikely that
+   * users will need to override this method.
+   */
   protected def runtimeConfig: unsafe.IORuntimeConfig = unsafe.IORuntimeConfig()
 
+  /**
+   * The entry point for your application. Will be called by the runtime
+   * when the process is started. If the underlying runtime supports it,
+   * any arguments passed to the process will be made available in the
+   * `args` parameter. The numeric value within the resulting [[ExitCode]]
+   * will be used as the exit code when the process terminates unless
+   * terminated exceptionally or by interrupt.
+   *
+   * @param args The arguments passed to the process, if supported by the
+   *        underlying runtime. For example, `java com.company.MyApp --foo --bar baz`
+   *        or `node com-mycompany-fastopt.js --foo --bar baz` would each
+   *        result in `List("--foo", "--bar", "baz")`.
+   * @see [[IOApp.Simple!.run:cats\.effect\.IO[Unit]*]]
+   */
   def run(args: List[String]): IO[ExitCode]
 
   final def main(args: Array[String]): Unit = {
@@ -92,9 +248,15 @@ trait IOApp {
 
 object IOApp {
 
+  /**
+   * A simplified version of [[IOApp]] for applications which ignore their
+   * process arguments and always produces [[ExitCode.Success]] (unless
+   * terminated exceptionally or interrupted).
+   *
+   * @see [[IOApp]]
+   */
   trait Simple extends IOApp {
     def run: IO[Unit]
     final def run(args: List[String]): IO[ExitCode] = run.as(ExitCode.Success)
   }
-
 }