How not to code

NormalCode.scala

for {
 app  <- distinctBundles
 day  <- dateRange(inputStart, inputEnd)
 hour <- (0 to 23)
 path = buildPath(inputPath, hour, day, app)
} yield (path, fileSystem.isDirectory(new Path(path)))

ValdoDelirium.scala

// app, day, hour
val appsAndDates: fs2.Stream[fs2.Pure, (AppAndOs, LocalDate)] = fs2.Stream
  .emits(distinctBundles)
  .flatMap(
    app =>
      fs2.Stream
        .emits(dateRange(inputStart, inputEnd))
        .map((app, _))
  )

appsAndDates
  .flatMap {
    case (app, day) =>
      fs2.Stream
        .emits((0 to 23))
        .map(buildPath(inputPath, _, day, app))
  }
  .map(ensureTrailingSlash)
  .covary[IO]
  .parEvalMapUnordered(10)((path: String) => IO.delay(fileSystem.isDirectory(new Path(path))).map((path, _)))
  .collect { case (path, true) => path }
  .compile
  .toList
  .unsafeRunSync()