Marker for blocking operations that need to be disallowed on top of JavaScript engines, or other platforms that don't support the blocking of threads.

As sample, lets implement a low-level blocking operation; but kids, don't do this at home, since this is error prone and you already have Scala's Await.result, this sample being shown for pedagogical purposes:

import monix.execution.schedulers.CanBlock
import java.util.concurrent.CountDownLatch
import scala.concurrent.{ExecutionContext, Future}
import scala.util.Try

def block[A](fa: Future[A])
  (implicit ec: ExecutionContext, permit: CanBlock): Try[A] = {

  var result = Option.empty[Try[A]]
  val latch = new CountDownLatch(1)

  fa.onComplete { r =>
    result = r
    latch.countDown()
  }

  latch.await()
  result.get
}

And then for JavaScript engines (Scala.js) you could describe the same function, with the same signature, but without any implementation, since this operation isn't supported:

def block[A](fa: Future[A])
  (implicit ec: ExecutionContext, permit: CanBlock): Try[A] =
  throw new UnsupportedOperationException("Cannot block threads on top of JavaScript")

Now in usage, when the caller is invoking block as described, it will work without issues on top of the JVM, but when compiled with Scala.js it will trigger a message like this:

[error] Playground.scala:30:8: Blocking operations aren't supported
[error] on top of JavaScript, because it cannot block threads!
[error] Please use asynchronous API calls.
[error]   block(Future(1))
[error]        ^