DeadlockError

Standard library class · Extends RuntimeError

A DeadlockError is what Emerald raises when every task is waiting for something, and nothing is going to happen that could end any of the waits. Instead of leaving the program hanging forever, Emerald stops it and says what each task was waiting for, and where.

Tasks.run { tasks =>
    const numbers: Channel[Int] = Channel()
    print(numbers.receive())
}

Output

tasks.em:3:11: DeadlockError: these tasks are waiting for each other
      print(numbers.receive())
            ^^^^^^^^^^^^^^^^^
At `tasks.em:3`, the group is waiting to receive from channel 1.
Check the task results they wait for.
in a block, called at tasks.em:1:1

Here nothing will ever send into the channel, so waiting for a message can never end. The message names the place each task is stuck, which is usually enough to see what is missing: a send that was never written, a channel that was never closed, or two tasks each waiting for the other’s result.

A DeadlockError is a RuntimeError, so catch error: RuntimeError catches it too. It is a mistake in the program’s design rather than a problem to recover from, so it is usually better to fix the waiting than to catch the error.

A program that is waiting for a timer, a file, a web request, or input isn’t deadlocked, since those can still end the wait. Emerald raises this error only when nothing outside the program could wake it either.

message: String

What went wrong, in words.