Walnut/Distributed Computing/Promises

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Promises

When you make an eventual send to an object (referred to hereafter simply as a send, which contrasts with a call to a local object that waits for the action to complete), even though the action may not occur for a long time, you immediately get back a promise for the result of the action:

 # E syntax
 def carVow := makeCar <- ("Mercedes")
 carVow <- moveTo(2,3)

In this example, we have sent the makeCar function the default "run" message. Eventually the carMaker will create the new car on the same computer where the carMaker resides; in the meantime, we get back a promise for the car.

Once we've got a promise, pipelining comes into the picture: We can immediately start making eventual sends to the promise just as if it were indeed the car. Enormous queues of operations can be shipped across the poor-latency comm network while waiting for the car to be created, confident that those operations will be executed as soon as possible.

But don't try to make an immediate call on the promise. Don't try it even if the car maker (and therefore the car it creates) actually live on the same computer with the program. To make immediate calls on the promised object, you must set up an action to occur when the promise resolves, using a when-catch construct:

 # E syntax
 def temperatureVow := carVow <- getEngineTemperature()
 when (temperatureVow) -> {
     println(`The temperature of the car engine is: $temperatureVow`)
 } catch prob {
     println(`Could not get engine temperature: $prob`)
 }
 println("execution of the when-catch waits for resolution of the promise,")
 println("but the program moves on immediately to these printlns")

We can read the when-catch statement as, "when the promise for a temperature becomes done, and therefore the temperature is locally available, perform the main action block... but if something goes wrong, catch the problem in variable prob and perform the problem block". In this example, we have requested the engine temperature from the carVow. Eventually the carVow resolves to a car and receives the request for engine temperature; then eventually the temperatureVow resolves into an actual temperature. The when-catch construct waits for the temperature to resolve into an actual (integer) temperature, but only the when-catch construct waits (i.e., the when catch will execute later, out of order, when the promise resolves). The program itself does not wait: rather, it proceeds on, with methodical determination, to the next statement following the when-catch.

Inside the when-catch statement, we say that the promise has been resolved. A resolved promise is either fulfilled or broken. If the promise is fulfilled, the main body of the when-catch is activated. If the promise is broken, the catch clause is activated.

Notice that after temperatureVow resolves to temperature, the when clause treats temperature as a local object. This is always safe to do when using vows. A vow is a reference that always resolves to a local object. In the example, variable names suffixed with "vow" remind us and warn others that this reference is a vow. We can reliably make eventual sends on the vow and immediate calls on what the vow resolves to, but we cannot reliably make immediate calls to the vow itself. A more detailed discussion of these types of warnings can be found in the Naming Conventions section later.

Not all references can guarantee to resolve to a local object. A receiver is a reference that can resolve to a local or remote object. Since we cannot ascertain beforehand whether it is local or remote, we must treat it as a remote object, i.e., we must interact with it only using eventual sends. We can use either the suffix Rcvr on the variable name as a reminder, or we can use the rcvr guard when defining the variable as a reminder. If you use Rcvr as a suffix, you will be more reliably reminded not to use immediate calls, but it does produce long variable names.

 # E syntax
 def car :rcvr := makeCarRcvr <- ("Mercedes")
 car <- moveTo(2,3)

In this example, makeCarRcvr is a reference to a makeCar function that we have reason to believe may be remote, and therefore can only be spoken to reliably with eventual sends. We could name the vehicle a carRcvr, since it's a reference to a car local to the makeCar function, which therefore is also probably remote to us (if makeCar is remote, so is the car). Instead, we have chosen to use the rcvr guard to remind us to only use eventual sends when interacting with this car.

When-catch, like try-catch, has an optional finally clause that always executes:

 # E syntax
 when (tempVow) -> {
     #...use tempVow
 } catch prob {
     #.... report problem
 } finally {
     #....log event
 }