Category: programming

Scala function error “Type mismatch” in Intellij Idea vs Scala Eclipse

Using Scala we can define a function in a form when name and parameters are specified followed by the definition of the function body.

For example:

def abs(x: Double) = if (x < 0) -x else x
As you can see, curly braces are not mandatory when all those elements are put in one line.
When you put that code in Scala worksheet both in Intellij Idea and Scala Eclipse (Scala Eclipse IDE) everything is all right. Code compiles and runs.

The previous example was very simple. However, functions are more complex in real life so there is no way to stick them in one line. It is not a problem at all, you define multiline functions simply by embracing the function body in curly braces:

def listLength(list: List[_]): Int = {
  if (list == Nil) 0 else 1 + listLength(list.tail)
}

Code of course compiles and runs both in Intellij Idea and Scala Eclipse.

What happens if you omit the braces?

def listLength(list: List[_]): Int = if (list == Nil) 0 else 1 + listLength(list.tail)
That is the point where I spot difference in behavior between those two IDE.
While Scala Eclipse runs that code without any error, Intellij Idea gets stuck with the following error message:
error: type mismatch;
   found   : Unit
   required: Int
             if (list == Nil) 0
             ^
I am sure that that error message does not tell much to Scala beginners about the source of the problem.
Let’s investigate the error message more deeply.

Firstly, find out what type Unit is.
Type Unit is similar to what is known in Java as void. In Java void means that function does not return anything. However, every method in Scala returns a value. To deal with the situation when Scala function does not have anything to return, type Unit was introduced. Eventually Unit is defined on the bytecode level as void, but in Scala source level it is still a type.Come back to the error message. It says that expected returned type is Int but the function returns Unit in fact. What probably happened is situation, when only line:

if (list == Nil) 0

is interpreted, Scala adds the else statement in that form:

else ()

where the mentioned rule of returning Unit when there is nothing to return, is applied.
So the rewritten if finally looks as follows:

if (list == Nil) 0 else ()

If part returns Int, while else part returns Unit. More general type is taken and the whole function tries to return Unit while expecting Int from function definition.

I met that problem when trying to run in Intellij Idea the code example presented in Scala Eclipse. Multiline function was coded without curly braces and only Intellij Idea thrown the error presented.

ActiveMQ-CPP bug

I was faced with strange bug in ActiveMQ-CPP library recently.
I just wanted to make sure that message broker (ActiveMQ) redelivers not acknowledged messages to the consumer applying some delay. For example 10 seconds.
Let’s say that consumer processes message and some error occurs like there is no connectivity to the external system where processed message is stored. I do not want the message to be lost but rather returned to the broker and redelivered after some time.
Quick look at the API and documentation and it is clear that such behavior can be implemented using RedeliveryPolicy.
There are some properties which can be used to adjust the policy:

Option Name Default Description
cms.RedeliveryPolicy.backOffMultiplier 5.0 Used to extend the redelivery window when the _useExponentialBackOff option is enabled.
cms.RedeliveryPolicy.collisionAvoidancePercent 15 Percentage of randomness to add to redelivery backoff to keep multiple consumers in a rollback from redelivering at the same time.
cms.RedeliveryPolicy.initialRedeliveryDelay 1000 Amount of time to wait before starting redeliveries, in milliseconds.
cms.RedeliveryPolicy.maximumRedeliveries 6 Maximum number of time to attempt a redelivery before giving up and NAck’ing the message.
cms.RedeliveryPolicy.useCollisionAvoidance false Should the Random avoidance percentage be added into the redelivery delay.
cms.RedeliveryPolicy.useExponentialBackOff false Should redeliveries be slowed on each cycle.

Unfortunately, even if I tried almost all possible combination I could not forced the broker to redeliver messages after specified redelivery delay. Messages came back to the consumer immediately. It caused really high processor utilization and, what is more, log files were growing very quickly.

As last resort I dived into library source code and that is what I found there:

if( internal->redeliveryDelay > 0 && !this->internal->unconsumedMessages->isClosed() ) {
    // TODO - Can't do this until we can control object lifetime.
    // Start up the delivery again a little later.
    // this->internal->scheduler->executeAfterDelay(
    // new StartConsumerTask(this), internal->redeliveryDelay);
    start();
} else {
    start();
}

That must be a joke… API gives you a way to set redelivery delay, documentation says it can be done… But that is what you find in the source code.
Just to be proper I use 3.4.5 version of the library and the bug is fixed in 3.5.0 version.