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Sebastian Sardina FAQ for using SARL + SWI/JPL Prolog + Maven

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FAQ for using SARL + SWI Prolog + Maven

This is a collection of questions/issues that arose from my teaching of Agent Oriented Programming, where I have assessments in SARL and SWI Prolog, all packaged with Apache Maven.

If you think a question is worth putting it here, please let me know, I am happy to add it! THANKS!

ECLIPSE

How do I make ECLIPSE know about environment variables (e.g., SARL_VERSION)?

I can imagine there are other ways, but the way I made it work is by starting ECLIPSE from CLI with the variable exported already:

$ export SARL_ECLIPSE=0.7.2
$ ./eclipse-sarl

If you find another way that ECLIPSE can gather the environment variables (without re-defining them one by one), let me know!

What plugins are useful to develop on SARL in ECLIPSE?

On the SARL ECLIPSE distribution I install:

SWI-PROLOG

Making JPL work under Mac OS

(notes from student The Champion in the context of my AOPD course in 2018)

There is an issue with using JPL in Mac OS due to a linking error in JPL. Here Theo describes his solution (edited with some extra info):

The problem was that SWI-Prolog come as a relocatable app and the JPL library is therefore run-path dependent, causing the linking error when loaded without the proper run-path search paths. That is JPL does not know where SWI is.

So, one way to solve this would to use a not relocatable version of SWI-prolog such as the homebrew or macport installs. However, those do not include the JPL library. :-(

The only option is therefore to manually remove the run-path dependencies from the lib using the install_name_tool: 

install_name_tool <old_path> <new_path> dynlib

So the first step is to install a not relocatable version of SWI-prolog:

brew install swi-prolog

This will install prolog in /usr/local/Cellar/swi-prolog

As the homebrew version does not come with the JPL library (an issue on the homebrew github as been opened about this), the second step is therefore to buid the library from source. Alternatively, you can download the already compiled libjpl.dylib and copy it in:

/usr/local/Cellar/swi-prolog/7.6.4/libexec/lib/swipl-7.6.4/lib/

Now we need to ensure all dependencies link of the lib are valid (and absolute). Then cd into this directory and check all the dependencies paths using the command:

otool -L libjpl.dylib

For example:

libjpl.dylib:
/Applications/SWI-Prolog.app/Contents/swipl/lib/x86_64-darwin15.6.0/libjpl.dylib:
    @rpath/libjsig.dylib (compatibility version 1.0.0, current version 1.0.0)
    @rpath/libjvm.dylib (compatibility version 1.0.0, current version 1.0.0)
    @executable_path/../swipl/lib/x86_64-darwin15.6.0/libswipl.dylib (compatibility version 0.0.0, current version 7.6.4)
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1226.10.1)

If an entry is not a valid path on your system you can change it using the command:

install_name_tool -change <the_invalid_path> <your_new_valid_path> libjpl.dylib

Above, for example, libswipl.dylib is not installed in @executable_path/../swipl/lib/x86_64-darwin15.6.0/libswipl.dylib but instead in /usr/local/lib/swipl-7.7.19/lib/x86_64-darwin17.7.0/libswipl.dylib So we can do:

install_name_tool -change @rpath/libjsig.dylib /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/server/libjsig.dylib libjpl.dylib
install_name_tool -change @rpath/libjvm.dylib /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/server/libjvm.dylib libjpl.dylib
install_name_tool -change @executable_path/../swipl/lib/x86_64-darwin15.6.0/libswipl.dylib /usr/local/lib/swipl-7.7.19/lib/x86_64-darwin17.7.0/libswipl.dylib libjpl.dylib

As you can see the goal here is to replace of the run-path dependent part (@rpath or @executable_path) with an absolute path on your system.

After doing this step using install_name_tool, your lib should not depend on any external run-path:

libjpl.dylib:
 /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/server/libjsig.dylib (compatibility version 1.0.0, current version 1.0.0)
 /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/server/libjvm.dylib (compatibility version 1.0.0, current version 1.0.0)
 /usr/local/lib/swipl-7.7.19/lib/x86_64-darwin17.7.0/libswipl.dylib (compatibility version 0.0.0, current version 7.7.19)
 /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.50.4)

Once all entries are valid, you can add the path to this library:

/usr/local/Cellar/swi-prolog/7.6.4/libexec/lib/swipl-7.6.4/lib/

to your java.path.libray and it will, hopefully, link correctly: :-)

java -Djava.library.path=/usr/local/Cellar/swi-prolog/7.6.4/libexec/lib/swipl-7.6.4/lib/x86_64-darwin17.3.0/

The Java Virtual Machine (JVM) uses the java.library.path property in order to locate and load native libraries. When a Java application loads a native library using the System.loadLibrary() method, the java.library.path is scanned for the specified library. If the JVM is not able to detect the requested library, it throws an UnsatisfiedLinkError. See this tutorial for more explanation on that variable.

You can also do this in Eclipse:

  1. Select your project in the Package Explorer area and press a right click on it.
  2. Select Build Path → Configure Build Path... option.
  3. In the appearing window, select the Libraries tab.
  4. Then, expand the JRE System library option and select the Native library location.
  5. Click on the Edit... button at the right panel.
  6. Locate the required library and then click OK.

And that's it, your project will link and run properly on Mac OS ;)

Thanks Theo!

MAVEN

How do I install a JAR dependency manually?

Suppose your application need version 1.2.0.7.2 of artifact sarl-agtcity-mw from group id org.bitbucket.ssardina-research, but for some reason it cannot be gathered automatically from the cloud. Suppose you can get a hold of that dependency and obtain file sarl-agtcity-mw.jar. How do you install it in the repo? Here it is:

mvn install:install-file -Dfile=sarl-agtcity-mw.jar -DgroupId=org.bitbucket.ssardina-research \
    -DartifactId=sarl-agtcity-mw -Dversion=1.2.0.7.2 -Dpackaging=jar

How do I tell maven to work offline (not check anything on the internet)?

Use the -o option for offline mode (e.g., mvn -o clean package): https://www.packtpub.com/mapt/book/application_development/9781785286124/8/ch08lvl1sec81/working-in-offline-mode

Note you will still need to build the system online at least one, so that your system has the chance to get all the dependencies and store them under ~/.m2/

Why maven is not downloading the sources of dependencies?

ECLPSE IDE seems to download all sources by default. To get the sources via CLI: mvn dependency:sources

To state the POM to get all sources (and javadocs) of dependencies, check this post

The version of the maven-compiler-plugin should not be higher than 3.6.2 because the newer versions are using Java 1.9. And, it may cause problems during the compilation process.

In reality, the maven-compiler-plugin is not mandatory at all in the POM because Maven is supposed to download one when needed for compilation. If you encounter strange error related to the use of an Java 9 class from the Google library, then you have to specify the maven-compiler-plugin in order to avoid it:

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.6.2</version>
            <configuration>
                <source>${compiler.level}</source>
                <target>${compiler.level}</target>
                <encoding>${project.build.sourceEncoding}</encoding>
            </configuration>
        </plugin>

How to assemble my whole SARL application in a JAR file?

You can use maven-assembly-plugin build pluin:

    <plugin>
            <artifactId>maven-assembly-plugin</artifactId>
            <version>3.1.0</version>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <mainClass>BootMAS</mainClass>
                    </manifest>
                </archive>
                <descriptorRefs>
                    <descriptorRef>jar-with-dependencies</descriptorRef>
                </descriptorRefs>
            </configuration>

            <!--  EXECUTIONS -->
            <executions>
                <!--  Assemble an application bundle or distribution from an assembly descriptor.  -->
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>single</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

Note that with version 3.1.0 this may take quite longer to compile, as all dependencies are packaged into a single, often large, JAR file.

I get "no compiler is provided" error, why?

Make sure you:

  1. Have JDK installed in your system (the Java development framework that comes with the compiler javac), not just the Java runtime environment JRE.
  2. Your PATH environemnt variable points to the directory where java and javac of the JDK where installed.
  3. Your JAVA_HOME points to the directory where the JDK was installed. Alternative, you will need to specify it in your pom.xm; see here .

SARL

SARL is similar to Java but has different syntax in many places, how come?

Because it uses and builts on XTEND framework.

Seems my SARL application cannot find JPL!

There could be many reasons and it may depend on the OS you are using.

First of all, do you have the JPL package installed in the system?

  • In Windows, it is easy as you can click to install JPL Java<-->SWI interface at installation time.
    • You should get jpl.dll file in %SWI_HOME_DIR%/bin and jpl.jar in %SWI_HOME_DIR%/lib.
  • In Linux, you need to make sure libjpl.so and jpl.jar are somewhere! If you cannot find it, then you may need to install JPL.
    • In Ubuntu, it is provided with package swi-prolog-java.
    • In ARCH, you can generate it and install it using AUR package builder and swi-prolog-git package. Running the default PKG build file is enough to get JPL installed.
    • In MAC, we don't know how to make it work, as there is a glitch in one of the packages built for Mac... :-( [if you make it work, please let me know!]

Once JPL is in your system, you need to make the system is aware of it!

  • In Windows, make sure:
    • A system variable SWI_HOME_DIR points to the root of SWI install.
    • System variable Path includes %SWI_HOME_DIR%\bin and %SWI_HOME_DIR%\lib\jpl.jar.
  • In Linux, make sure:
    • export LD_PRELOAD=libswipl.so:$LD_PRELOAD
    • export LD_LIBRARY_PATH=/usr/lib/swi-prolog/lib/amd64/ (or wherever libjpl.so is located)

With this, your SARL application should be able to recognize where JPL is (hopefully!) :-)

You may also refer to the instructions of the SARL Prolog Capacity

Strange error in CLI when using occurence on left hand side of assignments

Consider this code:

on CarArrivedPercept {
    cars.get(occurrence.car).floor = occurrence.floor
}

We know that occurrence is static, so cannot be changed. However, in the above code, occurrence.car, is not being changed/assigned, but just used to refer to another entity where assignment is performed. However, SARL compiler will think that you are changing static element occurrence and complain with error. Instead do this:

on CarArrivedPercept {
    val n = occurrence.car
    cars.get(n).floor = occurrence.floor
}

Can we document SARL code for JAVADOC?

Presumably yes, but I was not yet successful (have not tried much though!). Check this post: https://groups.google.com/forum/#!topic/sarl/E_mhpFWyBFo

How do I control the log-level of the Logging built-in capacity?

Use setLogLevel() as explained here: http://www.sarl.io/docs/official/reference/bic/Logging.html

There used to be a bug in that debug() was passed to the system logging which if it was at level INFO it would not display it. It should now be fixed; see this post: https://github.com/sarl/sarl/issues/803

How can we know when an agent has been created fully after being spawn?

An event AgentSpawned will be emitted when an agent has been created and can be handled, say by a coordinator, to know the agent is now alive! Fo example:

on AgentSpawned {
    info("Agent {0} of type {1} has been created successfully and is now alive!",
        occurrence.agentIdentifiers, occurrence.agentType)
}

Equality and identity comparison (==, ===, !=, !==) in SARL and checking for null: same as Java ?

SARL's developer states that the mapping of the operator from SARL to Java are:

  • a === b becomes a == b
  • a !== b becomes a != b
  • a == b becomes a == null ? (b == null) : a.equals(b)
  • a != b becomes !Objects.equals(a,b). This is null-safe (part of Google API) and the code of the function is a == b || (a != null && a.equals(b))

It is always better to test valid against null with the === or !== operators.

Because the SARL == operator is mapped to the Java equals() function, and the === and !== operators to the Java == and != operators, it is better/safer, and a best practice, to use === and !== when one of the operands is of primitive type, e.g. null, number constants, primitive type variables. These operators are not replaced neither operator_equals nor operator_notEquals within the Java code.

Ususally, the SARL compiler generates a warning to push you to use === in place of ==. But with null == value, an ambiguous call error occurs before the warning is generated.

I believe SARL mapping this is because SARL comparison operators follow the which means that:

  • In Xtend the equals operators (==, !=) are bound to Object.equals.
  • Java’s identity equals semantic is mapped to the tripple-equals operators === and !== in Xtend.

Check xtend doc here for more details and SARL doc here.

Note directly from SARL developer (Sept 2018): SARL extends a part of Xtend dialect in order to have benefits of several background features, such as the validation tests for type inheritance. Since Xtend and SARL uses Xbase, a large part of the syntax is the same, especially within the blocks of code. (The rest of the syntax is defined in parallel. SARL was inspired by languages such as Scala, Python and Ruby. Several bugs or incoherencies of Xtend are fixed in SARL.

There is still a bit of an issue inside the Xbase library; check this https://github.com/sarl/sarl/issues/852#issuecomment-420842088

I have created an issue regarding null == value: Issue #854. According to the associated discussion within the Xbase group #300, the error message will be updated to push the developers to use ===.

How to return two values? Pairs in SARL

Java comes with a Pair<A,B> class to build an object for storing two values, nicknamed "key" and "value", but they have no meaning as key/value. It comes useful when a method has to return two values instead of just one. For example, the following function returns the next floor and direction that an elevator has to serve:

def kb_getNextJob() : Pair<Integer, Direction> {
    ...........
    }

As of Java 8, and as part of JavaFX, Java provides this Pair<A,B> class; check here and here. Note Pairs are different from Map, which can be seen as a collection of Pairs and with a proper key/value semantics.

There exist more advanced implementations of Pair, for example from Apache. See here, here and here.

SARL itself have compact syntax do deal with Pair, by using a -> b to create a Pair object (a,b). There are also compact ways of manipulating Collection and Maps.

Check SARL documentation on that here.

I cannot pass data to the Initialize of a Behaivor, occurrence.paramters is always empty!

From SARL developer in this post](https://groups.google.com/forum/#!topic/sarl/E0TuX-V123U):

Currently, there is no way to set the occurrence.parameters values with the current API.

The Initialize event is fired by the registerBehavior function. As you could see, this function does not provide a mean for passing initialization parameters. I think the ability to set the Initialize parameters when registering the behaviors is a missed feature from the SARL API.

The issue has been resolved in this issue) and should be available in SARL 0.8.0+. For 0.7.x, you could use the following work around:

behavior MyBehavior {

   var initParam : Object[]

   new (owner : Agent, initParam : Object*) {
      super(owner)
      this.initParam = initParam
   }

   on Initialize {
      // Do something with this.initParam
   }

}

agent MyAgent {

   uses Behaviors

   on Initialize {
      var beh = new MyBehavior(this, "p1")
      beh.registerBehavior
   }
}

Note that because registration happens after the creation of the behavior module, the initParam will be available at time of Initialize execution.

Cannot access agent owner of a skill in Initialize behavior, why?

Summary of See this thread

The owner of the skill is set when the skill is attached to the agent. It means that the owner cannot be known within the constructor of the skill. The function install is invoked just after the skill is installed into the agent. So:

new {
   super()
   assert this.owner === null
}

def install {
    assert this.owner !== null
}

Importantly, the initialization (via Initialize event) is not at creation time (constructor) but when the entity is attached to the agent.

Within SARL developers' perspective, constructor statements should be redefined only if you want to set the value of a field that is final (val keyword), because it is mandatory regarding the semantic of the val keyword. Consequently, the best practice is that all initializations should be within the on Initialize. They should avoid constructor definition.

So, the "clean" way to access the agent owner is in the install method:

skill SWIJPL_KB_Prolog implements KB_Prolog {
    var module_name : String

         def install {
           module_name = this.owner.ID.toString
         }
... 
}

SARL is giving lots of WARNINGS at compile time, how can I avoid them?

You can use @SupressWarning(..) annotations in the entities you do not want to be warned. For example, a typical warning SARL will give is lack of synchronization for variables that can be accessed/edited concurrently:

[WARNING] The field noToSpawn should be synchronized for avoiding value inconsistency due to parallel execution. [/home/ssardina/git/soft/agents/MASSIM/AgentsInCity/sarl-agtcity-base.git/src/main/sarl/au/edu/rmit/agtgrp/agtcity/sarl/agents/dummy/BootMultiSWIAgents.sarl:70]

To get rid of such warnings, assuming you are aware of the potentiall issue and have planned for it, you can do:

@SuppressWarnings("potential_field_synchronization_problem")
agent BootMultiSWIAgents {
..
}

See the Issue Codes for a complete list of what can be supressed.

I cannot define a static field in an agent tye declaration (agent, skill, behavior), why?

This is a design choice given that our entities are agents and as such they should not "share" data unless done explicitly in an agent-oriented manner, for example via resources or communication channels.
Having static fields in agents or skills would break the "independency" of agents.

It is most probable that such static data can be seen as a resource outside the skill or agent, and as such it should be managed outside it (for example within a class). This is the case for exampe if you want a synchronization (lock) among all instances of the skill: the lock should be managed by the resource itself, not by the skill.

Can I make SARL wait in the execution on some thread?

Yes, use something like TimeUnit.SECONDS.sleep(5)

Intialize behavior in agents do not terminate!

The on Initialize behavior handler in agents is a bit special, as it is the code ran when an agent is born. As such, its execution is more "synchronous" than other on-behavior rules. In particular:

  1. Any event emitted within an on Initialize, will not be processed until that on Initialize code finishes. So, your agent initialization should not depend (and wait) on any emited event being processed, as they won't!
  2. When spawning an agent in on Initialize, the spawn instructions will not return until all the Initialize behaviors of the spawned agent have been executed fully. 
import io.sarl.core.Initialize
import io.sarl.core.Logging
import io.sarl.core.Lifecycle

agent Agent1 {
    uses Logging, Lifecycle

    var agent_name = "agent1"

    on Initialize {
        info(agent_name + " spawned")
        info(agent_name + " spawning Agent2")
        spawn(Agent2)
        info(agent_name + " end")
    }
}

agent Agent2 {
    uses Logging

    var agent_name = "agent2"

    on Initialize {
        info(agent_name + " spawned")
        info(agent_name + " sleeping")
        Thread.sleep(5000)
        info(agent_name + " woke up")
        info(agent_name + " end")
    }

    on Initialize {
        info(agent_name + " init2")
        info(agent_name + " init2 end")
    }
}

The result will be:

Launching the agent: Agent1
agent1 spawned
agent1 spawning Agent2
agent2 spawned
agent2 init2
agent2 sleeping
agent2 init2 end
agent2 woke up
agent2 end
agent1 end
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