typed_jcop_example

Typed JCop is a prototype language that supports typechecking several JCop features. It is also a implementation of ContextFJ_{<:}, which is a calculus supporting inheritance of layer inheritance, subtyping of layer types, first-class layers and layer swapping.

How to get compiler

1. Please get original JCop compiler from the website (https://www.hpi.uni-potsdam.de/hirschfeld/trac/Cop/wiki/JCop), and follow the instruction of installation.

2. Please go to https://github.com/h-inoue/JCop/releases , download jcop.jar, and replace the original one.

How to use the compiler

After setting JCOP_HOME environment, you can compile the example as follows.

cd transfersystem
jcopc.sh -d bin -sourcepath src main.Main

Then, you can run a program with normal java command.

java -cp bin main.Main

Requirements

You may need JRE 7. The compiler will cause an error on JRE 8.

Language Constructs

Layer Definition

TJCop supports only layer declaration as class-in-layer style.

[swappable] layer L [extends L1] [requires L2, L3]{
    partial method declarations...
}

Partial Methods Declaration

A name of partial methods must include full qualified class name. You can also include proceed() call in the partial method body..

public void pckg.C.m( args ){
    ....
}

Layer Activation/Swapping

with( expression ){
    ....
}

swap( LayerName, expression ){
    ....
}

Type Checking Examples

A distinct feature of Typed JCop is COP originated type checking. For example, considering to compile following code.

Simple Example

/** Weather.jcop **/
package main;
public layer Weather{
    public String main.Main.getWeather(){
	    return "not implemented";
    }
}

/** Main.jcop **/
package main;
public class Main{
    public static void main(String[] args) {
	    String str = new Main().getWeather();
	    System.out.println(str);
    }
}

Since the method getWeather() is newly declared in a layer Weather, the method call will cause following compile error.

$ jcopc.sh Main.jcop
CopCompileError: [/Users/hiro/Dropbox/work/src/jcop/simple1/./Main.jcop:9:
    Semantic Error: There is no valid method getWeather() in this context [].]

If we modify the sourcecode to activate an instance of Weather layer, it works well.

/** Main.jcop **/
package main;
public class Main{
    public static void main(String[] args) {
	    with(new Weather()){
		    String str = new Main().getWeather();
		    System.out.println(str);
        }
    }
}

Example of swapping

Here, let's consider an example of bank transfer system.

/** EncryptionLayer.jcop **/
package encryption;
import account.*;
public swappable layer EncryptionLayer {
    public float account.Account.getBalance() {
	    return Encryption.encrypt(proceed());
    }
}

Suppose that we want to deactivate the layer temporally. We can define such a feature by declaring a sublayer of EncryptionLayer.

/** TemporallyDecryptionLayer.jcop **/
package encryption;
public layer TemporallyDecryptionLayer extends EncryptionLayer{
    public float account.Account.getBalance() {
	    return proceed();
    }
}

swap(EncryptionLayer, tmplayer){
    /* temporally deactivate encryption */
}

Whole example sourcecode is in this repository.

Current Limitation

Current Typed JCop is very restricted language, compared with original JCop. Following features of original JCop would cause a error or an inconsistency of type checking.

• Activating layer list.
• Deactivating layer.(without)
• Layer declaration as layer-in-class style
• before/after modifier
• staticactive layer
• Layer local field and method
• Calling superproceed()
• Using thislayer
• Declarative layer composition
• Reflective layer composition
• JCop API features

Bug

• When proceed() call continues to find a method of super class, it will stack.

References

JCop