Alloy example1 - Linked list

Using Alloy, we can make an abstraction model of linked list. Linked list should have the following properties.

• It should have one header as a starting Node, it can link to another Node or null.
• Node has a link which can be another Node or null.
• There should be no cycle.

sig List and Node

The first an second property can be described with “sig”.

one sig List { header: lone Node }

The form of Alloy code looks like a class in OOP, but it’s actually a set. Alloy visually models the member as a label in the arrow, and List as a box.

List has a member named header which can be one of less than one (lone) Node member.

You can think header as a variable name, and Node as the type of the variable.

sig Node { link: lone Node }

Predicate

Then you can have a predicate that is true or false. You can use predicate for describing the acyclic property.

predicate looks like a method or function as it has parameters.

You can describe many ways to describe acyclic, one of them can be “all nodes should not be a link as null”.

And don’t forget about the header, so the link should start from header : link.header.*link

pred acyclic (l : List)

{

        all n : l.header.*link | n !in n.^link

}

l.header.*link means l.header, l.header.link …

n.^link means n.link …

We check the predicate with “run acyclic” as if predicate is an entity to execute.

Alloy returns a list and two nodes that doesn’t have a cycle from the header, but it’s not what we expect.

We missed the fact that all the links from the header should be a Node (that is, has a link of Node or null)

fact

{

        List.header.*link = Node

}

Now, we finally have what we expect.

Equivalence

You can describe the acyclic property in many forms, and you can prove the equivalence between them using Alloy.

pred acyclic2 (l : List)

{

        no l.header or

        some n: l.header.*link | no n.link // n.link is null

}

This predicate says that there can be a link without any element (base case), and some link from List.header.*link should have null. (induction case). Lone means zero or one, no means zero.

pred acyclic3 (l: List)

{

        no (l.header).~link and

        (all n: l.header.*link | lone n.~link)

}

~link means that something that refers link. It means that header’s link is not referenced, and all the links are referenced at most once.

You can run predicates, or you can assert the equivalence.

assert Equiv

{

        all l : List | acyclic[l] <=> acyclic2[l]

}

check Equiv for 5

This command says check the assert with at most 5 elements.

Alloy transforms the code into SAT format, and runs the SAT solver to find a counter example, but it can’t find solution to this SAT problem, so it says they are equivalent.

The idea behind alloy

The importance of abstraction

Software is built on abstractions.

No amount of refactoring can rescue a system built on flawed concepts.

First, you design the abstractions, then you develop its embodiments in code. Unfortunately, this approach rarely works.

The problem is wishful thinking. When you implement the abstractions, they turn out to be incoherent and inconsistent.

There are two solutions to the solution of wishful thinking: “extreme programming” and “formal specification”.

Formal specification has two obstacles: succinct mathematical notations hard to understand and the lack of tools. Theorem provers demand effort more than is feasible for most software projects.

Alloy replaces conventional analysis based on theorem proving with a fully automatic analysis that gives immediate feedback. Unlike theorem proving, this analysis is not “complete”: it examines only a finite space of cases.

Unlike testing, this analysis requires no test cases. The users instead provides a property to be checked.

Except from Daniel Jackson’s Software Abstraction book Chapter 1.

You can see an example from this blog post.

Headless RCP - issue using other plugins

Issues and Solutions

Most of the issues you encounter when you work with headless RCP is highly likely with the “additial plug-ins”.

Missing constraints

Theoretically, if you have run your RCP in IDE, it should run as standalone without a problem. If you have some issues such as “Missing constraints” You can use this site.

http://stackoverflow.com/questions/14366421/the-bundle-xyz-could-not-resolved-reason-missing-constraint-import-package-a/14367997#14367997

Unbound class path container error

You get this error because your java library is wrongly setup.

Go to Add Library session in Build Path/Configure Build Path and set from “Execution environment” to “Workspace …”.

http://stackoverflow.com/questions/6798281/unbound-class-path-container-error-in-eclipse

IllegalStateException: “Workbench has not been created yet”

This error message is misleading, and it actually means “there is some race condition happening”.

You can solve this issue by

1. add -clean option to the parameter
2. Following the “NoClassFoundError case 1”, that is remove all the dependencies and add them again. When you just keep adding dependencies for whatever reasons you may end up this state.

http://waheedtechblog.blogspot.com/2011/11/javalangillegalstateexception-workbench.html

NoClassFoundError case 1

You add this, and that, and suddenly you got an error “NoClassFound”, what you can do is go to the product file, open the Dependencies. Delete all the plug-ins using “Remove” and “Add Required plug-ins” with the application that you are going to make standalone

NoClassFoundError case 2

You successfully created the plugin (utilities for example), but you may have this kind of error “NoClassDefFoundError”, when you execute the plugin that contains other plugin.

First thing you need to understand is that reference is built up already for you.

Lets’s say in the course of plugin development, you add “utilities” in “Dependencies”.

It makes your plugin know how to refer to the utilities during its build, as it finds the utilities, it can build the plugin successfully.

The issue is actually not in the user’s side, but in the provider’s side. You had to export the package in the “utilities” plugin.

SOLID principle in OOP programming

http://en.wikipedia.org/wiki/SOLID_(object-oriented_design)

• Single responsibility principle (SRP)
  • An object should have only a single responsibility.
• Open/closed principle (OCP)
  • Software entitles … should be open for extension, but closed for modification.
• Liskov substitution principle (LSP)
  • Objects in a program should be replaceable with instance of their subtypes without altering the correctness of that program.
• Interface segregation principle (ISP)
  • Many client specific interfaces are better one general purpose interface
• Dependency inversion principle (DIP
  • Do not depend upon concretions. One should depend upon abstraction.

Closure in Java using Anonymous Inner Class - example

Java doesn't support closure, but using Anonymous Inner Class, we can experience closure in Java.

Here is the code.

class Action {
	public void doAction() {
		System.out.println("Hello, world");
	}
}

class A {
	public void doStuff(Action a)
	{
		a.doAction();
	}
    public void method() {
        final int i = 0;

        doStuff(new Action() {
            public void doAction() {
                System.out.println(i);   // or whatever
            }
        });
    }
}

class Closure {
	public static void main(String[] args) {
		new A().method();
	}
}

You should be careful that the local variable in the method() method should be final in order to prevent this error message.

Closure.java:17: error: local variable i is accessed from within inner class; needs to be declared final System.out.println(i); // or whatever ^ 1 error

Instead of giving closure function to doStuff() method, anonymous inner class is given. In its implementation, doStuff() uses methods in A class as if it's closure functions. You can give different code block to doStuff() if necessary.

Compared to python example or lisp example, Java needs full support closure support to make the code simpler and easier to read.

References

• How does java implement inner class closures?

Closure in Lisp - example

This is a code that I borrowed from the book Practical Common Lips for closure in Lisp.

(defun artist-selector (artist)
  #'(lambda (cd) (equal (gets cd :artist) artist)))

In the code, artist-selector returns a closure that closes a variable "artist" over it. We can use this closure as a parameter to "select" function. Lisp supports high order function decades ago to enable it possible. "*db*" is a global variable.

(defun select (selector-fn)
  (remove-if-not selector-fn *db*))

The usage of "select" method is as follows. You can create another closure with different parameter to the "artist-selector" function.

(select (artist-selector "Dixie Chicks"))




References

• 3. Practical: A Simple Database
• Clousre in Python

Closure in Python - example

Closure is a "function (code block)" with a "referencing environment". I interpret it as a function pointer with free variables. In Python terms, a variable defined in an enclosing function is a free variable inside any nested functions.
In this example, printer is a closure as it points to a code block with a free variable of "msg". The variable msg is closed over the printer() function.

def make_printer(msg):
    def printer():
        print msg
    return printer

printer = make_printer('Foo!')
printer()

This printer method is not a closure but nested function, as in this case "msg" variable is nothing but a local variable that is not "closed".

def make_printer(msg):
    def printer(msg=msg):
        print msg
    return printer

printer = make_printer("Foo!")
printer() # "Foo!"
printer("Hello") # --> "Hello"

References

• Closure (computer science)
• Closures in Python