Scope Rules for Variables and Attributes

In this chapter we will learn about scope rules and how Ring find variables.

Also we will learn about conflicts and how to solve/avoid them.

The next information are important once you start developing large applications using Ring

These application may uses

  • Global variables (Try to avoid them)

  • Classes (Object-Oriented)

  • braces { } to access objects

  • Declarative Programming

  • Natural Programming

Three Scopes

In Ring we have three scopes :-

  1. Public/Global Scope - Each variable you define in the statements part (before functions and classes)

  2. Object Scope - When you are inside an object (Inside class method or using { } to access the object )

  3. Local Scope - Related to functions and methods

Defining Variables and Variables Access

  1. Ring uses lexical scoping, i.e. the scope of the variable is based on where we defined the variable.

  2. Inside braces { } when you access an object, You will change the current active object scope to this object scope but you still can access the global scope and the local scope.

  3. After the ‘Class’ keyword and the class name, when you write variable names to be defined as attributes, You still can access the global scope.

In this region (class region - after the class name and before methods) we have

  • Global Scope —-> The Global Scope

  • Object Scope —-> The Object Scope

  • Local Scope —-> The Object Scope

Note

Since the local scope in the class region point also to the object scope in this region, we can use nested braces and still have access to the object scope of the class through the local scope.

Tip

You can create windows and controls as attibutes by defining them in this region.

Tip

In the class region if you created objects and used braces {} to access them then using self.attribute inside braces will use the class (not the object that you access) because you have access to the class through the local scope.

  1. Function Parameters are automatically defined in the local scope.

How Ring find the variable?

1 - Search First in the Local Scope

if not found !

2 - Search in the Object Scope

if not found !

3 - Search in the public scope

if not found —-> Runtime Error

if found —-> Check if we can do optimization to avoid searching next time (Cache / Pointers for performance).

Using Object.Attribute

When we use object.attribute the search will be in the object attributes only.

I.e. no search will be done in the local scope or in the global scope for the object attribute.

Note

Using self.attribute will search for the first self before searching for attributes.

The Self Object

The self object is a reference to the current object that we can use from the class methods.

When we are inside class method and use Self we mean the object that will be created from this class.

Inside the class methods if we used Braces { } this will change the current object scope and self will be changed also inside braces to reference the object that we access using Braces.

Inside the Class Region (after the class name and before any method) we have access to the object through the object scope and the local scope also. In this region using Self will always be a reference to the class object. if we used Braces to change the object scope then used Self inside Braces, Also self will be a reference to the class object (not the object that we already access using braces) because in the class region we have :-

  • Global Scope —> Global Scope

  • Object Scope —> Object Scope

  • Local Scope —> Object Scope

And using Braces changes the object scope only (not the local scope) and when Ring search for variables it will search in the Local Scope first so it will find self in the class that we are inside.

How Ring Define Variables and Attributes

Ring will use the variable name in the Assignment operation

1 - Search using the variable name

2 - If not found —> Avoid the runtime error and define the variable in the current scope

3 - If found —> Use the variable and don’t define anything in the current scope

  • In the global region (before any function or class) the current scope is the global scope.

  • In the class region (after the class name and before any method) the current scope is the object attributes.

  • In Functions and methods the current scope is the local scope.

Conflict between Global Variables and Class Attributes

Look at this example:

name = "test"
o1 = new person
see o1

class person
        name
        address
        phone

In the previous example we have a global variable called ‘name’ inside the class person.

when we use the variable ‘name’, Ring will start the search operation and will try to find it.

if found —> Use it

if not found —> Define new attribute

But the variable name is a global variable, so it will be found and used!

We will not have the attribute name! added to the object.

Solution (1) - Use the Main Function

func main
        name = "test"
        o1 = new person
        see o1

class person
        name
        address
        phone

Solution (2) - Use special mark for global variable names like $

$name = "test"
o1 = new person
see o1

class person
        name
        address
        phone

Solution (3) - Use the AddAttribute() Method

name = "test"
o1 = new person
see o1

class person
        AddAttribute(self,"name")
        address
        phone

Solution (4) - Use self before the attribute name

name = "test"
o1 = new person
see o1

class person
        self.name
        address
        phone

So what is the best solution to this conflict?

1 - Use the $ Mark for global variables

2 - Optional : Try to avoid global variables and use the Main function

In practice i do both of them.

The other solution

  • Use self before the attribute name or use AddAttribute()

Conflict between Class Attributes and Local Variables

This conflict may happen when we access the object using braces

Example:

func main
        name = "nice"
        o1 = new person {name="mahmoud" address="Egypt"  phone = 000 }
        see o1

class person
        name
        address
        phone

In the previous example we have the local variable name.

The value of this variable will be set to “mahmoud” instead of the object attribute.

Solution (1) : Just use Self

func main
        name = "nice"
        o1 = new person {self.name="mahmoud" address="Egypt"  phone = 000 }
        see o1

class person
        name
        address
        phone

Solution (2) : Change the Local variable name

func main
        cName = "nice"
        o1 = new person {name="mahmoud" address="Egypt"  phone = 000 }
        see o1

class person
        name
        address
        phone

Solution (3) : Change Braces and use the Dot operator

func main
        name = "nice"
        o1 = new person
        o1.name ="mahmoud"
        o1.address ="Egypt"
        o1.phone = 000
        see o1

class person
        name
        address
        phone

Using Braces to access objects inside Class Methods

Remember that we have Three scopes (Local Scope, Object Scope and Global Scope) and when we are inside a class method, we expect that we have access to the object attributes and methods and this is true until we use braces to access another object attributes and methods because in this case our object scope will be switched to another object.

new point { test() }

class point
        x=10 y=20
        func test
                see x + nl + y + nl # works fine
                myobj = new otherclass {
                        see name + nl
                        see x + nl + y + nl # error !
                }

class otherclass
        name = "test"

Output:

10
20
test

Line 8 Error (R24) : Using uninitialized variable : x
In method test() in file methodbraceerror.ring
called from line 5  in file methodbraceerror.ring

Now what we will do to solve the previous problem?

Solution (1) : Write the code that access the class attributes outside braces.

new point { test() }

class point
        x=10 y=20
        func test
                see x + nl + y + nl # works fine
                myobj = new otherclass {
                        see name + nl
                }
                see x + nl + y + nl # Outside braces - works fine


class otherclass
        name = "test"

Output:

10
20
test
10
20

Solution (2) : Don’t Use Braces

new point { test() }

class point
        x=10 y=20
        func test
                see x + nl + y + nl
                myobj = new otherclass
                see myobj.name
                see x + nl + y + nl

class otherclass
        name = "test"

Solution (3) : Copy the self object

We may use this solution if we want to use braces and get access to the class attributes (Just Reading).

new point { test() }

class point
        x=10 y=20
        func test
                oSelf = self
                see x + nl + y + nl
                myobj = new otherclass {
                        see name + nl
                        see oself.x + nl + oself.y + nl
                }

class otherclass
        name = "test"

Output:

10
20
test
10
20

Now look at this line

oself = self

The problem with the previous line is that we will have a new copy from the object Because in Ring the assignment operator copy lists and objects by value (not by reference).

When we access the new object attributes (reading) we don’t have problems

But if we modified the object attributes (Then we will modify the copy!).

Note

We can use braces again with the copy

new point { test() }

class point
        x=10 y=20
        func test
                oSelf = self
                see x + nl + y + nl
                myobj = new otherclass {
                        see name + nl
                        oSelf {
                                see x + nl + y + nl
                        }
                }

class otherclass
        name = "test"

In a GUI application, we may create a class contains the window objects as attributes to be able to access the controls from different methods. Remember the previous information when you try to access objects using braces inside methods because in this case you can’t access the object attributes directly and if you copied the self object you will work on a copy and the new controls that you create will be related to the copy and you can’t access them.

Accessing the class attributes from braces inside class methods

We access the class attributes directly from the class methods, also we have the choice to use the Self reference before the attribute/method name. Using Braces {} inside class method change the active object scope and prevent us from getting direct access to the class attributes. Also using Self will not help because the Self reference will be changed to the object that we access using Braces.

In this case if you want to read an attribute you have to copy the Self object before using Braces and if you want to modify an attribute you have to the copy from local variable to the object attribute after using Braces.

This case happens when you want to read/modify attribute insead braces.

Class MyApp

        oCon   # Attribute

        # some code here

        Func OpenDatabase
                # some code here
                new QSqlDatabase() {
                        oCon = addDatabase("QSQLITE") {
                                setDatabaseName("weighthistory.db")
                                open()
                        }
                }
                self.oCon = oCon
                # some code here

In the previous example we want to create the connection object and save it inside the oCon attribute.

The object is an output from the addDatabase() method that we use after accessing the QSQLDatabase() object.

Inside braces we can’t use the Self reference to use the object created from the MyApp class, Because the Self reference here will be to the object that we access using Braces.

We solved the problem in the previous example by creating a local variable called oCon then after Braces we copied that variable to the oCon attribute.

The next code is another solution.

Class MyApp

        oCon   # Attribute

        # some code here

        Func OpenDatabase
                # some code here
                oCon = new QSqlDatabase()
                oCon = oCon.addDatabase("QSQLITE") {
                        setDatabaseName("weighthistory.db")
                        Open()
                }
                # some code here

The next code is a better solution.

Class MyApp

        oCon   # Attribute

        # some code here

        Func OpenDatabase
                # some code here
                new QSqlDatabase() {
                        this.oCon = addDatabase("QSQLITE") {
                                setDatabaseName("weighthistory.db")
                                Open()
                        }
                }
                # some code here

Note

We used this.attribute to access the class attribute (oCon) while we are inside Braces.

Creating a Class for each Window in GUI applications

A good way for creating classes for windows is to define the window directly after the class name

In this area you can use nested braces without problems to define the window and the controls, and they will be attributes that you can access from methods.

Example:

Load "guilib.ring"

new qApp
{
        $ObjectName = "oFirstWindow"
        oFirstWindow = new FirstWindow

        $ObjectName = "oSecondWindow"
        oSecondWindow = new SecondWindow

        exec()
}

Class FirstWindow

        win = new qWidget() {
                setgeometry(0,50,300,200)
                setWindowTitle("First Window")
                label1 = new qLabel(win)
                {
                        setgeometry(10,10,300,30)
                        setText("0")
                }
                btn1 = new qPushButton(win)
                {
                        move(100,100)
                        setText("Increment")
                        setClickEvent($ObjectName+".increment()")
                }
                show()
        }

        Func Increment
                label1 {
                        setText( "" + ( 0 + text() + 1 ) )
                }


Class SecondWindow

        win = new qWidget() {
                setgeometry(400,50,300,200)
                setWindowTitle("Second Window")
                label1 = new qLabel(win)
                {
                        setgeometry(10,10,300,30)
                        setText("0")
                }
                btn1 = new qPushButton(win)
                {
                        move(100,100)
                        setText("Decrement")
                        setClickEvent($ObjectName+".decrement()")
                }
                show()
        }

        Func Decrement
                label1 {
                        setText( "" + ( 0 + text() - 1 ) )
                }

Conflict between self inside braces and self in the class region

In the class region (after the class name and before any methods) we define the attributes.

In this region we have access to the global scope and the local scope will point to the object scope.

Three Scopes

  • Global Scope —> Gloabl Scope

  • Object Scope —> Object Scope

  • Local Scope —> Object Scope

Look at this example

New Account {
        see aFriends
}

Class Account
        name = "Mahmoud"
        aFriends = []
        aFriends + new Friend {
                name = "Gal"
        }
        aFriends + new Friend {
                name = "Bert"
        }

Class Friend
        name

Output:

name: NULL
name: NULL

The problem in the previous example is that the Class account contains an attribute called “name” and the Friend class contains an attribue called “name” also.

If you tried using self.name inside braces you will get the same result!

New Account {
        see aFriends
}

Class Account
        name = "Mahmoud"
        aFriends = []
        aFriends + new Friend {
                self.name = "Gal"
        }
        aFriends + new Friend {
                self.name = "Bert"
        }

Class Friend
        name

So why using self.name inside braces doesn’t solve this conflict?

Because after the class region we have

  • global scope —> global scope

  • object scope —> object scope (Account Class)

  • local scope —> local scope (Account Class)

When we use braces we change the object scope, so we have

  • global scope —> global scope

  • object scope —> object scope (Friend Class)

  • local scope —> local scope (Account Class)

Ring search in the local scope first, so using self.name will use the Account class.

There are many solution

Solution (1) : Access the object through the list

New Account {
        see aFriends
}

Class Account
        name = "Mahmoud"
        aFriends = []
        aFriends + new Friend
        aFriends[len(aFriends)] {
                aFriends[len(aFriends)].name = "Gal"
        }
        aFriends + new Friend
        aFriends[len(aFriends)] {
                aFriends[len(aFriends)].name = "Bert"
        }

Class Friend
        name

Solution (2) : Create Method in the friend class to set the name attribute.

New Account {
        see aFriends
}

Class Account
        name = "Mahmoud"
        aFriends = []
        aFriends + new Friend {
                setname("Gal")
        }
        aFriends + new Friend {
                setname("Bert")
        }

Class Friend
        name
        func setname cName
                name = cName

Solution (3) : Create a method in the account class to set the attribute

New Account {
        see aFriends
}

Class Account
        name = "Mahmoud"
        aFriends = []
        friend("Gal")
        friend("Bert")

        func friend cName
                aFriends + new Friend {
                        name = cName
                }

Class Friend
        name

Solution (4) : Declarative Programming

New Account {
        name = "mahmoud"
        friend {
                name = "Gal"
        }
        friend {
                name = "Bert"
        }
        see aFriends
}

Class Account
        name
        aFriends = []
        friend
        func getfriend
                aFriends + new Friend
                return aFriends[len(aFriends)]

Class Friend
        name

Output:

name: Gal
name: Bert

Using braces to escape from the current object scope

Since braces change the current object scope to another object. we can use it to do some work without modifying the class attributes and using the same variable names.

new point {x=10 y=20 z=30 start() }
class point x y z
        func start
                see self # print the x y z values (10,20,30)
                new Local {
                        x = 100
                        y = 200
                        z = 300
                }
                see self # print the x y z values (10,20,30)
                see x + nl # will print 100
                see y + nl # will print 200
                see z + nl # will print 300
                Self {  # NO Advantage - Search is done in local scope first
                        see x + nl # will print 100
                        see y + nl # will print 200
                        see z + nl # will print 300
                }
                see self.x + nl # will print 10
                see self.y + nl # will print 20
                see self.z + nl # will print 30

class Local

Output:

x: 10.000000
y: 20.000000
z: 30.000000
x: 10.000000
y: 20.000000
z: 30.000000
100
200
300
100
200
300
10
20
30

The For Loops uses the local scope

Starting from Ring 1.8, when the For Loop defines new identifier (variable) it will define it in the local scope.

Example:

x = 10
? x             # Print 10
test1()
? x             # Print 10
test2()
? x             # Print 10

func test1
        for x = 1 to 5
        next
        ? x     # Print 6

func test2
        list = 1:5
        for x in list
        next
        ? x     # Print NULL (The "For In" loop will kill the reference after the loop)

Output:

10
6
10
NULL
10

Summary of Scope Rules

At first remember that

1 - Each programming language comes with it’s scope rules based on the language goals

2 - Programming in the small is different than Programming in the Large

3 - Some programming language are designed for developing small programs while others are designed for large programs

4 - In programming, If we have access to more than one scope - Then problems may come if we don’t manage things correctly

5 - It’s always more secure to reduce the number of visible scopes

6 - Some programming languages force you to manage the scope in some way, while others not!

In Ring

1 - Special and very simple scope rules that are designed for Flexibility first then Security

2 - Ring is designed to support programming in the small and programming in the large.

3 - The language provide the different programming paradigms that you may select from based on the project size. Errors comes only if you selected a bad paradigm for the target project or you are using the paradigm in a way that is not correct or at least not common.

4 - In Ring you have the choice, you can use global variables or avoid them. you can give them a special $ mark or leave them. you can use object-oriented or stay with procedures. you can use the class region (after the class name and before any method) just for attributes or use it for code too.

5 - Just read the next scope rules and think about them then use them in your favorite way.

Scope Rules:

1 - At any place in our program code we have only at maximum Three Scopes (Local Scope, Object Scope and Global Scope).

2 - When Ring find a variable it will search in the local scope first then in the object scope then in the global scope.

3 - At any time inside procedures or methods you can use braces { } to access an object and change the current object scope.

4 - In the class region (After the class name and before any method) this is a special region where both of the object scope and the local scope point to the object scope. I.e. No local variables where each variable you define in this region will become an attribute.

5 - Before defining any variable (in any scope and in the class region too) a search process will be done to use the variable if it’s found.

6 - Functions and Methods parameters are defined automatically as local variables to these functions or methods.

7 - Using Object.Attribute will search in the object attributes only.

8 - Using Self.Attribute will lead to a search for Self first then search in Self Attributes.

9 - The Self reference inside class region (after the class name and before any method) always point to the object scope created from the class.

10- The Self reference inside methods will be changed when we uses Braces to be a reference to the object that we access.

11- Writing variable names directly in the class region (after the class name and before any method) means using them or define then (in order).

12- Using self.attribute in the class region reduce search to the object scope (avoid conflict with global scope).

From these rules you can understand all types of conflicts and why you may have them and how to avoid them

Simple advices to avoid any conflict and use the scope rules in a better way

1 - Try to avoid global variables

2 - Use the Main Function - This will help you to avoid global variables

3 - If you are going to use many global variables use the $ mark before the variable name

4 - In the class region if you don’t respect the advice number three ($) then use self.attribute when you define your attributes

5 - You can use object.attribute and object.method() instead of object { attribute } and object { method() } if you don’t like changing the object scope.

6 - If you will use nested braces in a class - think about using the class region if possible because in this region you will have access to the object that you access using { } + access to the class attributes

7 - If you are inside a class method and used nested braces you will change the object scope with each brace and you will loss the access to the class attributes directly but you have access to the local scope before and after using brace { } , if you will read/modify the class attribute from braces then use This.Attribute because using ‘This’ means (The object created from this class) while using ‘Self’ means (The object in the current object scope).

After understanding all of the previous points, You will master this topic.