Index: doc/next-tutorial/next-tutorial.html =================================================================== diff -u -r367fd6fc64a1631117a00d57c15c33bc972fc41e -r69a2fff0fc8fe546a79002bb7b7e4b110483368b --- doc/next-tutorial/next-tutorial.html (.../next-tutorial.html) (revision 367fd6fc64a1631117a00d57c15c33bc972fc41e) +++ doc/next-tutorial/next-tutorial.html (.../next-tutorial.html) (revision 69a2fff0fc8fe546a79002bb7b7e4b110483368b) @@ -1,4163 +1,3464 @@ - - - - - -Tutorial for the Next Scripting Language - - - - - + +
+
+
+
+
+
Abstract
+

This document provides a tutorial for the Next Scripting +Language NX.

+
+

The Next Scripting Language (NX) is a highly flexible object oriented +scripting language based on Tcl [Ousterhout 1990]. NX is a successor +of XOTcl 1 [Neumann and Zdun 2000a] and was developed based on 10 +years of experience with XOTcl in projects containing several hundred +thousand lines of code. While XOTcl was the first language designed to +provide language support for design patterns, the focus of the Next +Scripting Framework and NX is on combining this with Language +Oriented Programming. In many respects, NX was designed to ease the +learning of the language for novices (by using a more mainstream +terminology, higher orthogonality of the methods, less predefined +methods), to improve maintainability (remove sources of common errors) +and to encourage developers to write better structured programs (to +provide interfaces) especially for large projects, where many +developers are involved.

+

The Next Scripting Language is based on the Next Scripting Framework +(NSF) which was developed based on the notion of language oriented +programming. The Next Scripting Frameworks provides C-level support +for defining and hosting multiple object systems in a single Tcl +interpreter. The name of the Next Scripting Framework is derived from +the universal method combinator "next", which was introduced in XOTcl. +The combinator "next" serves as a single instrument for method +combination with filters, per-object and transitive per-class mixin +classes, object methods and multiple inheritance.

+

The definition of NX is fully scripted (e.g. defined in +nx.tcl). The Next Scripting Framework is shipped with three language +definitions, containing NX and XOTcl 2. Most of the existing XOTcl 1 +programs can be used without modification in the Next Scripting +Framework by using XOTcl 2. The Next Scripting Framework requires Tcl +8.5 or newer.

+
+
+
+

1. NX and its Roots

+
+

Object oriented extensions of Tcl have quite a +long history. Two of the most prominent early Tcl based OO languages +were incr Tcl (abbreviated as itcl) and Object Tcl (OTcl +[Wetherall and Lindblad 1995]). While itcl provides a traditional +C++/Java-like object system, OTcl was following the CLOS approach and +supports a dynamic object system, allowing incremental class and +object extensions and re-classing of objects.

+

Extended Object Tcl (abbreviated as XOTcl [Neumann and Zdun 2000a]) +is a successor of OTcl and was the first language providing language +support for design patterns. XOTcl extends OTcl by providing namespace +support, adding assertions, dynamic object aggregations, slots and by +introducing per-object and per-class filters and per-object and +per-class mixins.

+

XOTcl was so far released in more than 30 versions. It is described in +its detail in more than 20 papers and serves as a basis for other +object systems like TclOO [Donal ???]. The scripting language NX and +the Next Scripting Framework [Neumann and Sobernig 2009] extend +the basic ideas of XOTcl by providing support for language-oriented +programming. The the Next Scripting Framework supports multiple +object systems concurrently. Effectively, every object system has +different base classes for creating objects and classes. Therefore, +these object systems can have different interfaces and can +follow different naming conventions for built-in methods. Currently, +the Next Scripting Framework is packaged with three object systems: +NX, XOTcl 2.0, and TclCool (the language introduced by TIP#279).

+
+
+Languages +
+
Figure 1. Language History of the Next Scripting Language
+
+

+

The primary purpose of this document is to introduce NX to beginners. +We expect some prior knowledge of programming languages, and some +knowledge about Tcl. In the following sections we introduce NX by +examples. In later sections we introduce the more advanced concepts of +the language. Conceptually, most of the addressed concepts are very +similar to XOTcl. Concerning the differences between NX and XOTcl, +please refer to the Migration Guide for the Next Scripting Language.

+
+
+
+

2. Introductory Overview Example: Stack

+
+

A classical programming example is the implementation of a stack, which +is most likely familiar to many readers from many introductory +programming courses. A stack is a last-in first-out data structure +which is manipulated via operations like push (add something to the +stack) and pop remove an entry from the stack. These operations are +called methods in the context of object oriented programming +systems. Primary goals of object orientation are encapsulation and +abstraction. Therefore, we define a common unit (a class) that defines +and encapsulates the behavior of a stack and provides methods to a user +of the data structure that abstract from the actual implementation.

+
+

2.1. Define a Class "Stack"

+

In our first example, we define a class named Stack with the methods +push and pop. When an instance of the stack is created (e.g. a +concrete stack s1) the stack will contain an instance variable named +things, initialized with the an empty list.

+
Listing 2: Class Stack

+
+
+
nx::Class create Stack {
+
+   #
+   # Stack of Things
+   #
+
+   :variable things {}
+
+   :public method push {thing} {
+      set :things [linsert ${:things} 0 $thing]
+      return $thing
+   }
+
+   :public method pop {} {
+      set top [lindex ${:things} 0]
+      set :things [lrange ${:things} 1 end]
+      return $top
+   }
+}
+

Typically, classes are defined in NX via nx::Class create, followed +by the name of the new class (here: Stack). The definition of the +stack placed between curly braces and contains here just the method +definitions. Methods of the class are defined via :method followed +by the name of the method, an argument list and the body of the +method, consisting of Tcl and NX statements.

+

When an instance of Stack is created, it will contain an instance +variable named things. If several Stack instances are created, +each of the instances will have their own (same-named but different) +instance variable. The instance variable things is used in our +example as a list for the internal representation of the stack. We +define in a next step the methods to access and modify this list +structure. A user of the stack using the provided methods does not +have to have any knowledge about the name or the structure of the +internal representation (the instance variable things).

+

The method push receives an argument thing which should be placed +on the stack. Note that we do not have to specify the type of the +element on the stack, so we can push strings as well as numbers or +other kind of things. When an element is pushed, we add this element +as the first element to the list things. We insert the element using +the Tcl command linsert which receives the list as first element, +the position where the element should be added as second and the new +element as third argument. To access the value of the instance +variable we use Tcl’s dollar operator followed by the name. The +names of instance variables are preceded with a colon :. Since the +name contains a non-plain character, Tcl requires us to put braces +around the name. The command linsert and its arguments are placed +between square brackets. This means that the function linsert is called and +a new list is returned, where the new element is inserted at the first +position (index 0) in the list things. The result of the linsert +function is assigned again to the instance variable things, which is +updated this way. Finally the method push returns the pushed thing +using the return statement.

+

The method pop returns the most recently stacked element and removes +it from the stack. Therefore, it takes the first element from the list +(using the Tcl command lindex), assigns it to the method-scoped +variable top, removes the element from the instance variable +things (by using the Tcl command lrange) and returns the value +popped element top.

+

This finishes our first implementation of the stack, more enhanced +versions will follow. Note that the methods push and pop are +defined as public; this means that these methods can be +used from all other objects in the system. Therefore, these methods +provide an interface to the stack implementation.

+
Listing 3: Using the Stack

+
+
+
#!/usr/bin/env tclsh
+package require nx
+
+nx::Class create Stack {
+
+   #
+   # Stack of Things
+   #
+   ....
+}
+
+Stack create s1
+s1 push a
+s1 push b
+s1 push c
+puts [s1 pop]
+puts [s1 pop]
+s1 destroy
+

Now we want to use the stack. The code snippet in Listing 3 shows how to use the class Stack in a script. +Since NX is based on Tcl, the script will be called with the Tcl shell +tclsh. In the Tcl shell we have to require package nx to use the +Next Scripting Framework and NX. The next lines contain the definition +of the stack as presented before. Of course, it is as well possible to +make the definition of the stack an own package, such we could simple +say package require stack, or to save the definition of a stack +simply in a file and load it via source.

+

In line 12 we create an instance of the stack, namely the stack object +s1. The object s1 is an instance of Stack and has therefore +access to its methods. The methods like push or pop can be invoked +via a command starting with the object name followed by the +method name. In lines 13-15 we push on the stack the values a, then +b, and c. In line 16 we output the result of the pop method +using the Tcl command puts. We will see on standard output the +value+c+ (the last stacked item). The output of the line 17 is the +value b (the previously stacked item). Finally, in line 18 we +destroy the object. This is not necessary here, but shows the life +cycle of an object. In some respects, destroy is the counterpart of +create from line 12.

+
+
+object-class-appclass.png +
+
Figure 4. Class and Object Diagram
+
+

+

Figure 4 shows the actual class and +object structure of the first Stack example. Note that the common +root class is nx::Object that contains methods for all objects. +Since classes are as well objects in NX, nx::Class is a +specialization of nx::Object. nx::Class provides methods for +creating objects, such as the method create which is used to create +objects (and classes as well).

+
+
+

2.2. Define an Object Named "stack"

+

The definition of the stack in Listing 2 +follows the traditional object oriented approach, found in +practically every object oriented programming language: Define a class +with some methods, create instances from this class, and use the +methods defined in the class in the instances of the class.

+

In our next example, we introduce generic objects and object +specific methods. With NX, we can define generic objects, which are +instances of the most generic class nx::Object (sometimes called +common root class). nx::Object is predefined and contains a +minimal set of methods applicable to all NX objects. In this example, +we define a generic object named stack and provide methods for this +object. The methods defined above were methods provided by a class for +objects. Now we define object specific methods, which are methods +applicable only to the object for which they are defined.

+
Listing 5: Object stack

+
+
+
nx::Object create stack {
+
+   :object variable things {}
+
+   :public object method push {thing} {
+      set :things [linsert ${:things} 0 $thing]
+      return $thing
+   }
+
+   :public object method pop {} {
+      set top [lindex ${:things} 0]
+      set :things [lrange ${:things} 1 end]
+      return $top
+   }
+}
+

The example in Listing 5 defines the +object stack in a very similar way as the class Stack. But the +following points are different.

+
    +
  • +

    +First, we use nx::Object instead of nx::Class to denote + that we want to create a generic object, not a class. +

    +
  • +
  • +

    +We use :object variable to define the variable things just for + this single instance (the object stack). +

    +
  • +
  • +

    +The definition for the methods push and pop are the same as + before, but here we defined these with object method. Therefore, + these two methods push and pop are object-specific. +

    +
  • +
+

In order to use +the stack, we can use directly the object stack in the same way as +we have used the object s1 in Listing 3 +the class diagram for this the object stack.

+
+
+object-stack.png +
+
Figure 6. Object stack
+
+

+

A reader might wonder when to use a class Stack or rather an object +stack. A big difference is certainly that one can define easily +multiple instances of a class, while the object is actually a +single, tailored entity. The concept of the object stack is similar to a module, +providing a certain functionality via a common interface, without +providing the functionality to create multiple instances. The reuse of +methods provided by the class to objects is as well a difference. If +the methods of the class are updated, all instances of the class will +immediately get the modified behavior. However, this does not mean that +the reuse for the methods of stack is not possible. NX allows for +example to copy objects (similar to prototype based languages) or to +reuse methods via e.g. aliases (more about this later).

+

Note that we use capitalized names for classes and lowercase names for +instances. This is not required and a pure convention making it easier +to understand scripts without much analysis.

+
+
+

2.3. Implementing Features using Mixin Classes

+

So far, the definition of the stack methods was pretty minimal. +Suppose, we want to define "safe stacks" that protect e.g. against +stack under-runs (a stack under-run happens, when more pop than +push operations are issued on a stack). Safety checking can be +implemented mostly independent from the implementation details of the +stack (usage of internal data structures). There are as well different +ways of checking the safety. Therefore we say that safety checking is +orthogonal to the stack core implementation.

+

With NX we can define stack-safety as a separate class using methods +with the same names as the implementations before, and "mix" this +behavior into classes or objects. The implementation of Safety in +stack under-runs and to issue error messages, when this happens.

+
Listing 7: Class Safety

+
+
+
nx::Class create Safety {
+
+  #
+  # Implement stack safety by defining an additional
+  # instance variable named "count" that keeps track of
+  # the number of stacked elements. The methods of
+  # this class have the same names and argument lists
+  # as the methods of Stack; these methods "shadow"
+  # the methods of class Stack.
+  #
+
+  :variable count 0
+
+  :public method push {thing} {
+    incr :count
+    next
+  }
+
+  :public method pop {} {
+    if {${:count} == 0} { error "Stack empty!" }
+    incr :count -1
+    next
+  }
+}
+

Note that all the methods of the class Safety end with next. +This command is a primitive command of NX, which calls the +same-named method with the same argument list as the current +invocation.

+

Assume we save the definition of the class Stack in a file named +Stack.tcl and the definition of the class Safety in a file named +Safety.tcl in the current directory. When we load the classes +Stack and Safety into the same script (see the terminal dialog in +e.g. a certain stack s2 as a safe stack, while all other stacks +(such as s1) might be still "unsafe". This can be achieved via the +option -mixin at the object creation time (see line 9 in +option -mixin mixes the class Safety into the new instance s2.

+
Listing 8: Using the Class Safety

+
+
+
% package require nx
+2.0
+% source Stack.tcl
+::Stack
+% source Safety.tcl
+::Safety
+% Stack create s1
+::s1
+% Stack create s2 -object-mixin Safety
+::s2
+% s2 push a
+a
+% s2 pop
+a
+% s2 pop
+Stack empty!
+
+% s1 info precedence
+::Stack ::nx::Object
+
+% s2 info precedence
+::Safety ::Stack ::nx::Object
+

When the method push of s2 is called, first the method of the +mixin class Safety will be invoked that increments the counter and +continues with next to call the shadowed method, here the method +push of the Stack implementation that actually pushes the item. +The same happens, when s2 pop is invoked, first the method of +Safety is called, then the method of the Stack. When the stack is +empty (the value of count reaches 0), and pop is invoked, the +mixin class Safety generates an error message (raises an exception), +and does not invoke the method of the Stack.

+

The last two commands in +Listing 8 +use introspection to query for the objects +s1 and s2 in which order the involved classes are processed. This +order is called the precedence order and is obtained via info +precedence. We see that the mixin class Safety is only in use for +s2, and takes there precedence over Stack. The common root class +nx::Object is for both s1 and s2 the base class.

+
+
+per-object-mixin.png +
+
Figure 9. Per-object Mixin
+
+

+

Note that in Listing 8, +the class Safety is only mixed into a single object (here +s2), therefore we refer to this case as a per-object mixin. +Figure 9 shows the class +diagram, where the class Safety is used as a per-object mixin for +s2.

+

The mixin class Safety can be used as well in other ways, such as e.g. for +defining classes of safe stacks:

+
Listing 10: Class SafeStack

+
+
+
#
+# Create a safe stack class by using Stack and mixin
+# Safety
+#
+nx::Class create SafeStack -superclasses Stack -mixins Safety
+
+SafeStack create s3
+

The difference of a per-class mixin and an per-object mixin is that +the per-class mixin is applicable to all instances of the +class. Therefore, we call these mixins also sometimes instance mixins. +In our example in Listing 10, +Safety is mixed into the definition of +SafeStack. Therefore, all instances of the class SafeStack (here +the instance s3) will be using the safety definitions.

+
+
+per-class-mixin.png +
+
Figure 11. Per-class Mixin
+
+

+

Figure 11 shows the class diagram +for this definition. +Note that we could use Safety as well as a per-class mixin on +Stack. In this case, all stacks would be safe stacks and we could +not provide a selective feature selection (which might be perfectly +fine).

+
+
+

2.4. Define Different Kinds of Stacks

+

The definition of Stack is generic and allows all kind of elements +to be stacked. Suppose, we want to use the generic stack definition, +but a certain stack (say, stack s4) should be a stack for integers +only. This behavior can be achieved by the same means as introduced +already in Listing 5, namely +object-specific methods.

+
Listing 12: Object Integer Stack

+
+
+
Stack create s4 {
+
+  #
+  # Create a stack with a object-specific method
+  # to check the type of entries
+  #
+
+  :public object method push {thing:integer} {
+    next
+  }
+}
+

The program snippet in Listing 12 defines an instance s4 of the class +Stack and provides an object specific method for push to implement +an integer stack. The method pull is the same for the integer stack +as for all other stacks, so it will be reused as usual from the class +Stack. The object-specific method push of s4 has a value +constraint in its argument list (thing:integer) that makes sure, +that only integers can be stacked. In case the argument is not an +integer, an exception will be raised. Of course, one could perform the +value constraint checking as well in the body of the method proc by +accepting an generic argument and by performing the test for the value +in the body of the method. In the case, the passed value is an +integer, the push method of Listing 12 calls next, and therefore calls the +shadowed generic definition of push as provided by Stack.

+
Listing 13: Class IntegerStack

+
+
+
nx::Class create IntegerStack -superclass Stack {
+
+  #
+  # Create a Stack accepting only integers
+  #
+
+  :public method push {thing:integer} {
+    next
+  }
+}
+

An alternative approach is shown in +Listing 13, where the class +IntegerStack is defined, using the same method definition +as s4, this time on the class level.

+
+
+

2.5. Define Object Specific Methods on Classes

+

In our previous examples we defined methods provided by classes +(applicable for their instances) and object-specific methods (methods +defined on objects, which are only applicable for these objects). In +this section, we introduce methods that are defined on the class +objects. Such methods are sometimes called class methods or +static methods.

+

In NX classes are objects, they are specialized objects with +additional methods. Methods for classes are often used for managing +the life-cycles of the instances of the classes (we will come to this +point in later sections in more detail). Since classes are objects, we +can use exactly the same notation as above to define class methods by +using object method. The methods defined on the class object are +in all respects identical with object specific methods shown in the +examples above.

+
Listing 14: Class Stack2

+
+
+
nx::Class create Stack2 {
+
+   :public object method available_stacks {} {
+      return [llength [:info instances]]
+   }
+
+   :variable things {}
+
+   :public method push {thing} {
+      set :things [linsert ${:things} 0 $thing]
+      return $thing
+   }
+
+   :public method pop {} {
+      set top [lindex ${:things} 0]
+      set :things [lrange ${:things} 1 end]
+      return $top
+   }
+}
+
+Stack2 create s1
+Stack2 create s2
+
+puts [Stack2 available_stacks]
+

The class Stack2 in Listing 14 consists of the +earlier definition of the class Stack and is extended by the +class-specific method available_stacks, which returns the +current number of instances of the stack. The final command puts +(line 26) prints 2 to the console.

+
+
+stack2.png +
+
Figure 15. Stack2
+
+

+

The class diagram in Figure 15 shows the +diagrammatic representation of the class object-specific method +available_stacks. Since every class is a specialization of the +common root class nx::Object, the common root class is often omitted +from the class diagrams, so it was omitted here as well in the diagram.

+
+
+
+
+

3. Basic Language Features of NX

+
+
+

3.1. Variables and Properties

+

In general, NX does not need variable declarations. It allows one to +create or modify variables on the fly by using for example the Tcl +commands set and unset. Depending on the variable name (or more +precisely, depending on the variable name’s prefix consisting of +colons ":") a variable is either local to a method, or it is an +instance variable, or a global variable. The rules are:

+
    +
  • +

    +A variable without any colon prefix refers typically to a method + scoped variable. Such a variable is created during the invocation + of the method, and it is deleted, when the method ends. In the + example below, the variable a is method scoped. +

    +
  • +
  • +

    +A variable with a single colon prefix refers to an instance + variable. An instance variable is part of the object; when the + object is destroyed, its instance variables are deleted as well. In the + example below, the variable b is an instance variable. +

    +
  • +
  • +

    +A variable with two leading colons refers to a global variable. The + lifespan of a globale variable ends when the variable is explicitly + unset or the script terminates. Variables, which are placed in Tcl + namespaces, are also global variables. In the example below, the + variable c is a global variable. +

    +
  • +
+
Listing 16: Scopes of Variables

+
+
+
nx::Class create Foo {
+
+  :public method foo args {...}
+    # "a" is a method scoped variable
+    set a 1
+    # "b" is an Instance variable
+    set :b 2
+    # "c" is a global variable/namespaced variable
+    set ::c 3
+  }
+}
+

Listing 16 shows a method foo +of some class Foo referring to differently scoped variables.

+
+

3.1.1. Properties: Configurable Instance Variables

+

As described above, there is no need to declare instance variables in +NX. In many cases, a developer might want to define some value +constraints for variables, or to provide defaults, or to make +variables configurable upon object creation. Often, variables are +"inherited", meaning that the variables declared in a general class +are also available in a more specialized class. For these purposes NX +provides variable handlers responsible for the management of +instance variables. We distinguish in NX between configurable +variables (called property) and variables that are not configurable +(called variable).

+
+
+

A property is a definition of a configurable instance variable.

+
+

The term configurable means that (a) one can provide at creation time of +an instance a value for this variable, and (b), one can query the +value via the accessor function cget and (c), one can change the +value of the variable via configure at runtime. Since the general +accessor function cget and configure are available, an application +developer does not have to program own accessor methods. When value +checkers are provided, each time, the value of the variable is to be +changed, the constrained are checked as well.

+
+
+person-student.png +
+
Figure 17. Classes Person and Student
+
+

+

The class diagram above defines the classes Person and +Student. For both classes, configurable instance variable are +specified by defining these as properties. The listing below shows +an implementation of this conceptual model in NX.

+
Listing 18: Properties

+
+
+
#
+# Define a class Person with properties "name"
+# and "birthday"
+#
+nx::Class create Person {
+  :property name:required
+  :property birthday
+}
+
+#
+# Define a class Student as specialization of Person
+# with additional properties
+#
+nx::Class create Student -superclass Person {
+  :property matnr:required
+  :property {oncampus:boolean true}
+}
+
+#
+# Create instances using configure parameters
+# for the initialization
+#
+Person create p1 -name Bob
+Student create s1 -name Susan -matnr 4711
+
+# Access property value via accessor method
+puts "The name of s1 is [s1 cget -name]"
+

By defining name and birthday as properties of Person, NX makes +these configurable. When we create an instance of Person named +p1, we can provide a value for e.g. the name by specifying -name +during creation. The properties result in non-positional configure parameters +which can be provided in any order. In our listing, we create an instance of +Person using the configure parameter name and provide the value of +Bob to the instance variable name.

+

The class Student is defined as a specialization of Person with +two additional properties: matnr and oncampus. The property +matnr is required (it has to be provided, when an instance of this +class is created), and the property oncampus is boolean, and is per +default set to true. Note that the class Student inherits the +properties of Person. So, Student has four properties in total.

+

The property definitions provide the configure parameters for +instance creation. Many other languages require such parameters to be +passed via arguments of a constructor, which is often error prone, +when values are to be passed to superclasses. Also in dynamic +languages, the relationships between classes can be easily changed, +and different superclasses might have different requirements in their +constructors. The declarative approach in NX reduces the need for +tailored constructor methods significantly.

+

Note, that the property matnr of class Student is required. This +means, that if we try to create an instance of Student, a runtime +exception will be triggered. The property oncamups is boolean and +contains a default value. Providing a default value means that +whenever we create an instance of this class the object will contain +such an instance variable, even when we provide no value via the +configure parameters.

+

In our listing, we create an instance of Student using the two +configure parameters name and matnr. Finally, we use method cget +to obtain the value of the instance variable name of object s1.

+
+
+

3.1.2. Non-configurable Instance Variables

+

In practice, not all instance variables should be configurable. But +still, we want to be able to provide defaults similar to +properties. To define non-configurable instance variables the +predefined method variable can be used. Such instance variables are +often used for e.g. keeping the internal state of an object. The +usage of variable is in many respects similar to property. One +difference is, that property uses the same syntax as for method +parameters, whereas variable receives the default value as a +separate argument (similar to the variable command in plain +Tcl). The introductory Stack example in Listing 2 uses already the method variable.

+
Listing 19: Declaring Variables

+
+
+
nx::Class create Base {
+  :variable x 1
+  # ...
+}
+
+nx::Class create Derived -superclass Base {
+  :variable y 2
+  # ...
+}
+
+# Create instance of the class Derived
+Derived create d1
+
+# Object d1 has instance variables
+# x == 1 and y == 2
+

Note that the variable definitions are inherited in the same way as +properties. The example in Listing 19 shows a +class Derived that inherits from Base. When an instance d1 is +created, it will contain the two instance variables x and y. +Note that the variable declarations from property and variable are +used to initialize (and to configure) the instances variables of an object.

+
Listing 20: Setting Variables in the Constructor

+
+
+
nx::Class create Base2 {
+ # ...
+ :method init {} {
+   set :x 1
+   # ....
+ }
+}
+
+nx::Class create Derived2 -superclass Base2 {
+ # ...
+ :method init {} {
+   set :y 2
+   next
+   # ....
+ }
+}
+
+# Create instance of the class Derived2
+Derived2 create d2
+

In many other object oriented languages, the instance variables are +initialized solely by the constructor (similar to class Derived2 in +Listing 20). This approach is certainly +also possible in NX. Note that the approach using constructors +requires an explicit method chaining between the constructors and is +less declarative than the approach in NX using property and variable.

+

Both, property and variable provide much more functionalities. One +can for example declare public, protected or private accessor +methods, or one can define variables to be incremental (for +e.g. adding values to a list of values), or one can define variables +specific behavior.

+
+
+
+

3.2. Method Definitions

+

The basic building blocks of an object oriented program are object and +classes, which contain named pieces of code, the methods.

+
+
+

Methods are subroutines (pieces of code) associated with objects +and/or classes. A method has a name, receives optionally arguments +during invocation and returns a value.

+
+

Plain Tcl provides subroutines, which are not associated with objects +or classes. Tcl distinguishes between +proc+s (scripted subroutines) +and commands (system-languages implemented subroutines).

+

Methods might have different scopes, defining, on which kind of +objects these methods are applicable to. These are described in more +detail later on. For the time being, we deal here with methods defined +on classes, which are applicable for the instance of these classes.

+
+

3.2.1. Scripted Methods

+

Since NX is a scripting language, most methods are most likely +scripted methods, in which the method body contains Tcl code.

+
Listing 21: Scripted method

+
+
+
# Define a class
+nx::Class create Dog {
+
+  # Define a scripted method for the class
+  :public method bark {} {
+    puts "[self] Bark, bark, bark."
+  }
+}
+
+# Create an instance of the class
+Dog create fido
+
+# The following line prints "::fido Bark, bark, bark."
+fido bark
+

In the example above we create a class Dog with a scripted method +named bark. The method body defines the code, which is executed when +the method is invoked. In this example, the method bar prints out a +line on the terminal starting with the object name (this is determined +by the built in command self) followed by "Bark, bark, bark.". This +method is defined on a class and applicable to instances of the class +(here the instance fido).

+
+
+

3.2.2. C-implemented Methods

+

Not all of the methods usable in NX are scripted methods; many +predefined methods are defined in the underlying system language, +which is typically C. For example, in Listing 21 we +used the method create to create the class Dog and to create the +dog instance fido. These methods are implemented in C in the next +scripting framework.

+

C-implemented methods are not only provided by the underlying +framework but might be as well defined by application developers. This +is an advanced topic, not covered here. However, application developer +might reuse some generic C code to define their own C-implemented +methods. Such methods are for example accessors, forwarders and +aliases.

+
+
+

An accessor method is a method that accesses instance +variables of an object. A call to an accessor +without arguments uses the accessor as a getter, obtaining the actual +value of the associated variable. A call to an accessor with an +argument uses it as a setter, setting the value of the associated +variable.

+
+

NX provides support for C-implemented accessor methods. Accessors have +already been mentioned in the section about properties. When +the option -accessor public|protected|private is provided to a +variable or property definition, NX creates automatically a +same-named accessors method.

+
Listing 22: Accessor Methods

+
+
+
nx::Class create Dog {
+ :public method bark {} { puts "[self] Bark, bark, bark." }
+ :method init {} { Tail create [self]::tail}
+}
+
+nx::Class create Tail {
+  :property -accessor public {length:double 5}
+  :public method wag {} {return Joy}
+}
+
+# Create an instance of the class
+Dog create fido
+
+# Use the accessor "length" as a getter, to obtain the value
+# of a property. The following call returns the length of the
+# tail of fido
+fido::tail length get
+
+# Use the accessor "length" as a setter, to alter the value
+# of a property. The following call changes the length of
+# the tail of fido
+fido::tail length set 10
+
+# Proving an invalid values will raise an error
+fido::tail length set "Hello"
+

Listing 22 shows an extended example, where every dog +has a tail. The object tail is created as a subobject of the dog in +the constructor init. The subobject can be accessed by providing the +full name of the subobject fido::tail. The method length is an +C-implemented accessor, that enforces the value constraint (here a +floating point number, since length uses the value constraint +double). Line 25 will therefore raise an exception, since the +provided values cannot be converted to a double number.

+
Listing 23: Forwarder Methods

+
+
+
nx::Class create Dog {
+  :public method bark {} { puts "[self] Bark, bark, bark." }
+  :method init {} {
+    Tail create [self]::tail
+    :public object forward wag [self]::tail wag
+  }
+}
+
+nx::Class create Tail {
+  :property {length 5}
+  :public method wag {} {return Joy}
+}
+
+# Create an instance of the class
+Dog create fido
+
+# The invocation of "fido wag" is delegated to "fido::tail wag".
+# Therefore, the following method returns "Joy".
+fido wag
+

Listing 23 again extends the example by adding a +forwarder named wag to the object (e.g. fido). The forwarder +redirects all calls of the form fido wag with arbitrary arguments to +the subobject fido::tail.

+
+
+

A forwarder method is a +C-implemented method that redirects an invocation for a certain method +to either a method of another object or to some other method of the +same object. Forwarding an invocation of a method to some other +object is a means of delegation.

+
+

The functionality of the forwarder can just as well be implemented as +a scripted method, but for the most common cases, the forward +implementation is more efficient, and the forward method expresses +the intention of the developer.

+

The method forwarder has several options to change e.g. the order of +the arguments, or to substitute certain patterns in the argument list +etc. This will be described in later sections.

+
+
+

3.2.3. Method-Aliases

+
+
+

An alias method is a means to register either an existing method, +or a Tcl proc, or a Tcl command as a method with the provided +name on a class or object.

+
+

In some way, the method alias is a restricted form of a forwarder, +though it does not support delegation to different objects or argument +reordering. The advantage of the method alias compared to a forwarder +is that it has close to zero overhead, especially for aliasing +c-implemented methods.

+
Listing 24: Method-Alias

+
+
+
nx::Class create Dog {
+  :public method bark {} { puts "[self] Bark, bark, bark." }
+
+  # Define a public alias for the method "bark"
+  :public alias warn [:info method handle bark]
+  # ...
+}
+
+# Create an instance of the class
+Dog create fido
+
+# The following line prints "::fido Bark, bark, bark."
+fido warn
+

Listing 24 extends the last example by defining an +alias for the method bark. The example only shows the bare +mechanism. In general, method aliases are very powerful means for +reusing pre-existing functionality. The full object system of NX and +XOTcl2 is built from aliases, reusing functionality provided by the +next scripting framework under different names. Method aliases +are as well a means for implementing traits in NX.

+
+
+
+

3.3. Method Protection

+

All kinds of methods might have different kind of protections in NX. +The call-protection defines from which calling context methods might +be called. The Next Scripting Framework supports as well redefinition +protection for methods.

+

NX distinguishes between public, protected and private methods, +where the default call-protection is protected.

+
+
+

A public method can be called from every context. A protected +method can only be invoked from the same object. A private method +can only be invoked from methods defined on the same entity +(defined on the same class or on the same object) via the invocation +with the local flag (i.e. ": -local foo").

+
+

All kind of method protections are applicable for all kind of methods, +either scripted or C-implemented.

+

The distinction between public and protected leads to interfaces for +classes and objects. Public methods are intended for consumers of +these entities. Public methods define the intended ways of providing +methods for external usages (usages, from other objects or +classes). Protected methods are intended for the implementor of the +class or subclasses and not for public usage. The distinction between +protected and public reduces the coupling between consumers and the +implementation, and offers more flexibility to the developer.

+
Listing 25: Protected Methods

+
+
+
nx::Class create Foo {
+
+  # Define a public method
+  :public method foo {} {
+    # ....
+    return [:helper]
+  }
+
+  # Define a protected method
+  :method helper {} {
+     return 1
+  }
+}
+
+# Create an instance of the class:
+Foo create f1
+
+# The invocation of the public method "foo" returns 1
+f1 foo
+
+# The invocation of the protected method "helper" raises an error:
+f1 helper
+

The example above uses :protected method helper …. We could have +used here as well :method helper …, since the default method +call-protection is already protected.

+

The method call-protection of private goes one step further and +helps to hide implementation details also for implementors of +subclasses. Private methods are a means for avoiding unanticipated name +clashes. Consider the following example:

+
Listing 26: Private Methods

+
+
+
nx::Class create Base {
+  :private method helper {a b} {expr {$a + $b}}
+  :public method foo     {a b} {: -local helper $a $b}
+}
+
+nx::Class create Sub -superclass Base {
+  :public method bar     {a b} {: -local helper $a $b}
+  :private method helper {a b} {expr {$a * $b}}
+  :create s1
+}
+
+s1 foo 3 4     ;# returns 7
+s1 bar 3 4     ;# returns 12
+s1 helper 3 4  ;# raises error: unable to dispatch method helper
+

The base class implements a public method foo using the helper +method named helper. The derived class implements a as well a public +method bar, which is also using a helper method named helper. When +an instance s1 is created from the derived class, the method foo +is invoked which uses in turn the private method of the base +class. Therefore, the invocation s1 foo 3 4 returns its sum. If +the local flag had not beed used in helper, s1 would +have tried to call the helper of Sub, which would be incorrect. For +all other purposes, the private methods are "invisible" in all +situations, e.g., when mixins are used, or within the next-path, etc.

+

By using the -local flag at the call site it is possible to invoke +only the local definition of the method. If we would call the method +without this flag, the resolution order would be the standard +resolution order, starting with filters, mixins, object methods +and the full intrinsic class hierarchy.

+

NX supports the modifier private for methods and properties. In all +cases private is an instrument to avoid unanticipated interactions +and means actually "accessible for methods defined on the same entity +(object or class)". The main usage for private is to improve +locality of the code e.g. for compositional operations.

+

In order to improve locality for properties, a private property +defines therefore internally a variable with a different name to avoid +unintended interactions. The variable should be accessed via the +private accessor, which can be invoked with the -local flag. In the +following example class D introduces a private property with the +same name as a property in the superclass.

+
Listing 27: Private Properties

+
+
+
#
+# Define a class C with a property "x" and a public accessor
+#
+nx::Class create C {
+  :property -accessor public {x c}
+}
+
+#
+# Define a subclass D with a private property "x"
+# and a method bar, which is capable of accessing
+# the private property.
+#
+nx::Class create D -superclass C {
+  :property -accessor private {x d}
+  :public method bar {p} {return [: -local $p get]}
+}
+
+#
+# The private and public (or protected) properties
+# define internally separate variable that do not
+# conflict.
+#
+D create d1
+puts [d1 x get]   ;# prints "c"
+puts [d1 bar x]   ;# prints "d"
+

Without the private definition of the property, the definition of +property x in class D would shadow the +definition of the property in the superclass C for its instances +(d1 x or set :x would return d instead of c).

+
+
+

3.4. Applicability of Methods

+

As defined above, a method is a subroutine defined on an object or +class. This object (or class) contains the method. If the object (or +class) is deleted, the contained methods will be deleted as well.

+
+

3.4.1. Instance Methods

+
+
+

Typically, methods are defined on a class, and the methods defined on the +class are applicable to the instances (direct or indirect) of this +class. These methods are called instance methods.

+
+

In the following example method, foo is an instance method defined +on class C.

+
Listing 28: Methods applicable for instances

+
+
+
nx::Class create C {
+  :public method foo {} {return 1}
+  :create c1
+}
+
+# Method "foo" is defined on class "C"
+# and applicable to the instances of "C"
+c1 foo
+

There are many programming languages that only allow these types of methods. +However, NX also allows methods to be defined on objects.

+
+
+

3.4.2. Object Methods

+
+
+

Methods defined on objects are object methods. Object +methods are only applicable on the object, on which they are defined. +Object methods cannot be inherited from other objects.

+
+

The following example defines an object method bar on the +instance c1 of class C, and as well as the object specific method +baz defined on the object o1. An object method is defined +via object method.

+

Note that we can define a object method that shadows (redefines) +for this object methods provided from classes.

+
Listing 29: Object Method

+
+
+
nx::Class create C {
+  :public method foo {} {return 1}
+  :create c1 {
+     :public object method foo {} {return 2}
+     :public object method bar {} {return 3}
+  }
+}
+
+# Method "bar" is an object specific method of "c1"
+c1 bar
+
+# object-specific method "foo" returns 2
+c1 foo
+
+# Method "baz" is an object specific method of "o1"
+nx::Object create o1 {
+  :public object method baz {} {return 4}
+}
+o1 baz
+
+
+

3.4.3. Class Methods

+
+
+

A class method is a method defined on a class, which is only +applicable to the class object itself. The class method is actually +an object method of the class object.

+
+

In NX, all classes are objects. Classes are in NX special kind of +objects that have e.g. the ability to create instances and to provide +methods for the instances. Classes manage their instances. The general +method set for classes is defined on the meta-classes (more about +this later).

+

The following example defines a public class method bar on class +C. The class method is specified by using the modifier object in +front of method in the method definition command.

+
Listing 30: Class Methods

+
+
+
nx::Class create C {
+  #
+  # Define a class method "bar" and an instance
+  # method "foo"
+  #
+  :public object method bar {} {return 2}
+  :public method foo {} {return 1}
+
+  #
+  # Create an instance of the current class
+  #
+  :create c1
+}
+
+# Method "bar" is a class method of class "C"
+# therefore applicable on the class object "C"
+C bar
+
+# Method "foo" is an instance method of "C"
+# therefore applicable on instance "c1"
+c1 foo
+
+# When trying to invoke the class method on the
+# instance, an error will be raised.
+c1 bar
+

In some other object-oriented programming languages, class methods +are called "static methods".

+
+
+
+

3.5. Ensemble Methods

+

NX provides ensemble methods as a means to structure the method name +space and to group related methods. Ensemble methods are similar in +concept to Tcl’s ensemble commands.

+
+
+

An ensemble method is a form of a hierarchical method consisting of +a container method and sub-methods. The first argument of the +container method is interpreted as a selector (the sub-method). Every +sub-method can be an container method as well.

+
+

Ensemble methods provide a means to group related commands together, +and they are extensible in various ways. It is possible to add +sub-methods at any time to existing ensembles. Furthermore, it is +possible to extend ensemble methods via mixin classes.

+

The following example defines an ensemble method for string. An +ensemble method is defined when the provide method name contains a +space.

+
Listing 31: Ensemble Method

+
+
+
nx::Class create C {
+
+    # Define an ensemble method "string" with sub-methods
+    # "length", "tolower" and "info"
+
+    :public method "string length"  {x} {....}
+    :public method "string tolower" {x} {...}
+    :public method "string info" {x} {...}
+    #...
+    :create c1
+}
+
+# Invoke the ensemble method
+c1 string length "hello world"
+
+
+

3.6. Method Resolution

+

When a method is invoked, the applicable method is searched in the +following order:

+Per-object Mixins -> Per-class Mixins -> Object -> Intrinsic Class Hierarchy +

In the case, no mixins are involved, first the object is searched for +an object method with the given name, and then the class hierarchy +of the object. The method can be defined multiple times on the search +path, so some of these method definitions might be shadowed by the +more specific definitions.

+
Listing 32: Method Resolution with Intrinsic Classes

+
+
+
nx::Class create C {
+  :public method foo {} {
+    return "C foo: [next]"
+  }
+}
+
+nx::Class create D -superclass C {
+
+  :public method foo {} {
+    return "D foo: [next]"
+  }
+
+   :create d1 {
+     :public object method foo {} {
+       return "d1 foo: [next]"
+     }
+   }
+}
+
+# Invoke the method foo
+d1 foo
+# result: "d1 foo: D foo: C foo: "
+
+# Query the precedence order from NX via introspection
+d1 info precedence
+# result: "::D ::C ::nx::Object"
+

Consider the example in +Listing 32. When the method +foo is invoked on object d1, the object method has the highest +precedence and is therefore invoked. The object methods shadows +the same-named methods in the class hierarchy, namely the method foo +of class D and the method foo of class C. The shadowed methods +can be still invoked, either via the primitive next or via method +handles (we used already method handles in the section about method +aliases). In the example above, next calls the shadowed method and +add their results to the results of every method. So, the final result +contains parts from d1, D and C. Note, that the topmost next +in method foo of class C shadows no method foo and simply +returns empty (and not an error message).

+

The introspection method info precedence provides information about +the order, in which classes processed during method resolution.

+
Listing 33: Method Resolution with Mixin Classes

+
+
+
nx::Class create M1 {
+  :public method foo {} { return "M1 foo: [next]"}
+}
+nx::Class create M2 {
+  :public method foo {} { return "M2 foo: [next]"}
+}
+
+#
+# "d1" is created based on the definitions of the last example
+#
+# Add the methods from "M1" as per-object mixin to "d1"
+d1 object mixins add M1
+
+#
+# Add the methods from "M2" as per-class mixin to class "C"
+C mixins add M2
+
+# Invoke the method foo
+d1 foo
+# result: "M1 foo: M2 foo: d1 foo: D foo: C foo: "
+
+# Query the precedence order from NX via introspection
+d1 info precedence
+# result: "::M1 ::M2 ::D ::C ::nx::Object"
+

The example in Listing 33 is +an extension of the previous example. We define here two additional +classes M1 and M2 which are used as per-object and per-class +mixins. Both classes define the method foo, these methods shadow +the definitions of the intrinsic class hierarchy. Therefore an +invocation of foo on object d1 causes first an invocation of +method in the per-object mixin.

+
Listing 34: Method Invocation Flags

+
+
+
#
+# "d1" is created based on the definitions of the last two examples,
+# the mixins "M1" and "M2" are registered.
+#
+# Define a public object method "bar", which calls the method
+# "foo" which various invocation options:
+#
+d1 public object method bar {} {
+   puts [:foo]
+   puts [: -local foo]
+   puts [: -intrinsic foo]
+   puts [: -system foo]
+}
+
+# Invoke the method "bar"
+d1 bar
+

In the first line of the body of method bar, the method foo is +called as usual with an implicit receiver, which defaults to the +current object (therefore, the call is equivalent to d1 foo). The +next three calls show how to provide flags that influence the method +resolution. The flags can be provided between the colon and the method +name. These flags are used rather seldom but can be helpful in some +situations.

+

The invocation flag -local means that the method has to be resolved +from the same place, where the current method is defined. Since the +current method is defined as a object method, foo is resolved as +a object method. The effect is that the mixin definitions are +ignored. The invocation flag -local was already introduced int the +section about method protection, where it was used to call private +methods.

+

The invocation flag -intrinsic means that the method has to be resolved +from the intrinsic definitions, meaning simply without mixins. The +effect is here the same as with the invocation flag -local.

+

The invocation flag -system means that the method has to be resolved +from basic - typically predefined - classes of the object system. This +can be useful, when script overloads system methods, but still want to +call the shadowed methods from the base classes. In our case, we have +no definitions of foo on the base clases, therefore an error message +is returned.

+

The output of Listing 34 is:

+
+
+
   M1 foo: M2 foo: d1 foo: D foo: C foo:
+   d1 foo: D foo: C foo:
+   d1 foo: D foo: C foo:
+   ::d1: unable to dispatch method 'foo'
+
+
+
+

3.7. Parameters

+

NX provides a generalized mechanism for passing values to either +methods (we refer to these as method parameters) or to objects +(these are called configure parameters). Both kind of parameters +might have different features, such as:

+
    +
  • +

    +Positional and non-positional parameters +

    +
  • +
  • +

    +Required and non-required parameters +

    +
  • +
  • +

    +Default values for parameters +

    +
  • +
  • +

    +Value-checking for parameters +

    +
  • +
  • +

    +Multiplicity of parameters +

    +
  • +
+

TODO: complete list above and provide a short summary of the section

+

Before we discuss method and configure parameters in more detail, we +describe the parameter features in the subsequent sections based on +method parameters.

+
+

3.7.1. Positional and Non-Positional Parameters

+

If the position of a parameter in the list of formal arguments +(e.g. passed to a function) is significant for its meaning, this is a +positional parameter. If the meaning of the parameter is independent +of its position, this is a non-positional parameter. When we call a +method with positional parameters, the meaning of the parameters (the +association with the argument in the argument list of the method) is +determined by its position. When we call a method with non-positional +parameters, their meaning is determined via a name passed with the +argument during invocation.

+
Listing 35: Positional and Non-Positional Method Parameters

+
+
+
nx::Object create o1 {
+
+  #
+  # Method foo has positional parameters:
+  #
+  :public object method foo {x y} {
+    puts "x=$x y=$y"
+  }
+
+  #
+  # Method bar has non-positional parameters:
+  #
+  :public object method bar {-x -y} {
+    puts "x=$x y=$y"
+  }
+
+  #
+  # Method baz has non-positional and
+  # positional parameters:
+  #
+  :public object method baz {-x -y a} {
+    puts "x? [info exists x] y? [info exists y] a=$a"
+  }
+}
+
+# invoke foo (positional parameters)
+o1 foo 1 2
+
+# invoke bar (non-positional parameters)
+o1 bar -y 3 -x 1
+o1 bar -x 1 -y 3
+
+# invoke baz (positional and non-positional parameters)
+o1 baz -x 1 100
+o1 baz 200
+o1 baz -- -y
+

Consider the example in Listing 35. The method +foo has the argument list x y. This means that the first argument +is passed in an invocation like o1 foo 1 2 to x (here, the value +1), and the second argument is passed to y (here the value 2). +Method bar has in contrary just with non-positional arguments. Here +we pass the names of the parameter together with the values. In the +invocation o1 bar -y 3 -x 1 the names of the parameters are prefixed +with a dash ("-"). No matter whether in which order we write the +non-positional parameters in the invocation (see line 30 and 31 in +Listing 35) in both cases the variables x +and y in the body of the method bar get the same values assigned +(x becomes 1, y becomes 3).

+

It is certainly possible to combine positional and non-positional +arguments. Method baz provides two non-positional parameter (-y +and -y) and one positional parameter (namely a). The invocation in +line 34 passes the value of 1 to x and the value of 100 to a. +There is no value passed to y, therefore value of y will be +undefined in the body of baz, info exists y checks for the +existence of the variable y and returns 0.

+

The invocation in line 35 passes only a value to the positional +parameter. A more tricky case is in line 36, where we want to pass +-y as a value to the positional parameter a. The case is more +tricky since syntactically the argument parser might consider -y as +the name of one of the non-positional parameter. Therefore we use -- +(double dash) to indicate the end of the block of the non-positional +parameters and therefore the value of -y is passed to a.

+
+
+

3.7.2. Optional and Required Parameters

+

Per default positional parameters are required, and non-positional +parameters are optional (they can be left out). By using parameter +options, we can as well define positional parameters, which are +optional, and non-positional parameters, which are required.

+
Listing 36: Optional and Required Method Parameters

+
+
+
nx::Object create o2 {
+
+  #
+  # Method foo has one required and one optional
+  # positional parameter:
+  #
+  :public object method foo {x:required y:optional} {
+    puts "x=$x y? [info exists y]"
+  }
+
+  #
+  # Method bar has one required and one optional
+  # non-positional parameter:
+  #
+  :public object method bar {-x:required -y:optional} {
+    puts "x=$x y? [info exists y]"
+  }
+}
+
+# invoke foo (one optional positional parameter is missing)
+o2 foo 1
+

The example in Listing 36 defined method foo +with one required and one optional positional parameter. For this +purpose we use the parameter options required and optional. The +parameter options are separated from the parameter name by a colon. If +there are multiple parameter options, these are separated by commas +(we show this in later examples).

+

The parameter definition x:required for method foo is equivalent +to x without any parameter options (see e.g. previous example), +since positional parameters are per default required. The invocation +in line 21 of Listing 36 will lead to an +undefined variable y in method foo, because no value us passed to +the optional parameter. Note that only trailing positional parameters might be +optional. If we would call method foo of Listing 35 with only one argument, the system would raise an +exception.

+

Similarly, we define method bar in Listing 36 with one required and one optional non-positional +parameter. The parameter definition -y:optional is equivalent to +-y, since non-positional parameter are per default optional. +However, the non-positional parameter -x:required is required. If we +invoke bar without it, the system will raise an exception.

+
+
+

3.7.3. Default Values for Parameters

+

Optional parameters might have a default value. This default value is used, +when no argument is provided for the corresponding parameter. Default values can be +specified for positional and non-positional parameters.

+
Listing 37: Method Parameters with Default Values

+
+
+
nx::Object create o3 {
+
+  #
+  # Positional parameter with default value:
+  #
+  :public object method foo {{x 1} {y 2}} {
+    puts "x=$x y=$y"
+  }
+
+  #
+  # Non-positional parameter with default value:
+  #
+  :public object method bar {{-x 10} {-y 20}} {
+    puts "x=$x y=$y"
+  }
+}
+
+# use default values
+o3 foo
+o3 bar
+

In order to define a default value for a parameter, the parameter +specification must be of the form of a 2 element list, where the +second argument is the default value. See for an example in +Listing 37.

+
+
+

3.7.4. Value Constraints

+

NX provides value constraints for all kind of parameters. By +specifying value constraints a developer can restrict the permissible +values for a parameter and document the expected values in the source +code. Value checking in NX is conditional, it can be turned on or off +in general or on a per-usage level (more about this later). The same +mechanisms can be used not only for input value checking, but as well +for return value checking (we will address this point as well later).

+
+
Built-in Value Constraints
+

NX comes with a set of built-in value constraints, which can be +extended on the scripting level. The built-in checkers are either the +native checkers provided directly by the Next Scripting Framework (the +most efficient checkers) or the value checkers provided by Tcl through +string is …. The built-in checkers have as well the advantage that +they can be used also at any time during bootstrap of an object +system, at a time, when e.g. no objects or methods are defined. The +same checkers are used as well for all C-implemented primitives of NX +and the Next Scripting Framework.

+
+
+value-checkers.png +
+
Figure 38. General Applicable Value Checkers in NX
+
+

+

Figure 38 shows the built-in +general applicable value checkers available in NX, which can be used +for all method and configure parameters. In the next step, we show how to +use these value-checkers for checking permissible values for method +parameters. Then we will show, how to provide more detailed value +constraints.

+
Listing 39: Method Parameters with Value Constraints

+
+
+
nx::Object create o4 {
+
+  #
+  # Positional parameter with value constraints:
+  #
+  :public object method foo {x:integer o:object,optional} {
+    puts "x=$x o? [info exists o]"
+  }
+
+  #
+  # Non-positional parameter with value constraints:
+  #
+  :public object method bar {{-x:integer 10} {-verbose:boolean false}} {
+    puts "x=$x verbose=$verbose"
+  }
+}
+
+# The following invocation raises an exception, since the
+# value "a" for parameter "x" is not an integer
+o4 foo a
+

Value constraints are specified as parameter options in the parameter +specifications. The parameter specification x:integer defines x as +a required positional parameter which value is constraint to an +integer. The parameter specification o:object,optional shows how to +combine multiple parameter options. The parameter o is an optional +positional parameter, its value must be an object (see +Listing 39). Value constraints are +specified exactly the same way for non-positional parameters (see +method bar in Listing 39).

+
Listing 40: Parameterized Value Constraints

+
+
+
#
+# Create classes for Person and Project
+#
+nx::Class create Person
+nx::Class create Project
+
+nx::Object create o5 {
+  #
+  # Parameterized value constraints
+  #
+  :public object method work {
+     -person:object,type=Person
+     -project:object,type=Project
+   } {
+    # ...
+  }
+}
+
+#
+# Create a Person and a Project instance
+#
+Person create gustaf
+Project create nx
+
+#
+# Use method with value constraints
+#
+o5 work -person gustaf -project nx
+

The native checkers object, class, metaclass and baseclass can +be further specialized with the parameter option type to restrict +the permissible values to instances of certain classes. We can use for +example the native value constraint object either for testing +whether an argument is some object (without further constraints, as in +Listing 37, method foo), or we can +constrain the value further to some type (direct or indirect instance +of a class). This is shown by method work in +Listing 40 which requires +the parameter -person to be an instance of class Person and the +parameter -project to be an instance of class Project.

+
+
+
Scripted Value Constraints
+

The set of predefined value checkers can be extended by application +programs via defining methods following certain conventions. The user +defined value checkers are defined as methods of the class nx::Slot +or of one of its subclasses or instances. We will address such cases +in the next sections. In the following example we define two new +value checkers on class nx::Slot. The first value checker is called +groupsize, the second one is called choice.

+
Listing 41: Scripted Value Checker for Method Parameters

+
+
+
#
+# Value checker named "groupsize"
+#
+::nx::Slot method type=groupsize {name value} {
+  if {$value < 1 || $value > 6} {
+    error "Value '$value' of parameter $name is not between 1 and 6"
+  }
+}
+
+#
+# Value checker named "choice" with extra argument
+#
+::nx::Slot method type=choice {name value arg} {
+  if {$value ni [split $arg |]} {
+    error "Value '$value' of parameter $name not in permissible values $arg"
+  }
+}
+
+#
+# Create an application class D
+# using the new value checkers
+#
+nx::Class create D {
+  :public method foo {a:groupsize} {
+    # ...
+  }
+  :public method bar {a:choice,arg=red|yellow|green b:choice,arg=good|bad} {
+    # ...
+  }
+}
+
+D create d1
+
+# testing "groupsize";
+# the second call (with value 10) will raise an exception:
+d1 foo 2
+d1 foo 10
+
+# testing "choice"
+# the second call (with value pink for parameter a)
+# will raise an exception:
+d1 bar green good
+d1 bar pink bad
+

In order to define a checker groupsize a method of the name +type=groupsize is defined. This method receives two arguments, +name and value. The first argument is the name of the parameter +(mostly used for the error message) and the second parameter is +provided value. The value checker simply tests whether the provided +value is between 1 and 3 and raises an exception if this is not the +case (invocation in line 36 in Listing 41).

+

The checker groupsize has the permissible values defined in its +method’s body. It is as well possible to define more generic checkers +that can be parameterized. For this parameterization, one can pass an +argument to the checker method (last argument). The checker choice +can be used for restricting the values to a set of predefined +constants. This set is defined in the parameter specification. The +parameter a of method bar in Listing 41 +is restricted to the values red, yellow or green, and the +parameter b is restricted to good or bad. Note that the syntax +of the permissible values is solely defined by the definition of the +value checker in lines 13 to 17. The invocation in line 39 will be ok, +the invocation in line 40 will raise an exception, since pink is not +allowed.

+

If the same checks are used in many places in the program, +defining names for the value checker will be the better choice since +it improves maintainability. For seldom used kind of checks, the +parameterized value checkers might be more convenient.

+
+
+
+

3.7.5. Multiplicity

+
+
+

Multiplicity is used to define whether a parameter should receive +single or multiple values.

+
+

A multiplicity specification has a lower and an upper bound. A lower +bound of 0 means that the value might be empty. A lower bound of 1 +means that the parameter needs at least one value. The upper bound +might be 1 or n (or synonymously *). While the upper bound of +1 states that at most one value has to be passed, the upper bound of +n says that multiple values are permitted. Other kinds of +multiplicity are currently not allowed.

+

The multiplicity is written as parameter option in the parameter +specification in the form lower-bound..upper-bound. If no +multiplicity is defined the default multiplicity is 1..1, which +means: provide exactly one (atomic) value (this was the case in the +previous examples).

+
Listing 42: Method Parameters with Explicit Multiplicity

+
+
+
nx::Object create o6 {
+
+  #
+  # Positional parameter with an possibly empty
+  # single value
+  #
+  :public object method foo {x:integer,0..1} {
+    puts "x=$x"
+  }
+
+  #
+  # Positional parameter with an possibly empty
+  # list of values value
+  #
+  :public object method bar {x:integer,0..n} {
+    puts "x=$x"
+  }
+
+  #
+  # Positional parameter with a non-empty
+  # list of values
+  #
+  :public object method baz {x:integer,1..n} {
+    puts "x=$x"
+  }
+}
+

Listing 42 contains three examples for +positional parameters with different multiplicities. Multiplicity is +often combined with value constraints. A parameter specification of +the form x:integer,0..n means that the parameter x receives a list +of integers, which might be empty. Note that the value constraints are +applied to every single element of the list.

+

The parameter specification x:integer,0..1 means that x might be +an integer or it might be empty. This is one style of specifying that +no explicit value is passed for a certain parameter. Another style is +to use required or optional parameters. NX does not enforce any +particular style for handling unspecified values.

+

All the examples in Listing 42 are for +single positional parameters. Certainly, multiplicity is fully +orthogonal with the other parameter features and can be used as well +for multiple parameters, non-positional parameter, default values, +etc.

+
+
+

3.7.6. Defaults substitution

+

Optional object and method parameters can set a default value. Recall +that default values can be specified for positional and non-positional +parameters, alike. This default value is used to define a +corresponding method-local and object variable, respectively, and to +set it to the default value. Normally, the default value is taken +literally. Default values can also be preprocessed into a final value +using Tcl substitution in the sense of [subst]. For this, a parameter +can be decorated with the parameter option substdefault in addition +to a default value.

+
Listing 43: Default-value substitution using substdefault

+
+
+
nx::Class create ::D
+nx::Class create ::C {
+   # default: all substitutions (command, variable, control
+   # characters) active!
+  :property {d:object,type=::D,substdefault {[::D new]}};
+  :create ::c; # final values are computed and set at instantiation time
+}
+
+::c cget -d;
+

The listing in Listing 43 employs substdefault +to establish a reference named d between an instance of class ::C +and an instance of ::D when an instance of ::C is created. By +default, all substitution kinds are active: command, variable, and +backslash substitution. Beyond this default, substdefault can be +parametrized to include or to exclude any combination of substitution types:

+
    +
  • +

    +all active (default): substdefault=0b111 +

    +
  • +
  • +

    +backslashes only (-novariables, -nocommands): substdefault=0b100 +

    +
  • +
  • +

    +variables only (-nobackslashes, -nocommands): substdefault=0b010 +

    +
  • +
  • +

    +commands only (-nobackslashes, -novariables): substdefault=0b001 +

    +
  • +
  • +

    +none (-nobackslashes, -nocommands, -novariables): substdefault=0b000 +

    +
  • +
+
+
+
+
+
+

4. Advanced Language Features

+
+

+
+

4.1. Objects, Classes and Meta-Classes

+

+
+
+

4.2. Resolution Order and Next-Path

+

+
+
+

4.3. Details on Method and Configure Parameters

+

The parameter specifications are used in NX for the following +purposes. They are used for

+
    +
  • +

    +the specification of input arguments of methods and commands, for +

    +
  • +
  • +

    +the specification of return values of methods and commands, and for +

    +
  • +
  • +

    +the specification for the initialization of objects. +

    +
  • +
+

We refer to the first two as method parameters and the last one as +configure parameters. The examples in the previous sections all parameter +specification were specifications of method parameters.

+
+
+

Method parameters specify properties about permissible values passed +to methods.

+
+

The method parameter specify how methods are invoked, how the +actual arguments are passed to local variables of the invoked method +and what kind of checks should be performed on these.

+
+
+

Configure parameters are parameters that specify, how objects +can be parameterized upon creation.

+
+

Syntactically, configure parameters and method parameters are the same, +although there are certain differences (e.g. some parameter options +are only applicable for objects parameters, the list of object +parameters is computed dynamically from the class structures, object +parameters are often used in combination with special setter methods, +etc.). Consider the following example, where we define the two +application classes Person and Student with a few properties.

+
Listing 44: Configure Parameters

+
+
+
#
+# Define a class Person with properties "name"
+# and "birthday"
+#
+nx::Class create Person {
+  :property name:required
+  :property birthday
+}
+
+#
+# Define a class Student as specialization of Person
+# with and additional property
+#
+nx::Class create Student -superclass Person {
+  :property matnr:required
+  :property {oncampus:boolean true}
+}
+
+#
+# Create instances using configure parameters
+# for the initialization
+#
+Person create p1 -name Bob
+Student create s1 -name Susan -matnr 4711
+
+# Access property value via "cget" method
+puts "The name of s1 is [s1 cget -name]"
+

The class Person has two properties name and birthday, where the +property name is required, the property birthday is not. The +class Student is a subclass of Person with the additional required +property matnr and an optional property oncampus with the +default value true (see Listing 44). The class diagram below visualizes these +definitions.

+
+
+configure-parameter.png +
+
Figure 45. System and Application Classes
+
+

+

In NX, these definitions imply that instances of the class of Person +have the properties name and birthday as non-positional object +parameters. Furthermore it implies that instances of Student will +have the configure parameters of Person augmented with the object +parameters from Student (namely matnr and oncampus). Based on +these configure parameters, we can create a Person named Bob and a +Student named Susan with the matriculation number 4711 (see line +23 and 24 in <<xmp-object-parameters, +instance variables name, matnr and oncampus (the latter is +initialized with the default value).

+
+

4.3.1. Configure Parameters available for all NX Objects

+

The configure parameters are not limited to the application defined +properties, also NX provides some predefined definitions. Since +Person is a subclass of nx::Object also the configure parameters of +nx::Object are inherited. In the introductory stack example, we used +-mixins applied to an object to denote per-object mixins (see +Listing 8). Since mixins +is defined as a parameter on nx::Object it can be used as an object +parameter -mixins for all objects in NX. To put it in other words, +every object can be configured to have per-object mixins. If we would +remove this definition, this feature would be removed as well.

+

As shown in the introductory examples, every object can be configured +via a scripted initialization block (the optional scripted block +specified at object creation as last argument; see +Listing 5 or +Listing 12). The +scripted block and its meaning are as well defined by the means of +configure parameters. However, this configure parameter is positional (last +argument) and optional (it can be omitted). The following listing shows +the configure parameters of Person p1 and Student s1.

+
Listing 46: Computed Actual Configure Parameter

+
+
+
Configure parameters for Person p1:
+   Command:
+      p1 info lookup syntax configure
+   Result:
+      -name /value/ ?-birthday /value/? ?-object-mixins /mixinreg .../?
+      ?-class /class/? ?-object-filters /filterreg .../? ?/__initblock/?
+
+Configure parameter for Student s1:
+   Command:
+      s1 info lookup syntax configure
+   Result:
+      ?-oncampus /boolean/? -matnr /value/ -name /value/
+      ?-birthday /value/? ?-object-mixins /mixinreg .../? ?-class /class/?
+      ?-object-filters /filterreg .../? ?/__initblock/?
+

The given parameter show, how (a) objects can be configured +at runtime or (b) how new instances can be configured +at creation time via the new or create methods. +Introspection can be used to obtain the configuration +parameters from an object via +p1 info lookup parameters configure +(returning the configure parameters currently applicable for +configure or cget) or from a class +Person info lookup parameters create on a class +(returning the configure parameters applicable when an object +of this class is created)

+

The listed configure parameter types mixinreg and +filterreg are for converting definitions of filters and mixins. The +last value __initblock says that the content of this variable +will be executed in the context of the object being created (before +the constructor init is called). More about the configure parameter +types later.

+
+
+

4.3.2. Configure Parameters available for all NX Classes

+

Since classes are certain kind of objects, classes are parameterized +in the same way as objects. A typical parameter for a class definition +is the relation of the class to its superclass.In our example, we have +specified, that Student has Person as superclass via the +non-positional configure parameter -superclass. If no superclass is +specified for a class, the default superclass is +nx::Object. Therefore nx::Object is the default value for the +parameter superclass.

+

Another frequently used parameter for classes is -mixins to denote +per-class mixins (see e.g. the introductory Stack example in +Listing 10), which is defined in +the same way.

+

Since Student is an instance of the meta-class nx::Class it +inherits the configure parameters from nx::Class (see class diagram +Figure 45). +Therefore, one can use e.g. -superclass in the definition of classes.

+

Since nx::Class is a subclass of nx::Object, the meta-class +nx::Class inherits the parameter definitions from the most general +class nx::Object. Therefore, every class might as well be configured +with a scripted initialization block the same way as objects can be +configured. We used actually this scripted initialization block in +most examples for defining the methods of the class. The following +listing shows (simplified) the parameters applicable for Class +Student.

+
Listing 47: Parameters for Classes

+
+
+
Configure parameter for class nx::Class
+   Command:
+      nx::Class info lookup syntax configure
+   Result:
+      ?-superclass /class .../? ?-mixins /mixinreg .../?
+      ?-filters /filterreg .../? ?-object-mixins /mixinreg .../?
+      ?-class /class/? ?-object-filters /filterreg .../? ?/__initblock/?
+
+
+

4.3.3. User defined Parameter Types

+

More detailed definition of the configure parameter types comes here.

+
+
+

4.3.4. Slot Classes and Slot Objects

+

In one of the previous sections, we defined scripted (application +defined) checker methods on a class named nx::Slot. In general NX +offers the possibility to define value checkers not only for all +usages of parameters but as well differently for method parameters or +configure parameters

+
+
+slots.png +
+
Figure 48. Slot Classes and Objects
+
+

+
+
+

4.3.5. Attribute Slots

+

Still Missing

+
    +
  • +

    +return value checking +

    +
  • +
  • +

    +switch +

    +
  • +
  • +

    +initcmd … +

    +
  • +
  • +

    +subst rules +

    +
  • +
  • +

    +converter +

    +
  • +
  • +

    +incremental slots +

    +
  • +
+
+
+
+
+
+

5. Miscellaneous

+
+

+
+

5.1. Profiling

+

+
+
+

5.2. Unknown Handlers

+

NX provides two kinds of unknown handlers:

+
    +
  • +

    +Unknown handlers for methods +

    +
  • +
  • +

    +Unknown handlers for objects and classes +

    +
  • +
+
+

5.2.1. Unknown Handlers for Methods

+

Object and classes might be equipped +with a method unknown which is called in cases, where an unknown +method is called. The method unknown receives as first argument the +called method followed by the provided arguments

+
Listing 49: Unknown Method Handler

+
+
+
::nx::Object create o {
+  :object method unknown {called_method args} {
+    puts "Unknown method '$called_method' called"
+  }
+}
+
+# Invoke an unknown method for object o:
+o foo 1 2 3
+
+# Output will be: "Unknown method 'foo' called"
+

Without any provision of an unknown method handler, an error will be +raised, when an unknown method is called.

+
+
+

5.2.2. Unknown Handlers for Objects and Classes

+

The next scripting framework provides in addition to unknown method +handlers also a means to dynamically create objects and classes, when +these are referenced. This happens e.g. when superclasses, mixins, or +parent objects are referenced. This mechanism can be used to implement +e.g. lazy loading of these classes. Nsf allows one to register multiple +unknown handlers, each identified by a key (a unique name, different +from the keys of other unknown handlers).

+
Listing 50: Unknown Class Handler

+
+
+
::nx::Class public object method __unknown {name} {
+  # A very simple unknown handler, showing just how
+  # the mechanism works.
+  puts "***** __unknown called with <$name>"
+  ::nx::Class create $name
+}
+
+# Register an unknown handler as a method of ::nx::Class
+::nsf::object::unknown::add nx {::nx::Class __unknown}
+
+::nx::Object create o {
+  # The class M is unknown at this point
+
+  :object mixins add M
+  # The line above has triggered the unknown class handler,
+  # class M is now defined
+
+  puts [:info object mixins]
+  # The output will be:
+  #     ***** __unknown called with <::M>
+  #     ::M
+}
+

The Next Scripting Framework allows one to add, query, delete and list unknown handlers.

+
Listing 51: Unknown Handler registration

+
+
+
# Interface for unknown handlers:
+# nsf::object::unknown::add /key/ /handler/
+# nsf::object::unknown::get /key/
+# nsf::object::unknown::delete /key/
+# nsf::object::unknown::keys
+
References
    +
  • +

    + U. Zdun, M. Strembeck, G. Neumann: + Object-Based and Class-Based Composition of Transitive Mixins, + Information and Software Technology, 49(8) 2007 . +

    +
  • +
  • +

    + G. Neumann and U. Zdun: Filters as a + language support for design patterns in object-oriented scripting + languages. In Proceedings of COOTS’99, 5th Conference on + Object-Oriented Technologies and Systems, San Diego, May 1999. +

    +
  • +
  • +

    + G. Neumann and U. Zdun: Implementing + object-specific design patterns using per-object mixins. In Proc. of + NOSA`99, Second Nordic Workshop on Software Architecture, Ronneby, + Sweden, August 1999. +

    +
  • +
  • +

    + G. Neumann and U. Zdun: Enhancing + object-based system composition through per-object mixins. In + Proceedings of Asia-Pacific Software Engineering Conference (APSEC), + Takamatsu, Japan, December 1999. +

    +
  • +
  • +

    + G. Neumann and U. Zdun: XOTCL, an + object-oriented scripting language. In Proceedings of Tcl2k: The + 7th USENIX Tcl/Tk Conference, Austin, Texas, February 2000. +

    +
  • +
  • +

    + G. Neumann and U. Zdun: Towards the Usage + of Dynamic Object Aggregations as a Form of Composition In: + Proceedings of Symposium of Applied Computing (SAC’00), Como, + Italy, Mar 19-21, 2000. +

    +
  • +
  • +

    + G. Neumann, S. Sobernig: XOTcl 2.0 - A + Ten-Year Retrospective and Outlook, in: Proceedings of the Sixteenth + Annual Tcl/Tk Conference, Portland, Oregon, October, 2009. +

    +
  • +
  • +

    + J. K. Ousterhout: Tcl: An embeddable command + language. In Proc. of the 1990 Winter USENIX Conference, January 1990. +

    +
  • +
  • +

    + J. K. Ousterhout: Scripting: Higher Level + Programming for the 21st Century, IEEE Computer 31(3), March 1998. +

    +
  • +
  • +

    + D. Wetherall and C. J. Lindblad: Extending Tcl for + Dynamic Object-Oriented Programming. Proc. of the Tcl/Tk Workshop '95, + July 1995. +

    +
  • +
+
+
+
+
+
+

+ + +