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).
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.
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.
#!/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.
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.
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::Objectinstead ofnx::Classto denote that we want to create a generic object, not a class. -
We use
:object variableto define the variablethingsjust for this single instance (the objectstack). -
The definition for the methods
pushandpopare the same as before, but here we defined these withobject method. Therefore, these two methodspushandpopare 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.
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.
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} then { 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.
% 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.
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:
# # Create a safe stack class by using Stack and mixin # Safety # nx::Class create SafeStack -superclass Stack -mixin 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.
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.
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.
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.
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.
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 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
ais 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
bis 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
cis a global variable.
nx::Class create Foo { :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.
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.
# # 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.
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.
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.
# 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.
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.
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.
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.
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:
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.
# # 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.
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.
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.
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.
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:
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.
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.
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.
# # "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.
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.
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, which will be used, when not value is provided for this parameter. Default values can be specified for positional and non-positional parameters.
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.
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.
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 contraints are specified as parameter options in the parameter
specifications. The parameter specification x:integer defines x as
a required positional parmeter 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).
# # 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.
# # 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
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).
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.
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.
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.
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.
# # 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 43). The class diagram below visualizes these
definitions.
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.
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 paramter 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 44).
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.
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
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
::nx::Object create o { :object method unknown {called_method args} { puts "Unknown method '$called_method' called" } } # Invoke an unknown method for object o: o foo 1 2 3 # Output will be: "Unknown method 'foo' called"
Without any provision of an unknown method handler, an error will be raised, when an unknown method is called.
5.2.2. Unknown Handlers for Objects and Classes
The next scripting framework provides in addition to unknown method handlers also a means to dynamically create objects and classes, when these are referenced. This happens e.g. when superclasses, mixins, or parent objects are referenced. This mechanism can be used to implement e.g. lazy loading of these classes. Nsf allows to register multiple unknown handlers, each identified by a key (a unique name, different from the keys of other unknown handlers).
::nx::Class public object method __unknown {name} { # A very simple unknown handler, showing just how # the mechanism works. puts "***** __unknown called with <$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 to add, query, delete and list unknown handlers.
# Interface for unknown handlers: # nsf::object::unknown::add /key/ /handler/ # nsf::object::unknown::get /key/ # nsf::object::unknown::delete /key/ # nsf::object::unknown::keys
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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.
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