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--- at:tutorial:objects [2007/07/02 13:44] – jorge
+++ at:tutorial:objects [2007/07/09 21:13] – rewrote tvcutsem
@@ Line 4: / Line 4: @@
 In AmbientTalk, objects are not instantiated from
 classes. Rather, they are either created ex-nihilo or by cloning
-and adapting existing objects, like prototypes in the SELF programming language. The definition of such a prototypical object contains a number of fields and methods that represent the object's state and behaviour respectively.
+and adapting existing objects, in the spirit of prototype-based programming such as in the SELF programming language. The definition of a prototypical object contains a number of fields and methods that represent the object's state and behaviour respectively.
 The following code illustrates the ex-nihilo creation of an object:
 <code>
-> def point := object: {
+def Point := object: {
-    def x := 0;
+  def x := 0;
-    def y := 0;
+  def y := 0;
-    def init(aX,aY) {
+  def init(aX,aY) {
-      x := aX;
+    x := aX;
-      y := aY;
+    y := aY;
-    };
+  };
-    def sumOfSquares() { x*x + y*y };
+  def sumOfSquares() { x*x + y*y };
-  }
+}
 </code>
-As all definitions in AmbientTalk, objects, fields and methods are defined using the **def** keyword. Fields are defined using a ''def name := value'' syntax while methods are defined using a ''name(parameters) {body}'' syntax.
+The above code defines an //ex-nihilo// created point object and binds it to the variable ''Point''. The object itself does not carry a name (i.e. it is "anonymous"). Like all definitions in AmbientTalk, fields and methods are defined using the ''def'' keyword. Fields are defined using a ''def name := value'' syntax while methods are defined using a ''def name(parameters) {body}'' syntax.
-<note important>
+In the example above, the state of the point object is composed of ''x'' and ''y'' fields while its behaviour corresponds to the ''init'' and ''sumOfSquares'' methods.
-AmbientTalk not only supports traditional canonical syntax (e.g. ''o.m(a,b,c)'') but also keyworded syntax (e.g. ''o.at: key put: value'') for method definitions and message sends, as in SmallTalk.
+<note>
+As already explained in the [[at:tutorial:basic|basic programming]] part of the tutorial, AmbientTalk not only supports traditional canonical syntax (e.g. ''o.m(a,b,c)'') but also keyworded syntax (e.g. ''o.at: key put: value''). Keyworded syntax can be used both for method definitions and for message sends.
+For Smalltalk/Self programmers: note that a keyworded message send does require a message sending operator (like ''.'') in between the receiver and the message, which is different from Smalltalk and Self. As will be described in later chapters, AmbientTalk features more than one message sending operator, so the programmer must explicitly specify which one to use.
 </note>
-In the example above, the state of the ''point'' object is composed of ''x'' and ''y'' fields while its behaviour corresponds to the ''init'' and ''sumOfSquares'' methods.
 ===== Sending messages =====
@@ Line 32: / Line 34: @@
 <code>
-> point.x
+> Point.x
->>2
+>>0
-> point.sumOfSquares()
+> Point.sumOfSquares()
->>13
+>>0
 </code>
-This code shows two messages sent to the ''point'' object defined above in this section. The ''x'' message acts as an accessor for the ''x'' field. The ''sumOfSquares'' message selects the ''sumOfSquares'' method and evaluates its body.
+This code shows two messages sent to the point object defined above. The ''x'' message acts as an accessor for the ''x'' field. The ''sumOfSquares'' message looks up the ''sumOfSquares'' method in the object and applies it.
+Note that the "prototypical" point object defined above can act as a stand-alone object. This is different from a class in a class-based language, which often requires the use of ''static'' fields or methods to be used stand-alone.
 ===== Cloning and instantiation =====
-As said before in this section, AmbientTalk objects are created [[objects#Objects,_fields_and_methods|ex-nihilo]] or by cloning and adapting an existing object. The code below shows the instatiation of a new ''point'' object by using the cloning semantics.
+As noted above, AmbientTalk objects are created [[#Objects,_fields_and_methods|ex-nihilo]] or by cloning and adapting an existing object. The code below shows the instatiation of a new point object by //instantiating// the ''Point'' prototype:
 <code>
-> def anotherPoint := point.new(2,3)
+def anotherPoint := Point.new(2,3)
 </code>
-Every object understands the message ''new'', which creates a clone (a shallow copy) of the receiver object and initializes the clone by invoking its ''init'' method with the arguments that were passed to new (''aX'' and ''aY'' in the example of the ''point'' object). Hence, the ''init'' method plays the role of “constructor” for AmbientTalk objects. AmbientTalk’s object instantiation protocol closely corresponds to class instantiation in class-based languages, except that the new object is a clone of an existing object, rather than an empty object allocated from a class.
+Every object understands the message ''new'', which creates a clone (a shallow copy) of the receiver object and initializes the clone by invoking its ''init'' method with the arguments that were passed to new (''aX'' and ''aY'' in the example code). Hence, the ''init'' method plays the role of "constructor" for AmbientTalk objects. Hence, in the code above, ''anotherPoint'' shares its methods with its ''Point'' prototype, but it has its own set of fields:
-AmbientTalk also provides a ''clone'' language contsruct which only creates a clone of the receiver object without calling the ''init'' method (as a matter of fact the ''new'' message desribed above does nothing more but invoking this construct and the ''init'' method subsequently).
+<code>
+> anotherPoint.x
+>> 2
+> Point.x
+>> 0
+> anotherPoint.x := 3
+>> nil
+> Point.x
+>> 0
+</code>
+<note>
+AmbientTalk's object instantiation protocol closely corresponds to class instantiation in class-based languages. The major difference lies in the evaluation context of the ''init'' method: in a class-based language, the constructor is ran in the context of an empty object, freshly allocated from the class blueprint. In AmbientTalk, the ''init'' method is ran in the context of a shallow copy of an original object. Hence, in the ''init'' method, fields do not necessarily contain ''nil'' values: they have the value of the clonee. This can sometimes be useful to express the state of a clone as a delta w.r.t. the state of its clonee.
+</note>
+AmbientTalk also provides a ''clone:'' construct which only creates a clone of the receiver object without calling the ''init'' method ((As a matter of fact the ''new'' message desribed above does nothing more but invoking this construct and the ''init'' method subsequently.)).
 <code>
-> def clonedPoint := clone: point
+def clonedPoint := clone: Point
+> clonedPoint.x
+>> 0
+> clonedPoint.x := 2
+>> nil
+> Point.x
+>> 0
 </code>
@@ Line 105: / Line 130: @@
 ===== First-class delegation =====
-AmbientTalk provides an explicit delegation operator ''^''. The code below illustrates the use of this operator in the implementation of the ''init'' method of the ''point3D'' object.
+AmbientTalk provides an explicit delegation operator ''^'' (the "caret" or "hat" symbol). The code below illustrates the use of the ''^'' operator in the implementation of the ''init'' method of the ''point3D'' object.
 <code>
@@ Line 117: / Line 142: @@
 </code>
-A message sent to an object using this symbol (e.g. to the parent object in the example above) will first look for the method that matches the selector indicated in the message in this object (and its parents) and then execute the method body in the lexical scope of the message sender.
+A message sent to an object using the ''^'' symbol (e.g. to the parent object in the example above) will start the method lookup in this object (and its parents) and then execute the method body in the lexical scope of the message sender (''self'' is bound to the message sender).
+<note warning>
+The delegation operator does not have the same semantics as the dot notation. A message sent to ''super'' using the dot notation will not only start the method lookup in the object bound the ''super'' field but also bind the ''self'' pseudo variable to this object.
+</note>
 ===== Encapsulation =====