at:tutorial:objects
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at:tutorial:objects [2007/06/27 14:28] – jorge | at:tutorial:objects [2013/05/17 20:23] (current) – updated tvcutsem | ||
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In AmbientTalk, | In AmbientTalk, | ||
classes. Rather, they are either created ex-nihilo or by cloning | classes. Rather, they are either created ex-nihilo or by cloning | ||
- | and adapting existing objects, | + | and adapting existing objects, |
The following code illustrates the ex-nihilo creation of an object: | The following code illustrates the ex-nihilo creation of an object: | ||
< | < | ||
- | > def Point := object: { | + | def Point := object: { |
- | def x := 0; | + | def x := 0; |
- | def y := 0; | + | def y := 0; |
- | def init(xCoord,yCoord) { | + | def init(aX,aY) { |
- | x := xCoord; | + | x := aX; |
- | y := yCoord; | + | y := aY; |
- | }; | + | }; |
- | def sumOfSquares() { x*x + y*y }; | + | def sumOfSquares() { x*x + y*y }; |
- | } | + | } |
- | >>< | + | |
</ | </ | ||
- | As all definitions in AmbientTalk, objects, fields and methods are defined using the **def** keyword. Fields are defined using a '' | + | The above code defines an // |
- | < | + | In the example above, the state of the point object is composed of '' |
- | Note that AmbientTalk not only supports traditional canonical syntax (e.g. '' | + | |
+ | < | ||
+ | As already explained in the [[at: | ||
+ | |||
+ | For Smalltalk/ | ||
</ | </ | ||
- | In the example above, the state of the '' | + | ===== Sending messages ===== |
+ | In AmbientTalk, computation | ||
< | < | ||
- | > def aPoint := Point.new(2,3) | + | > Point.x |
- | >>< | + | >>0 |
+ | > Point.sumOfSquares() | ||
+ | >>0 | ||
</ | </ | ||
- | Every object understands | + | This code shows two messages sent to the point object |
- | ===== Sending messages ===== | + | Note that the " |
- | In AmbientTalk, | + | |
- | access to | + | ===== Cloning and instantiation ===== |
+ | As noted above, AmbientTalk objects are created [[# | ||
- | access to the fields and methods of the objects. | + | < |
+ | def anotherPoint := Point.new(2,3) | ||
+ | </ | ||
- | Object's fields | + | Every object understands the message |
< | < | ||
- | > aPoint.x | + | > anotherPoint.x |
- | >>2 | + | >> 2 |
- | > aPoint.sumOfSquares() | + | > Point.x |
- | >>13 | + | >> 0 |
+ | > anotherPoint.x := 3 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> | ||
</ | </ | ||
+ | < | ||
+ | AmbientTalk' | ||
+ | </ | ||
+ | AmbientTalk also provides a '' | ||
- | ===== Cloning and instantiation ===== | + | < |
+ | def clonedPoint := clone: Point | ||
+ | > clonedPoint.x | ||
+ | >> 0 | ||
+ | > clonedPoint.x := 2 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | </ | ||
+ | ===== Delegation and Cloning ===== | ||
+ | In order to support code reuse and modular extensions between objects, AmbientTalk features // | ||
+ | |||
+ | Delegation implies that, if a message is sent to an object, but that object has no definition for the message' | ||
+ | |||
+ | AmbientTalk distinguishes between **two kinds** of delegation relationships, | ||
+ | |||
+ | An **IS-A** delegation relationship between two objects signifies that the child object " | ||
+ | |||
+ | < | ||
+ | def Point3D := extend: Point with: { | ||
+ | def z := 0; | ||
+ | def sumOfSquares() { | ||
+ | super^sumOfSquares() + z*z; | ||
+ | }; | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In this example, '' | ||
+ | |||
+ | A **SHARES-A** relationship between two objects signifies that an object only delegates to another object purely for reasons of code or state sharing. The delegation link has no other semantics, and conceptually both parent and child can exist without one another. | ||
+ | |||
+ | The following code shows how to extend objects with a **SHARES-A** delegation relationship. It uses the '' | ||
+ | |||
+ | < | ||
+ | def Collection := share: Enumerable with: { | ||
+ | def elements := []; | ||
+ | ... | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In this code example, the '' | ||
+ | |||
+ | The **IS-A** and **SHARES-A** delegation relationships differ in their semantics for cloning child objects. Whereas cloning an **IS-A** child also clones its parent, a **SHARES-A** child shares its parent object with the clonee (see the figure below). | ||
+ | |||
+ | {{: | ||
+ | |||
+ | This cloning semantics reinforces the semantics of **IS-A** as promoting a unique link between a parent and a child object. **IS-A** delegation most closely corresponds to class-based inheritance. | ||
===== Delegation and Dynamic Inheritance ===== | ===== Delegation and Dynamic Inheritance ===== | ||
- | ===== Delegation | + | In AmbientTalk, |
+ | |||
+ | Because '' | ||
+ | |||
+ | < | ||
+ | def openConnection := object: { | ||
+ | def send(msg) { ... }; | ||
+ | }; | ||
+ | def closedConnection := object: { | ||
+ | def send(msg) { ... }; | ||
+ | }; | ||
+ | def connection := object: { | ||
+ | def init() { | ||
+ | super := closedConnection; | ||
+ | }; | ||
+ | def open() { | ||
+ | super := openConnection; | ||
+ | }; | ||
+ | def close() { | ||
+ | super := closedConnection; | ||
+ | }; | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In the above example, the '' | ||
+ | |||
+ | < | ||
+ | In AmbientTalk, | ||
+ | </ | ||
===== First-class Delegation ===== | ===== First-class Delegation ===== | ||
+ | |||
+ | AmbientTalk provides a special message-sending operator '' | ||
+ | |||
+ | < | ||
+ | def Enumerable := object: { | ||
+ | def collect: closure { | ||
+ | def c := self.new([]); | ||
+ | self.each: { |v| | ||
+ | c.add(closure(v)); | ||
+ | }; | ||
+ | c; | ||
+ | }; | ||
+ | }; | ||
+ | def Array := object: { | ||
+ | def elements := []; | ||
+ | def init(a) { elements := a; }; | ||
+ | def add(v) { elements := elements + [v]; self }; | ||
+ | def collect: closure { | ||
+ | Enumerable^collect: | ||
+ | }; | ||
+ | def each: clo { | ||
+ | 1.to: elements.length do: { |i| | ||
+ | clo(elements[i]); | ||
+ | }; | ||
+ | }; | ||
+ | }; | ||
+ | </ | ||
+ | |||
+ | A message sent to an object using the '' | ||
+ | |||
+ | < | ||
+ | Array.add(1).add(2).add(3) | ||
+ | def c := Array.collect: | ||
+ | c.each: { |v| system.print(v)} // prints 234 | ||
+ | </ | ||
+ | |||
+ | Of course, the example above is a bit contrived: we could have just assigned '' | ||
+ | |||
+ | Having described the semantics of '' | ||
+ | |||
+ | < | ||
+ | def Point3D := extend: Point with: { | ||
+ | def z := 0; | ||
+ | def init(aX, aY, aZ) { | ||
+ | super^init(aX, | ||
+ | z := aZ; | ||
+ | }; | ||
+ | }; | ||
+ | </ | ||
+ | |||
+ | <note warning> | ||
+ | AmbientTalk, | ||
+ | |||
+ | Keep in mind, however, that '' | ||
+ | </ | ||
===== Encapsulation ===== | ===== Encapsulation ===== | ||
+ | AmbientTalk has no notion of " | ||
+ | |||
+ | < | ||
+ | def makeBankAccount(balance) { | ||
+ | object: { | ||
+ | def deposit(amnt) { | ||
+ | balance := balance + amnt; | ||
+ | " | ||
+ | }; | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | Because the bank account object encapsulates its '' | ||
+ | |||
+ | < | ||
+ | > makeBankAccount(100).balance; | ||
+ | >> | ||
+ | < | ||
+ | </ | ||
+ | |||
+ | This pattern of creating objects by means of " | ||
+ | |||
+ | < | ||
+ | def b := makeBankAccount(100); | ||
+ | def b2 := b.new(); // shares its balance field with b! | ||
+ | b.deposit(10); | ||
+ | </ | ||
+ | |||
+ | In order to prevent this kind of errors, it is considered best practice to override '' | ||
+ | |||
+ | < | ||
+ | def makeBankAccount(balance) { | ||
+ | object: { | ||
+ | def new(@args) { makeBankAccount(@args) }; | ||
+ | def deposit(amnt) { /* as before */ }; | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | By overriding '' | ||
+ | |||
+ | ===== Uniform Access ===== | ||
+ | |||
+ | AmbientTalk inherits from languages like Self, Eiffel and Ruby the property that field access is made indistinguishable from invoking a nullary method. This property is often referred to as the [[Wp> | ||
+ | |||
+ | < | ||
+ | def o := object: { | ||
+ | def x := 5 | ||
+ | }; | ||
+ | > o.x | ||
+ | >> 5 | ||
+ | o := object: { | ||
+ | def x() { 5 } | ||
+ | }; | ||
+ | > o.x | ||
+ | >> 5 | ||
+ | </ | ||
+ | |||
+ | Hence, the general rule is that '' | ||
+ | |||
+ | < | ||
+ | def o := object: { | ||
+ | def x := 5 | ||
+ | def xPlus1() { x + 1 } | ||
+ | }; | ||
+ | > o.xPlus1 | ||
+ | >> 6 | ||
+ | o := object: { | ||
+ | def x() { 5 } | ||
+ | def xPlus1() { x + 1 } | ||
+ | }; | ||
+ | > o.xPlus1 | ||
+ | >> 6 | ||
+ | </ | ||
+ | |||
+ | Note that the variable lookup '' | ||
+ | |||
+ | ==== UAP and closures ==== | ||
+ | |||
+ | One may wonder how the uniform access principle interacts with closures. That is, if '' | ||
+ | |||
+ | < | ||
+ | def o := object: { | ||
+ | def x := { 5 } | ||
+ | }; | ||
+ | > o.x | ||
+ | >> < | ||
+ | </ | ||
+ | |||
+ | The rationale is that '' | ||
+ | |||
+ | < | ||
+ | > o.x() | ||
+ | >> 5 | ||
+ | </ | ||
+ | |||
+ | When explicitly // | ||
+ | |||
+ | ==== UAP and Assignment ==== | ||
+ | |||
+ | In order to uphold the uniform access principle, special care is required with respect to assignment. If clients should be fully able to abstract over the implementation of slots as either fields or accessor methods, it should be possible to unify field assignment with mutator invocation. In AmbientTalk, | ||
+ | |||
+ | < | ||
+ | def realField := 5; | ||
+ | def o := object: { | ||
+ | def virtualField() { realField }; | ||
+ | def virtualField: | ||
+ | }; | ||
+ | > o.virtualField | ||
+ | >> 5 | ||
+ | > o.virtualField := 3 // interpreted as o.virtualField: | ||
+ | >> 3 | ||
+ | </ | ||
+ | |||
+ | In essence, assigning the field of an object is treated as a message send triggering a mutator method. Here's a more useful example of virtual fields: | ||
+ | |||
+ | < | ||
+ | def time := object: { | ||
+ | def seconds := 60; | ||
+ | // a field implicitly represents an accessor method seconds() | ||
+ | // and a mutator method seconds: | ||
+ | |||
+ | def minutes() { seconds / 60 }; | ||
+ | def hours() | ||
+ | |||
+ | def minutes: | ||
+ | seconds := mins * 60; mins | ||
+ | }; | ||
+ | def hours: | ||
+ | seconds := hours * 3600; hours | ||
+ | }; | ||
+ | }; | ||
+ | > time.minutes | ||
+ | >> 1 | ||
+ | > time.hours := 1 | ||
+ | >> 1 | ||
+ | > time.seconds | ||
+ | >> 3600 | ||
+ | > time.seconds := 180 | ||
+ | >> 180 | ||
+ | > time.minutes | ||
+ | >> 2 | ||
+ | </ | ||
+ | |||
+ | If no mutator method is defined, the field assignment will fail with a '' | ||
+ | |||
+ | < | ||
+ | AmbientTalk has no direct notion of '' | ||
+ | </ | ||
+ | |||
+ | The uniform access principle for assignments equally holds for lexically visible methods. In other words, the code '' | ||
+ | |||
+ | <note warning> | ||
+ | The parser currently does not support the explicit invocation of a mutator as a method or function (as in '' | ||
+ | </ |
at/tutorial/objects.1182947320.txt.gz · Last modified: 2007/06/27 14:35 (external edit)