at:tutorial:objects
Differences
This shows you the differences between two versions of the page.
Both sides previous revisionPrevious revisionNext revision | Previous revisionNext revisionBoth sides next revision | ||
at:tutorial:objects [2007/06/29 08:35] – jorge | at:tutorial:objects [2007/07/27 16:28] – * tvcutsem | ||
---|---|---|---|
Line 4: | Line 4: | ||
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(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 }; |
- | } | + | } |
- | >>< | + | |
</ | </ | ||
- | 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 // |
- | <note important> | + | In the example above, the state of the point object is composed of '' |
- | AmbientTalk not only supports traditional canonical syntax (e.g. '' | + | |
- | </ | + | |
- | In the example above, the state of the '' | + | < |
+ | As already explained in the [[at: | ||
+ | |||
+ | For Smalltalk/ | ||
+ | </ | ||
===== Sending messages ===== | ===== Sending messages ===== | ||
Line 33: | Line 34: | ||
< | < | ||
- | > aPoint.x | + | > Point.x |
- | >>2 | + | >>0 |
- | > aPoint.sumOfSquares() | + | > Point.sumOfSquares() |
- | >>13 | + | >>0 |
</ | </ | ||
- | This code shows two messages sent to the '' | + | This code shows two messages sent to the point object defined above. The '' |
+ | |||
+ | Note that the " | ||
===== Cloning and instantiation ===== | ===== Cloning and instantiation ===== | ||
- | As said before in this section, AmbientTalk objects are created [[objects# | + | As noted above, AmbientTalk objects are created [[# |
< | < | ||
- | > def aPoint | + | def anotherPoint |
- | >>< | + | |
</ | </ | ||
- | Every object understands the message '' | + | Every object understands the message '' |
+ | |||
+ | < | ||
+ | > anotherPoint.x | ||
+ | >> 2 | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | > anotherPoint.x := 3 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | </ | ||
+ | |||
+ | < | ||
+ | AmbientTalk's object instantiation protocol closely corresponds to class instantiation in class-based languages. The major difference lies in the evaluation context of the '' | ||
+ | </ | ||
+ | |||
+ | AmbientTalk also provides a '' | ||
+ | |||
+ | < | ||
+ | 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 | ||
+ | |||
+ | 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 | ||
+ | |||
+ | 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 | ||
+ | |||
+ | {{: | ||
+ | |||
+ | This cloning semantics reinforces the semantics of **IS-A** as promoting | ||
===== 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 + 1 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)} | ||
+ | 234 | ||
+ | >>nil | ||
+ | </ | ||
+ | |||
+ | 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; | ||
+ | >> | ||
+ | < | ||
+ | </ | ||
+ | |||
+ | ===== 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 | ||
+ | > 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.txt · Last modified: 2013/05/17 20:23 by tvcutsem