at:tutorial:objects
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at:tutorial:objects [2007/07/09 21:25] – explained tvcutsem | at:tutorial:objects [2007/07/27 16:29] – * tvcutsem | ||
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</ | </ | ||
- | Every object understands the message '' | + | Every object understands the message '' |
< | < | ||
Line 86: | Line 86: | ||
Delegation implies that, if a message is sent to an object, but that object has no definition for the message' | 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, | + | AmbientTalk distinguishes between **two kinds** of delegation relationships, |
+ | |||
+ | An **IS-A** delegation relationship between two objects signifies that the child object " | ||
< | < | ||
Line 92: | Line 94: | ||
def z := 0; | def z := 0; | ||
def sumOfSquares() { | def sumOfSquares() { | ||
- | super^sumOfSquares() + z*z | + | super^sumOfSquares() + z*z; |
- | } | + | }; |
} | } | ||
</ | </ | ||
- | In this example, '' | + | In this example, '' |
- | These relationships define two different semantics for clonning child objects. Whereas clonning a **IS-A** child also clones its parent, **SHARE-A** | + | 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 |
+ | The following code shows how to extend objects with a **SHARES-A** delegation relationship. It uses the '' | ||
- | {{:at:tutorial:isaversussharea.png|:at: | + | < |
+ | def Collection | ||
+ | def elements | ||
+ | | ||
+ | } | ||
+ | </ | ||
+ | In this code example, the '' | ||
- | The following code shows how to extend objects with a **IS-A** | + | The **IS-A** |
+ | {{: | ||
+ | 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. | ||
- | The following code shows how to extend | + | ===== Delegation and Dynamic Inheritance ===== |
+ | |||
+ | In AmbientTalk, | ||
+ | |||
+ | Because '' | ||
< | < | ||
- | > def point3D | + | def openConnection |
- | def z := 0; | + | def send(msg) { ... }; |
- | def sumOfSquares() { | + | }; |
- | super^sumOfSquares() + z*z | + | def closedConnection |
- | } | + | def send(msg) { ... }; |
- | } | + | }; |
+ | def connection := object: { | ||
+ | | ||
+ | super := closedConnection; | ||
+ | }; | ||
+ | def open() { | ||
+ | | ||
+ | | ||
+ | | ||
+ | super := closedConnection; | ||
+ | }; | ||
+ | } | ||
</ | </ | ||
- | ===== Delegation | + | In the above example, the '' |
- | The parent of an object is bound to a field named '' | + | |
+ | < | ||
+ | In AmbientTalk, | ||
+ | </ | ||
+ | |||
+ | ===== First-class | ||
+ | |||
+ | AmbientTalk provides | ||
< | < | ||
- | > def openConnection | + | def Enumerable |
- | > def closedConnection := object: {...}; | + | def collect: closure |
- | > def connection | + | def c := self.new([]); |
- | | + | |
- | | + | |
}; | }; | ||
- | def close() { | + | |
- | | + | }; |
+ | }; | ||
+ | def Array := object: { | ||
+ | def elements := []; | ||
+ | | ||
+ | def add(v) { elements | ||
+ | def collect: closure { | ||
+ | Enumerable^collect: | ||
+ | }; | ||
+ | def each: clo { | ||
+ | 1.to: elements.length + 1 do: { |i| | ||
+ | clo(elements[i]); | ||
}; | }; | ||
- | } | + | }; |
+ | }; | ||
</ | </ | ||
- | <note important> | + | A message sent to an object using the '' |
- | In AmbientTalk, | + | |
- | </ | + | |
- | + | ||
- | ===== First-class delegation ===== | + | |
- | AmbientTalk provides | + | |
< | < | ||
- | > def point3D | + | > Array.add(1).add(2).add(3) |
- | def z := 0; | + | >> < |
- | def init(aX, aY, aZ) { | + | > def c := Array.collect: { |v| v+1 } |
- | super^init(aX, aY); | + | >> <object:14179973> |
- | z := aZ; | + | > c.each: |
- | | + | 234 |
- | } | + | >>nil |
</ | </ | ||
- | A message sent to an object using the '' | + | Of course, |
+ | |||
+ | Having described | ||
+ | |||
+ | < | ||
+ | def Point3D := extend: Point with: { | ||
+ | def z := 0; | ||
+ | def init(aX, aY, aZ) { | ||
+ | super^init(aX, | ||
+ | z := aZ; | ||
+ | }; | ||
+ | }; | ||
+ | </ | ||
<note warning> | <note warning> | ||
- | The delegation operator | + | AmbientTalk, |
+ | |||
+ | Keep in mind, however, that '' | ||
</ | </ | ||
===== Encapsulation ===== | ===== Encapsulation ===== | ||
- | In AmbientTalk, all fields and methods are " | + | |
+ | AmbientTalk | ||
< | < | ||
- | > def makeObject(hidden) { | + | def makeBankAccount(balance) { |
- | object: { | + | object: { |
- | def foo() { /* use hidden */ } | + | def deposit(amnt) { |
- | } | + | balance := balance + amnt; |
+ | " | ||
+ | }; | ||
} | } | ||
+ | } | ||
</ | </ | ||
- | Due to the encapsulation of this object the following | + | Because |
< | < | ||
- | > makeObject(5).hidden; | + | > 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