at:tutorial:reflection
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====== Reflective Programming ====== | ====== Reflective Programming ====== | ||
- | Reflection is an integral part of the AmbientTalk programming language. Through the use of reflection, the core language can be extended with both programming support as well as new language constructs. Both examples require a different kind of reflective access. The introduction of programming support (e.g. to visualise AmbientTalk objects) relies on **introspection**, | + | [[wp> |
- | The reflective model of AmbientTalk is based on **mirrors**, meta-level objects which allow one to reflect on an objects | + | The reflective model of AmbientTalk is based on [[http:// |
+ | |||
+ | ===== Mirrors ===== | ||
+ | |||
+ | As we have mentioned in the introduction, | ||
+ | |||
+ | A convenience primitive exists that allows AmbientTalk programmers to acquire a mirror on an object without explicitly having to consult the mirror factory (the primitive does so in the programmer' | ||
+ | |||
+ | Once a mirror has been created, it can be used to inspect an object as a collection of so-called //slot// objects, objects which bind a name to a value (a method slot is simply a slot that binds a name to a method object). | ||
+ | |||
+ | < | ||
+ | def Point := object: { | ||
+ | def x := 0; | ||
+ | def y := 0; | ||
+ | def distanceToOrigin() { (x*x + y*y).sqrt }; | ||
+ | }; | ||
+ | def p := Point.new(2, | ||
+ | // request a mirror on p via the mirror factory | ||
+ | > def mirrorOnP := reflect: p; | ||
+ | >>< | ||
+ | |||
+ | > | ||
+ | >> | ||
+ | </ | ||
+ | |||
+ | The code excerpt presented above uses the mirror to // | ||
+ | |||
+ | {{: | ||
+ | |||
+ | The code excerpt below shows how one can add and remove slots to and from an object, and how one can explicitly access values and invoke methods upon an object, reflectively: | ||
+ | |||
+ | < | ||
+ | // let's add a z coordinate to our point | ||
+ | def [zaccessor, zmutator] := lobby.at.lang.values.createFieldSlot(`z, | ||
+ | // we only add the accessor, so the slot is read-only | ||
+ | mirrorOnP.addSlot(zaccessor); | ||
+ | // let's test it: | ||
+ | > p.z | ||
+ | >> 0 | ||
+ | // we can also read slots reflectively: | ||
+ | > def x : | ||
+ | >> < | ||
+ | // and we can also invoke methods reflectively: | ||
+ | > mirrorOnP.invoke(p, | ||
+ | >> 3.605551275463989 | ||
+ | // finally, we can remove slots... | ||
+ | > mirrorOnP.removeSlot(`z); | ||
+ | </ | ||
+ | |||
+ | The following example contains the core of a unit testing framework by showing how to select all zero-argument methods of an object whose name starts with '' | ||
+ | |||
+ | < | ||
+ | >def isTestMethod(meth) { | ||
+ | | ||
+ | { meth.parameters.length == 0 } }; | ||
+ | >>< | ||
+ | >def retainTestMethods(obj) { | ||
+ | | ||
+ | | ||
+ | >>< | ||
+ | >def runTest(obj) { | ||
+ | | ||
+ | | ||
+ | >>< | ||
+ | > | ||
+ | ok | ||
+ | >>nil | ||
+ | </ | ||
+ | |||
+ | This part of the tutorial has provided a basic feeling of how AmbientTalk' | ||
+ | |||
+ | < | ||
+ | defaultMirror.listSlots.map: | ||
+ | </ | ||
+ | |||
+ | |||
+ | A complete overview of all meta-operations will be presented near the end of this chapter. | ||
+ | |||
+ | ===== Mirages ===== | ||
+ | |||
+ | Extending the AmbientTalk core language requires adding objects which have a different implementation for some of the default meta-operations. In this part of the tutorial, we describe how a programmer can redefine the programming language' | ||
+ | |||
+ | As a simple example, we show how to trace all method calls made on an object. The first step is to define a //mirror// object that encapsulates this logging behaviour. A mirror object must implement the complete AmbientTalk MOP. To make it convenient to make small changes to the MOP, AmbientTalk provides the '' | ||
+ | |||
+ | < | ||
+ | def createTracingMirror(baseObject) { | ||
+ | extend: defaultMirror.new(baseObject) with: { | ||
+ | def invoke(slf, invocation) { | ||
+ | system.println(" | ||
+ | super^invoke(slf, | ||
+ | } | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | The primitive '' | ||
+ | |||
+ | < | ||
+ | def TracingMirror := mirror: { | ||
+ | def invoke(slf, invocation) { | ||
+ | system.println(" | ||
+ | super^invoke(slf, | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | The next step is to create objects whose method calls can be traced. These objects will require the above tracing mirror as their implicit mirror. We call such objects with a custom implicit mirror **mirages**. Mirages are constructed using a variation on the '' | ||
+ | |||
+ | < | ||
+ | def mirage := object: { | ||
+ | def foo() { 42 }; | ||
+ | } mirroredBy: | ||
+ | </ | ||
+ | |||
+ | In the code above, the closure passed to '' | ||
+ | |||
+ | Another alternative is to just pass a mirror prototype to '' | ||
+ | |||
+ | < | ||
+ | def mirage := object: { | ||
+ | def foo() { 42 }; | ||
+ | } mirroredBy: TracingMirror; | ||
+ | </ | ||
+ | |||
+ | The AmbientTalk VM will then call '' | ||
+ | |||
+ | When invoking the method '' | ||
+ | |||
+ | < | ||
+ | > mirage.foo(); | ||
+ | invoked foo on < | ||
+ | >> 42 | ||
+ | </ | ||
+ | |||
+ | The picture below gives an overview of the different objects involved in the actor. | ||
+ | |||
+ | {{: | ||
+ | |||
+ | Whereas the example provided above may seem a little contrived, the reflective capabilities of AmbientTalk allow it to be extended with many abstraction relating to distributed computing for mobile ad hoc networks (AmbientTalk' | ||
+ | |||
+ | ===== The Metaobject Protocol ===== | ||
+ | |||
+ | The Meta-Object Protocol of AmbientTalk can be divided into a series of independent protocols. Whereas the full semantics and signature of the meta-methods can be found in the [[http:// | ||
+ | |||
+ | The **Message Invocation Protocol** consists of methods to deal with both synchronous and asynchronous method invocation. It provides necessary hooks to intercept both the reception of asynchronous messages and the invocation of synchronous messages. Moreover, it provides a hook to intercept asynchronous messages being sent by the object, allowing the object to add additional metadata to the message. The '' | ||
+ | |||
+ | {{: | ||
+ | |||
+ | The **Object Marshalling Protocol** consists of two methods '' | ||
+ | |||
+ | The **Slot Access and Modification Protocol** consists of operations which allow trapping both dynamic access and modification to slots. For instance, '' | ||
+ | |||
+ | The **Structural Access Protocol** reifies the structure of an AmbientTalk objects as a collection of slot objects. It consists of operations used list all available slots, get access to a first-class slot representation and to add new slots to an existing object. The '' | ||
+ | |||
+ | The **Object Instantiation Protocol** consists of the '' | ||
+ | |||
+ | The **Relational Testing Protocol** consists of the methods '' | ||
+ | |||
+ | The **Type Tag Protocol** consists of the methods '' | ||
+ | |||
+ | The **Evaluation Protocol** ensures that any AmbientTalk object can be part of a parse tree, and therefore every object provides meaningful implementations of the '' |
at/tutorial/reflection.1177618522.txt.gz · Last modified: 2007/04/27 12:58 (external edit)