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 state and behaviour. How to create such mirrors and how they can be used is demonstrated in the first part of the tutorial. The second part of the tutorial showcases how to construct mirages, objects which override the default meta-level operations with custom behaviour. This tutorial concludes with a brief overview of the meta-level operations which are offered by AmbientTalk mirrors. | + | The reflective model of AmbientTalk is based on [[http:// |
===== Mirrors ===== | ===== Mirrors ===== | ||
- | AmbientTalk uses a mirror-based architecture to provide reflective access to its objects. The basic principle of a mirror-based architecture is that all reflective facilities are encapsulated in a mirror object which provides reflective access to precisely one object, its reflectee. Moreover, the mirror of the object is not directly accessible as a slot of the object. Instead, a separate factory must be used to create mirrors, which allows the program to hand out different mirrors according to the dynamic call chain, the requesting object etc. The factory can be used implicitly using the '' | + | As we have already mentioned in the introduction, |
< | < | ||
- | def baseObject | + | >def mirrorOnOne |
- | def field := nil; | + | >>< |
- | def canonicalMethod() { nil }; | + | > |
- | def keyworded: arg1 method: | + | >> |
- | }; | + | <native method:<=>>, <native method:to:do:>, ...] |
- | def mirror | + | |
- | def slots := mirror.listSlots(); | + | |
- | slots.each: { | slot | system.println(slot) }; | + | |
</ | </ | ||
- | The code excerpt presented above uses the mirror to // | + | The code excerpt presented above uses the mirror to // |
- | In addition to allowing | + | When reflecting upon a user-defined object, we can observe that every object has some implictly defined methods and fields, |
< | < | ||
- | def invokeUserMethod(object) { | + | >def inspectable := object: { |
- | def userInput | + | def map(arg1, @restArgs) { restArgs.map(arg1); |
- | // This example assumes that the user typed a single symbol | + | >>< |
- | (reflect: | + | >def mirrorOnInspectable |
- | }; | + | >>< |
+ | > | ||
+ | >> | ||
+ | > | ||
+ | >> | ||
+ | < | ||
+ | >def method := mirrorOnInspectable.grabMethod(`map); | ||
+ | >>< | ||
+ | > | ||
+ | >> | ||
+ | </code> | ||
+ | |||
+ | Using a mirror on an object, it is possible to get access to a representation of the object' | ||
+ | |||
+ | In addition to allowing a program to reason about the structure of its objects, mirrors can also be used to perform operations such as method invocation in a first-class manner. The following example shows how to select all zero-argument methods whose name starts with '' | ||
+ | |||
+ | < | ||
+ | >def isTestMethod(meth) { | ||
+ | | ||
+ | { meth.parameters == [] } }; | ||
+ | >>< | ||
+ | >def retainTestMethods(obj) { | ||
+ | | ||
+ | | ||
+ | >>< | ||
+ | >def runTest(obj) { | ||
+ | | ||
+ | | ||
+ | >>< | ||
+ | > | ||
+ | ok | ||
+ | >>nil | ||
</ | </ | ||
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} | } | ||
</ | </ | ||
+ | |||
+ | < | ||
This mirror overrides the default implementation of the meta-operation '' | This mirror overrides the default implementation of the meta-operation '' | ||
Line 78: | Line 108: | ||
The **Slot Access and Modification Protocol** consists of operations which allow trapping both access and modification to slots. These operations are further refined based on whether they transitively search the dynamic or lexical parent chain. For instance, for the lookup of a variable, '' | The **Slot Access and Modification Protocol** consists of operations which allow trapping both access and modification to slots. These operations are further refined based on whether they transitively search the dynamic or lexical parent chain. For instance, for the lookup of a variable, '' | ||
- | The **Structural Access Protocol** 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 **Structural Access Protocol** 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 **Instantiation Protocol** consists of the '' | The **Instantiation Protocol** consists of the '' |
at/tutorial/reflection.txt · Last modified: 2010/11/16 16:32 by tvcutsem