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--- at:tutorial:reflection [2007/07/23 15:52] – elisag
+++ at:tutorial:reflection [2008/09/15 17:26] – * tvcutsem
@@ Line 1: / Line 1: @@
-<note> The tutorial is still under heavy construction! </note>
 ====== Reflective Programming ======
 [[wp>Reflection_(computer_science)|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**, the ability for a program to inspect and reason about parts of its own state. This particular flavour of reflection is quite popular and is available in most contemporary programming languages. AmbientTalk goes beyond introspection and also allows objects to supply alternative semantics for the default meta-level operations. This particular form of reflection, called **intercession**, allows enriching AmbientTalk from within the language itself.
-The reflective model of AmbientTalk is based on [[http://bracha.org/mirrors.pdf|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://bracha.org/mirrors.pdf|mirrors]], meta-level objects which allow one to reflect on an object's 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.
 ===== 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 ''reflect:'' primitive. Once a mirror has been created, it can be used for instance to produce a listing of an object's slots, as is exemplified below.
+As we have mentioned in the introduction, 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 //mirror factory// must be used to create mirrors, which allows the program to create different mirrors depending on dynamic conditions (e.g. what object made the request to reflect upon the object).
+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's stead). This primitive is called ''reflect:''.
+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).
 <code>
-def baseObject := object: {
+def Point := object: {
-  def field := nil;
+  def x := 0;
-  def canonicalMethod() { nil };
+  def y := 0;
-  def keyworded: arg1 method: arg2 { nil };
+  def distanceToOrigin() { (x*x + y*y).sqrt };
 };
-def mirror := reflect: baseObject;
+def p := Point.new(2,3);
-def slots := mirror.listSlots();
+// request a mirror on p via the mirror factory
-slots.each: { | slot | system.println(slot) };
+> def mirrorOnP := reflect: p;
+>><mirror on:<object:2493837>{x,x:=,...}>
+>mirrorOnP.listSlots().map: {|slot| slot.name };
+>>[super, super:=, x, x:=, y, y:=, distanceToOrigin]
 </code>
-The code excerpt presented above uses the mirror to //introspect// on an object and uses the ''listSlots'' meta-method to obtain a collection of the slots contained in this object, to wit ''field'', ''field:='', ''canonicalMethod'', ''keyworded:method:'' and the primitive slots ''=='', ''new'' and ''init'' which are implicitly present in every AmbientTalk object.
+The code excerpt presented above uses the mirror to //introspect// on an object and uses the ''listSlots'' meta-method. The result is a table of the slots (fields and methods) provided by this object. Notice that fields are represented as a combination of an accessor and a mutator method, conforming to the Uniform Access Principle as discussed in chapter 5. Also note that the object has a field called ''super'', although this field was not explicitly defined. In AmbientTalk, ''super'' is defined implicitly for every object.
-In addition to allowing a program to reason about the structure of its objects, mirrors can also be used to write operations such as message sending in a first-class manner. The following example uses this power to invoke a zero-argument method, whose name is specified at runtime by requesting input from the user.
+<code>
+>def inspectable := object: {
+ def map(arg1, @restArgs) { restArgs.map(arg1); } };
+>><obj:{super,super:=,map}>
+>def mirrorOnInspectable := reflect: inspectable;
+>><mirror on:<obj:{super,super:=,map}>>
+>mirrorOnInspectable.listFields()
+>>[<field:super>]
+>mirrorOnInspectable.listMethods()
+>>[<method:map>, <primitive method:new>,
+   <primitive method:init>, <primitive method:==>]
+>def method := mirrorOnInspectable.grabMethod(`map);
+>><method:map>
+>method.bodyExpression
+>>restArgs.map(arg1)
+</code>
+Using a mirror on an object, it is possible to get access to a representation of the object's methods and fields, allowing the programmer to read the value of a field, or as is exemplified above inspect the body of a method. This type of reflection is quite useful to for instance construct an inspector for live AmbientTalk objects.
+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 ''test'' and invoke them.
 <code>
-def invokeUserMethod(object) {
+>def isTestMethod(meth) {
-  def userInput := read: (system.readln());
+   (meth.name.text ~= "test.*").and:
-  // This example assumes that the user typed a single symbol
+   { meth.parameters == [] } };
-  (reflect: object).invoke(object, userInput, []);
+>><closure:isTestMethod>
-};
+>def retainTestMethods(obj) {
+   (reflect: obj).listMethods()
+     .filter: &isTestMethod };
+>><closure:retainTestMethods>
+>def runTest(obj) {
+   retainTestMethods(obj).each: { | meth |
+     (reflect: obj).invoke(obj, `(.#(meth.name)())) } };
+>><closure:runTest>
+>runTest(object: {def testOne() { system.println(`ok) } });
+ok
+>>nil
 </code>
@@ Line 80: / Line 116: @@
 The **Object Passing Protocol** consists of two methods ''pass'' and ''resolve'', which allow an object to prescribe how it should be passed to other objects and how the object should subsequently be resolved upon arrival. The default semantics allow objects to be passed by copy if they are tagged with the ''Isolate'' type tag. Otherwise, objects are passed by handing out a far object reference.
-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, ''lookup'' traverses the lexical chain whereas ''select'' (which requires an additional receiver parameter) traverses the dynamic parent chain.
+The **Slot Access and Modification Protocol** consists of operations which allow trapping both dynamic access and modification to slots. For instance, ''o.x'' can be intercepted using the ''invokeField'' meta-method, while ''o.x := 5'' is trapped using ''invoke'' where the selector will equal ''x:=''.
-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 ''listSlots'' meta-method used in previous examples is an element of this protocol.
+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 ''listMethods'' and ''listFields'' meta-methods used in previous examples are elements of this protocol.
 The **Instantiation Protocol** consists of the ''clone'' and ''newInstance'' methods, which are implictly called when using base-level code of the form ''clone: object'' and ''object.new(@args)'' respectively. In the default implementation, ''newInstance'' calls ''clone()'' to create a clone of the current object, and subsequently invokes the base-level ''init'' method with the supplied arguments on the cloned object.