PageRenderTime 27ms CodeModel.GetById 8ms app.highlight 9ms RepoModel.GetById 1ms app.codeStats 1ms

/interpreter/tags/at2dist170907/src/edu/vub/at/objects/ATObject.java

http://ambienttalk.googlecode.com/
Java | 967 lines | 67 code | 57 blank | 843 comment | 0 complexity | 330b16a5f6e137a46e0eeeb390908c2e MD5 | raw file
  1/**
  2 * AmbientTalk/2 Project
  3 * ATObject.java created on 13-jul-2006 at 15:33:42
  4 * (c) Programming Technology Lab, 2006 - 2007
  5 * Authors: Tom Van Cutsem & Stijn Mostinckx
  6 * 
  7 * Permission is hereby granted, free of charge, to any person
  8 * obtaining a copy of this software and associated documentation
  9 * files (the "Software"), to deal in the Software without
 10 * restriction, including without limitation the rights to use,
 11 * copy, modify, merge, publish, distribute, sublicense, and/or
 12 * sell copies of the Software, and to permit persons to whom the
 13 * Software is furnished to do so, subject to the following
 14 * conditions:
 15 *
 16 * The above copyright notice and this permission notice shall be
 17 * included in all copies or substantial portions of the Software.
 18 *
 19 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 20 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 21 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 22 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 23 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 24 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 25 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 26 * OTHER DEALINGS IN THE SOFTWARE.
 27 */
 28package edu.vub.at.objects;
 29
 30import edu.vub.at.actors.ATAsyncMessage;
 31import edu.vub.at.exceptions.InterpreterException;
 32import edu.vub.at.exceptions.XArityMismatch;
 33import edu.vub.at.exceptions.XDuplicateSlot;
 34import edu.vub.at.exceptions.XIllegalQuote;
 35import edu.vub.at.exceptions.XIllegalUnquote;
 36import edu.vub.at.exceptions.XObjectOffline;
 37import edu.vub.at.exceptions.XSelectorNotFound;
 38import edu.vub.at.exceptions.XUnassignableField;
 39import edu.vub.at.exceptions.XUndefinedSlot;
 40import edu.vub.at.objects.coercion.ATConversions;
 41import edu.vub.at.objects.grammar.ATAssignmentSymbol;
 42import edu.vub.at.objects.grammar.ATSymbol;
 43import edu.vub.at.objects.mirrors.NATMirage;
 44import edu.vub.at.objects.mirrors.OBJMirrorRoot;
 45import edu.vub.at.objects.natives.NATCallframe;
 46import edu.vub.at.objects.natives.NATObject;
 47import edu.vub.at.objects.natives.NATText;
 48import edu.vub.at.objects.natives.NativeATObject;
 49import edu.vub.at.objects.symbiosis.JavaClass;
 50import edu.vub.at.objects.symbiosis.JavaObject;
 51
 52/**
 53 * ATObject represents the public interface common to any AmbientTalk/2 object.
 54 * Any value representing an ambienttalk object should implement this interface.
 55 * 
 56 * More specifically, this interface actually defines two interfaces all at once:
 57 * <ul>
 58 *  <li>A <b>base-level</b> interface to AmbientTalk objects, describing all
 59 *      methods and fields that a regular AmbientTalk object understands.
 60 *  <li>A <b>meta-level</b> interface to AmbientTalk objects, describing all
 61 *      methods and fields that the <b>mirror</b> on any AmbientTalk object
 62 *      understands.
 63 * </ul>
 64 * 
 65 * In the AmbientTalk/2 Interpreter implementation, there are only a few classes
 66 * that (almost) fully implement this interface. The principal implementors are:
 67 * 
 68 * <ul>
 69 *   <li>{@link NativeATObject}: provides a default implementation for all <i>native</i> data types.
 70 *   For example, native methods, closures, abstract grammar nodes, booleans, numbers, etc.
 71 *   are all represented as AmbientTalk objects with 'native' behaviour.
 72 *   <li>{@link NATCallframe}: overrides most of the default behaviour of {@link NativeATObject} to
 73 *   implement the behaviour of call frames, also known as <i>activation records</i>. In
 74 *   AmbientTalk, call frames are the objects that together define the runtime stack.
 75 *   They are objects with support for fields but without support for actual methods.
 76 *   <li>{@link NATObject}: extends the behaviour of call frames to include support for
 77 *   full-fledged, programmer-defined objects with support for methods and delegation.
 78 *   <li>{@link JavaClass} and {@link JavaObject}: adapt the behaviour of native AmbientTalk
 79 *   objects to engage in symbiosis with either a Java class or a Java object.
 80 *   This implementation makes use of the Java Reflection API to regard Java objects
 81 *   as though it were AmbientTalk objects.
 82 *   <li>{@link NATMirage} and {@link OBJMirrorRoot}: these two classes work in tandem to
 83 *   enable reflection on AmbientTalk objects. That is, because of these two classes, an
 84 *   AmbientTalk programmer can himself invoke the methods provided in this interface.
 85 *   {@link NATMirage} implements each operation in this interface by forwarding a
 86 *   downed invocation to a custom so-called <i>mirror</i> object. This mirror object
 87 *   can delegate to {@link OBJMirrorRoot}, which is a special object that implements
 88 *   each meta-level operation of this interface as a base-level operation. Hence, in
 89 *   a sense, {@link OBJMirrorRoot} also 'implements' this interface, but at the
 90 *   AmbientTalk level, rather than at the Java level.
 91 * </ul>
 92 * 
 93 * @author tvcutsem
 94 */
 95public interface ATObject extends ATConversions {
 96
 97    /* ------------------------------
 98      * -- Message Sending Protocol --
 99      * ------------------------------ */
100
101    /**
102     * This behavioural meta-level operation reifies the act of sending
103     * an asynchronous message.
104     * 
105     * When the base-level AmbientTalk code <code>rcv<-m()</code> is
106     * evaluated in the context of an object <tt>o</tt>, an asynchronous message
107     * <code><-m()</code> is first created by the current actor mirror.
108     * Subsequently, this message needs to be sent to the receiver. This
109     * meta-level operation is reified by this method, as if by invoking:
110     * <pre>(reflect: o).send(message)</pre>
111     * The default behaviour is to access the current actor's mirror and to
112     * ask the actor to send the message in this object's stead by invoking
113     * <pre>actor.send(message)</pre>
114     * @param receiver the object designated to receive the asynchronous message
115     * @param message the asynchronous message to be sent by this object
116     * 
117     * @return the result of message sending, which will be the value of an
118     * asynchronous message send expression.
119     */
120    public ATObject meta_send(ATObject receiver, ATAsyncMessage message) throws InterpreterException;
121    
122    /**
123     * This behavioural meta-level operation reifies the act of receiving
124     * an asynchronous message.
125     * 
126     * When an AmbientTalk object receives a message that was sent to it
127     * asynchronously, the message is delivered to the object's mirror by
128     * means of this meta-level operation.
129     * <p>
130     * The default behaviour of a mirror in response to the reception of a
131     * message <tt>msg</tt> is to invoke:
132     * <pre>msg.process(self)</pre>
133     * In turn, the default message processing behaviour is to invoke
134     * the method corresponding to the message's selector on this object.
135     * Hence, usually a <tt>receive</tt> operation is simply translated into
136     * a <tt>invoke</tt> operation. The reason for having a separate <tt>receive</tt>
137     * operation is that this enables the AmbientTalk meta-level programmer to
138     * distinguish between synchronously and asynchronously received messages.
139     * @param message the message that was asynchronously sent to this object
140     * @return by default, the value of invoking the method corresponding to the message
141     */
142    public ATObject meta_receive(ATAsyncMessage message) throws InterpreterException;
143   
144    /**
145     * This meta-level operation reifies synchronous message sending ("method invocation").
146     * Hence, the meta-level equivalent
147     * of the base-level code <code>o.m()</code> is:
148     * <pre>(reflect: o).invoke(o,`m,[])</pre>.
149     * 
150     * Method invocation comprises selector lookup and the application of the value
151     * bound to the selector. Selector lookup first queries an object's local
152     * fields, then the method dictionary:
153     * <ul>
154     *  <li>If the selector ends with <tt>:=</tt> and matches a field, the field
155     *  is assigned if a unary argument list is specified (i.e. the field is treated
156     *  as a mutator method).
157     *  <li>Otherwise, if the selector is bound to a field containing
158     * a closure, that closure is applied to the given arguments.
159     *  <li>If the field is not bound to a closure, the field value is returned provided no arguments were
160     * specified (i.e. the field is treated like an accessor method).
161     *  <li>If the selector is bound to a method, the method is applied.
162     *  <li>If the selector is not found, the search continues in the objects <i>dynamic parent</i>.
163     * </ul>
164     * <p>
165     * Note also that the first argument to <tt>invoke</tt> denotes the
166     * so-called "receiver" of the invocation. It is this object to which
167     * the <tt>self</tt> pseudo-variable should be bound during method execution.
168     * 
169     * @see #meta_doesNotUnderstand(ATSymbol) for what happens if the selector
170     * is not found.
171     *
172     * @param receiver the object to which <tt>self</tt> is bound during execution
173     * of the method
174     * @param selector a symbol denoting the name of the method, accessor or mutator to be invoked
175     * @param arguments the table of actual arguments to be passed to the method
176     * @return by default, the object returned from the invoked method
177     */
178    public ATObject meta_invoke(ATObject receiver, ATSymbol selector, ATTable arguments) throws InterpreterException;
179
180    /**
181     * This meta-level operation reifies "field selection".
182     * In other words, the base-level code
183     * <code>o.m</code>
184     * is interpreted at the meta-level as:
185     * <code>(reflect: o).invokeField(o, `m)</code>
186     * 
187     * This meta-level operation is nearly identical to {@link #meta_invoke(ATObject, ATSymbol, ATTable)} with one
188     * important difference. When the selector is bound to a field storing a closure, this meta-level operation
189     * does <b>not</b> auto-apply the closure, but returns the closure instead.
190     * 
191     * For all other cases, the following equality holds:
192     * <code>o.m == o.m()</code>
193     * or, at the meta-level:
194     * <code>(reflect: o).invokeField(o, `m) == (reflect: o).invoke(o, `m, [])</code>
195     * 
196     * This effectively means that for client objects, it should not matter whether
197     * a property is implemented as a field or as a pair of accessor/mutator methods.
198     * 
199     * @param receiver the base-level object from which the 'field' should be selected.
200     * @param selector a symbol denoting the name of the method, accessor or mutator to be invoked
201     * @return the value of a field, or the return value of a nullary method.
202     */
203    public ATObject meta_invokeField(ATObject receiver, ATSymbol selector) throws InterpreterException;
204    
205    /**
206     * This meta-level method is used to determine whether an object has a
207     * field or method corresponding to the given selector, without actually invoking
208     * or selecting any value associated with that selector.
209     * <p>
210     * The lookup process is the same as that for the <tt>invoke</tt> operation (i.e.
211     * not only the object's own fields and methods are searched, but also those of
212     * its dynamic parents).
213     * 
214     * @param selector a symbol denoting the name of a field (accessor or mutator) or method
215     * @return a boolean denoting whether the object responds to <tt>o.selector</tt>
216     */
217    public ATBoolean meta_respondsTo(ATSymbol selector) throws InterpreterException;
218
219    /**
220     * This behavioural meta-level operation reifies a failed dynamic method or field lookup.
221     * 
222     * When method invocation or field selection fails to find the selector in
223     * the dynamic parent chain of an object, rather than immediately raising an
224     * {@link XSelectorNotFound} exception, the mirror of the original receiver
225     * of the method invocation or field selection is asked to handle failed lookup.
226     * <p>
227     * The default behaviour of <tt>doesNotUnderstand</tt> is to raise an
228     * {@link XSelectorNotFound} exception.
229     * <p>
230     * This method is very reminiscent of Smalltalk's well-known
231     * <tt>doesNotUnderstand:</tt> and of Ruby's <tt>method_missing</tt> methods.
232     * There are, however, two important differences:
233     * <ul>
234     *  <li> <tt>doesNotUnderstand</tt> is a <b>meta</b>-level operation in AmbientTalk.
235     *  It is an operation defined on mirrors, not on regular objects.
236     *  <li> <tt>doesNotUnderstand</tt> in AmbientTalk relates to <i>attribute
237     *  selection</i>, not to <i>method invocation</i>. Hence, this operation is
238     *  more general in AmbientTalk than in Smalltalk: it intercepts both failed
239     *  method invocations as well as failed field selections. Hence, it can be used
240     *  to model "virtual" fields. This shows in the interface: this operation
241     *  does not consume the actual arguments of a failed method invocation. Moreover,
242     *  a closure should be returned which can subsequently be applied for failed invocations.
243     *  Failed selections can simply return this closure without application. Hence, arguments
244     *  should be consumed by means of currying, e.g. by making <tt>doesNotUnderstand</tt>
245     *  return a block which can then take the arguments table as its sole parameter.
246     * </ul>
247     *
248     * @param selector a symbol denoting the name of a method or field that could not be found
249     * @return by default, this operation does not return a value, but raises an exception instead.
250     * @throws edu.vub.at.exceptions.XSelectorNotFound the default reaction to a failed selection
251     */
252    public ATClosure meta_doesNotUnderstand(ATSymbol selector) throws InterpreterException;
253
254    /* -----------------------------
255     * -- Object Passing protocol --
256     * ----------------------------- */
257
258    /**
259     * This behavioural meta-level operation reifies object serialization.
260     * 
261     * When an AmbientTalk object crosses actor boundaries, e.g. by means of
262     * parameter passing, as a return value or because it was explicitly
263     * exported, this meta-level operation is invoked on the object's mirror.
264     * <p>
265     * This operation allows objects to specify themselves how they should
266     * be parameter-passed during inter-actor communication. The interpreter
267     * will never pass an object to another actor directly, but instead always
268     * parameter-passes the <i>return value</i> of invoing <tt>pass()</tt> on
269     * the object's mirror.
270     * <p>
271     * Mirrors on by-copy objects implement <tt>pass</tt> as follows:
272     * <pre>def pass() { base }</pre>
273     * Mirrors on by-reference objects implement <tt>pass</tt> by returning
274     * a far reference to their base-level object.
275     * 
276     * @return the object to be parameter-passed instead of this object. For objects,
277     * the default is a far reference to themselves. For isolates, the default is
278     * to return themselves.
279     */
280    public ATObject meta_pass() throws InterpreterException;
281
282    /**
283     * This behavioural meta-level operation reifies object deserialization.
284     * 
285     * When an AmbientTalk object has just crossed an actor boundary (e.g.
286     * because of inter-actor message sending) this meta-level operation
287     * is invoked on the object's mirror.
288     * <p>
289     * This meta-level operation gives objects a chance to tell the interpreter
290     * which object they actually represent, because the object retained
291     * after parameter passing is the return value of the <tt>resolve</tt>
292     * operation.
293     * <p>
294     * Mirrors on by-copy objects, like isolates, implement <tt>resolve</tt> as follows:
295     * <pre>def resolve() { base }</pre>
296     * In other words, by-copy objects represent themselves. By-reference objects
297     * are paremeter passed as far references. Mirrors on far references implement
298     * <tt>resolve</tt> by trying to resolve the far reference into a local, regular
299     * object reference (which is possible if the object they point to is located
300     * in the actor in which they just arrived). If it is not possible to resolve
301     * a far reference into a local object, the far reference remains a far reference.
302     * <p>
303     * Note that for isolates, this operation also ensures that the isolate's
304     * lexical scope is rebound to the lexical root of the recipient actor.
305     *  
306     * @return the object represented by this object
307     * @throws XObjectOffline if a far reference to a local object can no longer be resolved
308     * because the object has been taken offline 
309     */
310    public ATObject meta_resolve() throws InterpreterException;
311    
312    /* ------------------------------------------
313     * -- Slot accessing and mutating protocol --
314     * ------------------------------------------ */
315    
316    /**
317     * This meta-level operation reifies first-class field or method selection. Hence, the
318     * base-level evaluation of <code>o.&x</code> is interpreted at the meta-level as:
319     * <pre>(reflect: o).select(o, `x)</pre>
320     * 
321     * The selector lookup follows the same search rules as those for <tt>invoke</tt>.
322     * That is: first an object's local fields and method dictionary are searched,
323     * and only then the object's <i>dynamic parent</i>.
324     * <p>
325     * The <tt>select</tt> operation can be used to both select fields or methods from
326     * an object. When the selector is bound to a method, the return value of
327     * <tt>select</tt> is a closure that wraps the found method in the object in which
328     * the method was found. This ensures that the method retains its context information,
329     * such as the lexical scope in which it was defined and the value of <tt>self</tt>, which
330     * will be bound to the original receiver, i.e. the first argument of <tt>select</tt>.
331     * <p>
332     * If the selector matches a field, an accessor is returned. If the selector ends with
333     * <tt>:=</tt>, a mutator is returned instead. An accessor is a nullary closure which upon
334     * application yields the field's value. A mutator is a unary closure which upon
335     * application assigns the field to the specified value.
336	 * Even for fields already bound to a closure, selecting the field returns an accessor
337	 * closure, not the bound closure itself.
338     *
339     * @see #meta_doesNotUnderstand(ATSymbol) for what happens if the selector is not found.
340     *
341     * @param receiver the dynamic receiver of the selection. If the result of the selection is
342     * a method, the closure wrapping the method will bind <tt>self</tt> to this object.
343     * @param selector a symbol denoting the name of the field or method to select.
344     * @return if selector is bound to a field, an accessor or mutator for the field; otherwise if
345     * the selector is bound to a method, a closure wrapping the method.
346     */
347    public ATClosure meta_select(ATObject receiver, ATSymbol selector) throws InterpreterException;
348
349    /**
350     * This meta-level operation reifies field definition. Hence, the base-level
351     * code <code>def x := v</code> evaluated in a lexical scope <tt>lex</tt>
352     * is interpreted at the meta-level as:
353     * <pre>(reflect: lex).defineField(`x, v)</pre>
354     * 
355     * Invoking this meta-level operation on an object's mirror adds a new field
356     * to that object. An object cannot contain two or more fields with the
357     * same name.
358     *
359     * @param name a symbol denoting the name of the new field
360     * @param value the value of the new field
361     * @return nil
362     * @throws edu.vub.at.exceptions.XDuplicateSlot if the object already has a
363     * local field with the given name
364     */
365    public ATNil meta_defineField(ATSymbol name, ATObject value) throws InterpreterException;
366
367    /* -----------------------------------------
368      * -- Cloning and instantiation protocol --
369      * ---------------------------------------- */
370
371    /**
372     * This meta-level operation reifies the act of cloning the base-level object.
373     * Hence, the code <code>clone: o</code> is interpreted at the meta-level as
374     * <pre>(reflect: o).clone()</pre>
375     * 
376     * AmbientTalk's default cloning semantics are based on shallow copying.
377     * A cloned object has copies of the original object's fields, but the values
378     * of the fields are shared between the clones. A clone has the same methods
379     * as the original object. Methods added at a later stage to the original
380     * will not affect the clone's methods and vice versa. This means that each
381     * objects has its own independent fields and methods.
382     * <p>
383     * If the cloned AmbientTalk object contains programmer-defined field objects,
384     * each of these fields is re-instantiated with the clone as a parameter. The
385     * clone is intialized with the re-instantiated fields rather than with the
386     * fields of the original object. This property helps to ensure that each
387     * object has its own independent fields.
388     * <p>
389     * If the object has a <i>shares-a</i> relationship with its parent, the object
390     * and its clone will <b>share</b> the same parent object. Shares-a relationships
391     * are the default in AmbientTalk, and they match with the semantics of
392     * shallow copying: the dynamic parent of an object is a regular field, hence
393     * its contents is shallow-copied.
394     * <p>
395     * If the object has an <i>is-a</i> relationship with its parent object, a
396     * clone of the object will receive a clone of the parent object as its parent.
397     * Hence, is-a relationships "override" the default shallow copying semantics
398     * and recursively clone the parent of an object up to a shares-a relationship.
399     * <p>
400     * If a mirage is cloned, its mirror is automatically re-instantiated with
401     * the new mirage, to ensure that each mirage has its independent mirror.
402     * @return a clone of the mirror's <tt>base</tt> object
403     */
404    public ATObject meta_clone() throws InterpreterException;
405
406    /**
407     * This meta-level operation reifies instance creation. The default
408     * implementation of an AmbientTalk object's <tt>new</tt> method is:
409     * <pre>def new(@initargs) { (reflect: self).newInstance(initargs) }</pre>
410     * 
411     * Creating a new instance of an object is a combination of:
412     * <ul>
413     *  <li>creating a clone of the object
414     *  <li>initializing the clone by invoking its <tt>init</tt> method
415     * </ul>
416     * 
417     * The default implementation is:
418     * <pre>def newInstance(initargs) {
419     *  def instance := self.clone();
420     *  instance.init(@initargs);
421     *  instance;
422     *}
423     * </pre>
424     * 
425     * Instance creation in AmbientTalk is designed to mimick class instantiation
426     * in a class-based language. Instantiating a class <tt>c</tt> requires <i>allocating</i>
427     * a new instance <tt>i</tt> and then invoking the <i>constructor</i> on that
428     * new instance. In AmbientTalk, class allocation is replaced by object
429     * <i>cloning</i>. The benefit is that an instantiated object its variables are
430     * already initialized to useful values, being those of the object from which
431     * it is instantiated. The <tt>init</tt> method plays the role of "constructor"
432     * in AmbientTalk.
433     *
434     * @param initargs a table denoting the actual arguments to be passed to
435     * the <tt>init</tt> method
436     * @return the new instance
437     */
438    public ATObject meta_newInstance(ATTable initargs) throws InterpreterException;
439
440    /* ---------------------------------
441      * -- Structural Access Protocol  --
442      * --------------------------------- */
443
444    /**
445     * This structural meta-level operation adds a field object to the receiver mirror's
446     * base object. An object cannot contain two or more fields with the same name.
447     * 
448     * Note that the field object passed as an argument serves as a <i>prototype</i>
449     * object: the actual field object added is an <i>instance</i> of the passed field object.
450     * A field object should always have an <tt>init</tt> method that takes as an argument
451     * the new host object to which it is added. This is often useful, as the behaviour
452     * of a field may depend on the object in which it resides. Because <tt>addField</tt>
453     * creates a new instance of the field, this gives the field object a chance to
454     * properly refer to its new host. 
455     * <p>
456     * As an example, here is how to add a read-only field <tt>foo</tt> initialized
457     * to <tt>5</tt> to an object <tt>obj</tt>:
458     * <pre>def makeConstantField(nam, val) {
459     *   object: {
460     *     def new(newHost) { self }; // singleton pattern
461     *     def name := nam;
462     *     def readField() { val };
463     *     def writeField(newVal) { nil };
464     *   }
465     * };
466     * (reflect: obj).addField(makeConstantField(`foo, 5));
467     * </pre>
468     * 
469     * @param field the prototype field object whose instance should be added
470     * to the receiver's base object
471     * @return nil
472     * @throws XDuplicateSlot if the base object already has a field with the
473     * same name as the new field
474     */
475    public ATNil meta_addField(ATField field) throws InterpreterException;
476
477    /**
478     * This structural meta-level operation adds a method to the receiver
479     * mirror's base object. An object cannot contain two or more methods
480     * with the same name.
481     * 
482     * @param method a method object to add to the receiver's base object's
483     * method dictionary.
484     * @return nil
485     * @throws XDuplicateSlot if a method with the new method's selector already
486     * exists in the base object.
487     */
488    public ATNil meta_addMethod(ATMethod method) throws InterpreterException;
489
490    /**
491     * This structural meta-level operation allows the metaprogrammer to reify a
492     * field of the receiver mirror's base object. Hence, unlike <tt>select</tt>
493     * and <tt>lookup</tt>, <tt>grabField</tt> returns a <i>field object</i> rather
494     * than the <i>value</i> bound to the field. For example: one could express
495     * <code>obj.super := val</code> at the meta-level as:
496     * 
497     * <pre>
498     * def superField := (reflect: obj).grabField(`super);
499     * superField.writeField(val);
500     * </pre>
501     *
502     * Another important difference between <tt>select</tt>, <tt>lookup</tt> and
503     * <tt>grabField</tt> is that <tt>grabField</tt> only considers the fields
504     * <i>local</i> to the receiver's base object. Fields of lexical or dynamic
505     * parent objects are <i>not</i> considered.
506     *
507     * @param selector a symbol representing the name of the field to select.
508     * @return a mirror on this object's field slot.
509     * @throws XUndefinedSlot if the field cannot be found within the receiver's
510     * base object.
511     */
512    public ATField meta_grabField(ATSymbol selector) throws InterpreterException;
513
514    /**
515     * This structural meta-level operation allows the metaprogrammer to
516     * reify a method defined on the receiver mirror's base object. Note that,
517     * unlike the <tt>select</tt> or <tt>lookup</tt> operations, <tt>grabMethod</tt>
518     * returns the bare method object, i.e. <i>not</i> a closure wrapping the method.
519     * <p>
520     * Also, unlike <tt>select</tt> and <tt>lookup</tt>, <tt>grabField</tt> only
521     * considers the locally defined methods of an object, methods of lexical or
522     * dynamic parent objects are <i>not</i> considered.
523     *
524     * @param selector a symbol representing the name of the method to grab from
525     * the receiver's base object.
526     * @return the bare method object bound to the given selector.
527     * @throws XSelectorNotFound if the method object cannot be found within the
528     * receiver's base object.
529     */
530    public ATMethod meta_grabMethod(ATSymbol selector) throws InterpreterException;
531
532    /**
533     * This structural meta-level operation allows access to all of the
534     * fields defined on the receiver mirror's base object. Note that
535     * this method only returns the base object's <i>locally</i> defined
536     * fields. Fields from parent objects are not returned.
537     * 
538     * @see ATObject#meta_grabField(ATSymbol) for details about the returned
539     * field objects. 
540     * @return a table of field objects (of type {@link ATField}).
541     */
542    public ATTable meta_listFields() throws InterpreterException;
543
544    /**
545     * This structural meta-level operation allows access to all of the
546     * methods defined on the receiver mirror's base object. Note that
547     * this method only returns the base object's <i>locally</i> defined
548     * methods. Methods from parent objects are not returned.
549     * 
550     * @see ATObject#meta_grabMethod(ATSymbol) for details about the returned
551     * method objects.
552     * @return a table of method objects (of type {@link ATMethod}).
553     */
554    public ATTable meta_listMethods() throws InterpreterException;
555
556    /**
557     * This structural meta-level operation returns whether or not
558     * the receiver mirror's base object is an <i>extension</i> of its
559     * parent object.
560     * <p>
561     * In AmbientTalk, all objects are part of a dynamic parent delegation chain:
562     * each object has a <tt>super</tt> field that denotes the object to which to
563     * delegate messages the object cannot understand itself. There are, however,
564     * two kinds of delegation links:
565     * <ul>
566     *  <li><b>IS-A</b> links: this kind of link denotes that the child object is
567     *  a true extension of its parent, and cannot meaningfully exist without the
568     *  parent's state. When the child is cloned, its parent will be cloned as well.
569     *  <li><b>SHARES-A</b> links: this kind of link denotes that the child object
570     *  simply delegates to its parent for purposes of sharing or code reuse. The
571     *  child can meaningfully exist without the parent's state. When the child is
572     *  cloned, the clone will delegate to the same parent.
573     * </ul>
574     *
575     * Examples:
576     * <pre>(reflect: (extend: parent with: code)).isExtensionOfParent() => true
577     *(reflect: (share: parent with: code)).isExtensionOfParent() => false
578     * </pre>
579     * 
580     * Note that accessing the dynamic parent itself is not a meta-level operation,
581     * the dynamic parent can simply be accessed from the base level by performing
582     * <code>obj.super</code>.
583     * 
584     * @return whether the base object extends its parent object via an
585     * <b>IS-A</b> link or not.
586     */
587    public ATBoolean meta_isExtensionOfParent() throws InterpreterException;
588
589    /* ------------------------------------------
590      * -- Abstract Grammar evaluation protocol --
591      * ------------------------------------------ */
592
593    /**
594     * This behavioural meta-level operation reifies the evaluation of
595     * abstract grammar objects into values. For objects, this operation
596     * returns the base object itself, signifying that the evaluation
597     * function defined on objects is the identity function. In other words,
598     * objects are <i>self-evaluating</i>. Parse tree objects (first-class
599     * abstract grammar elements), however, have dedicated evaluation
600     * functions. For example, evaluating <code>x</code> is equivalent to
601     * evaluating <code>(reflect: `x).eval(ctx)</code> where <tt>ctx</tt>
602     * is a reification of the current evaluation context.
603     * 
604     * @param ctx a context object that stores the current lexical scope and
605     * the current value of <tt>self</tt>
606     * @return the value of the abstract grammar element denoted by this mirror's
607     * base object.
608     * @throws XIllegalUnquote if an unquote abstract grammar element is evaluated. Such
609     * abstract grammar elements should only be encountered in a quoted parse tree.
610     */
611    public ATObject meta_eval(ATContext ctx) throws InterpreterException;
612
613    /**
614     * This behavioural meta-level operation reifies the quotation of
615     * abstract grammar elements. Regular objects simply return themselves
616     * upon quotation. When an abstract grammar element is quoted, rather
617     * than tree-recursively invoking <tt>eval</tt> on the parse trees,
618     * <tt>quote</tt> is tree-recursively invoked. When encountering
619     * an unquote, <tt>eval</tt> is again invoked on the unquoted subtree,
620     * with the context passed as an argument to <tt>quote</tt>.
621     * 
622     * @param ctx a context object passed on to be used in subsequent evaluations.
623     * @throws XIllegalQuote exception whenever an unquote-splice unquotation is discovered
624     * in an Abstract Grammar node where the resulting table cannot be spliced.
625     */
626    public ATObject meta_quote(ATContext ctx) throws InterpreterException;
627
628    /**
629     * This behavioural meta-level operation reifies the act of printing
630     * the base object in the read-eval-print loop. This operation may be
631     * overridden by mirrors to customise the printed representation of
632     * their base object.
633     * 
634     * @return a text value denoting a human-readable representation of the object.
635     */
636    public NATText meta_print() throws InterpreterException;
637
638    /* ----------------------------------
639     * -- Object Relational Comparison --
640     * ---------------------------------- */
641    
642    /**
643     * This meta-level operation determines whether this mirror's base object
644     * is related to the parameter object by a combination of cloning and
645     * extension operators. The default implementation is:
646     * 
647     * <pre>def isRelatedTo(object) {
648     *  self.isCloneOf(object).or: { (reflect: base.super).isRelatedTo(object) }
649     *}</pre>
650     * 
651     * @param object the object to compare this mirror's base object to
652     * @return true if the given object is a clone of the base object or a clone
653     * of the base object's parents.
654     */
655    public ATBoolean meta_isRelatedTo(ATObject object) throws InterpreterException;
656    
657    /**
658     * This meta-level operation determines whether this mirror's base object
659     * is a clone of the parameter object. The <i>is-clone-of</i> relation is transitive,
660     * so if <tt>martin</tt> is a clone of <tt>sally</tt> and <tt>sally</tt> is a clone of
661     * <tt>dolly</tt>, then <tt>martin</tt> is a clone of <tt>dolly</tt> as well.
662     * The relation is reflexive: <tt>dolly</tt> is a clone of itself.
663     * The relation is symmetric: <tt>dolly</tt> is also a clone of <tt>sally</tt>.
664     * 
665     * @param other the object to check the is-clone-of relationship with.
666     * @return true if the base object and the parameter object are clones (i.e. one
667     * was created by cloning the other), false otherwise.
668     */
669    public ATBoolean meta_isCloneOf(ATObject other) throws InterpreterException;
670
671    /* ---------------------------------
672     * -- Type Testing and Querying --
673     * --------------------------------- */
674    
675    /**
676     * Tests whether the receiver mirror's base object is tagged as a particular type.
677     * 
678     * The default implementation first compares the object's local type tags to the given type
679     * by means of the {@link ATTypeTag#base_isSubtypeOf(ATTypeTag)} method. If no local type
680     * is found, the test is applied recursively on this object's dynamic parent. In code:
681     * <pre>def isTaggedAs(type) {
682     *  (nil != (self.tagsOf: object).find: { |localType|
683	 *    localType.isSubtypeOf(type)
684	 *  }).or: { (reflect: base.super).isTaggedAs(type) }
685	 * };
686     * </pre>
687     * 
688     * The primitive method <tt>is: obj taggedAs: type</tt> is defined in terms of this
689     * method:
690     * <pre>
691     * def is: obj taggedAs: type {
692     *  (reflect: obj).isTaggedAs(type)
693     *};
694     * </pre>
695     * 
696     * @param type the type tag object to check for
697     * @return true if this mirror's base object or one of its parent objects is tagged
698     * with a subtype of the given type, false otherwise.
699     */
700    public ATBoolean meta_isTaggedAs(ATTypeTag type) throws InterpreterException;
701    
702    /**
703     * Returns all of the local type tags of this object. The primitive method
704     * <tt>tagsOf: obj</tt> is defined in terms of this method:
705     * 
706     * <pre>
707     * def tagsOf: obj {
708     *  (reflect: obj).typeTags
709     *};
710     * </pre>
711     * 
712     * @return a table of the type tags that were attached directly to this mirror's base
713     * object. The type tags of its parent objects are not returned.
714     */
715    public ATTable meta_typeTags() throws InterpreterException;
716    
717     /* -------------------------------
718      * - Base Level Object interface -
719      * ------------------------------- */
720
721    /**
722     * Bound to the dynamic parent of this object.
723     * 
724     * The dynamic parent of an object is the object to which failed
725     * selection or invocation requests or type tests are delegated to.
726     * 
727     * @return the current dynamic parent of this object.
728     */
729    public ATObject base_super() throws InterpreterException;
730    
731    /**
732     * The identity operator. In AmbientTalk, equality of objects
733     * is by default pointer-equality (i.e. objects are equal only
734     * if they are identical).
735     * 
736     * This is a primitive method, present by default in every AmbientTalk
737     * object but redefinable by the programmer.
738     * 
739     * @return by default, true if the parameter object and this object are identical,
740     * false otherwise.
741     */
742    public ATBoolean base__opeql__opeql_(ATObject other) throws InterpreterException;
743
744    /**
745     * The object instantiation method. Note that in class-based OO languages,
746     * this method is usually at the level of the <i>class</i>. In AmbientTalk,
747     * this method is situated at the object-level directly. It can be overridden
748     * to e.g. enforce the singleton pattern or to return instances of other
749     * objects.
750     * 
751     * The default implementation of this method is:
752     * <pre>def new(@args) {
753     *  (reflect: self).newInstance(@args)
754     *};
755     * </pre>
756     * 
757     * This is a primitive method, present by default in every AmbientTalk
758     * object but redefinable by the programmer.
759     * 
760     * @see ATObject#meta_newInstance(ATTable) for a description of object instantiation.
761     * @param initargs the variable argument list to pass to the <tt>init</tt> method.
762     * @return by default, the new instance of this mirror's base object.
763     */
764    public ATObject base_new(ATObject[] initargs) throws InterpreterException;
765
766    /**
767     * The object initialisation method. In class-based languages, this method
768     * is often called the constructor. AmbientTalk only supports one constructor
769     * per object, but thanks to variable argument lists and optional parameters,
770     * the same flexibility as defining multiple constructors can often be achieved.
771     * Also, by overriding <tt>new</tt>, the developer may invoke additional methods
772     * on newly created objects if this is desirable.
773     * 
774     * The default implementation of this method is:
775     * <pre>def init(@args) {
776     *  super^init(@args)
777     *};
778     * </pre>
779     * 
780     * This is a primitive method, present by default in every AmbientTalk
781     * object but redefinable by the programmer.
782     * 
783     * @see ATObject#meta_newInstance(ATTable) for a description of object initialisation.
784     * @param initargs the arguments to the <tt>init</tt> constructor method.
785     * @return the return value of invoking the <tt>init</tt> method. Note that
786     * this value is <i>discarded</i> when <tt>init</tt> is invoked from the
787     * <tt>newInstance</tt> meta-level operation.
788     */
789    public ATObject base_init(ATObject[] initargs) throws InterpreterException;
790    
791    /* -----------------------------------------
792     * - Implementation-Level Object interface -
793     * ----------------------------------------- */
794    
795    /**
796     * The <tt>lexicalParent</tt> field of a mirror denotes the lexical parent
797     * pointer of the mirror's base object. The lexical parent is the enclosing
798     * <i>lexical scope</i> in which the object was defined.
799     * 
800     * @return the object denoting this mirror's base object's lexically
801     * enclosing scope.
802     */
803    public ATObject impl_lexicalParent() throws InterpreterException;
804    
805	/**
806	 * Interprets <code>o.x()</code> or <code>o.m(arg)</code>.
807	 * Implements slot access. This method is an implementation-level method (not part of the MOP).
808	 * @param receiver the dynamic receiver of the slot invocation.
809	 * @param selector a regular symbol denoting the slot accessor.
810	 * @param arguments the actual arguments to the slot invocation.
811	 * @return the result of applying the accessor.
812	 * @throws XArityMismatch if a field accessor is not invoked with exactly zero arguments.
813	 */
814	public ATObject impl_invokeAccessor(ATObject receiver, ATSymbol selector, ATTable arguments) throws InterpreterException;
815	
816    /**
817     * Interprets <code>o.x := v</code>.
818     * Implements slot mutation. This method is an implementation-level method (not part of the MOP).
819     * @param receiver the dynamic receiver of the slot invocation.
820	 * @param selector an assignment symbol denoting which slot to invoke.
821	 * @param arguments the actual arguments to the slot invocation.
822	 * @return the result of applying the mutator.
823	 * @throws XArityMismatch if a field mutator is not invoked with exactly one argument.
824     */
825	public ATObject impl_invokeMutator(ATObject receiver, ATAssignmentSymbol selector, ATTable arguments) throws InterpreterException;
826	
827	/**
828	 * Interprets <code>o.&m</code>.
829	 * Implements slot accessor selection. This method is an implementation-level method (not part of the MOP).
830	 * @param receiver the dynamic receiver of the slot selection.
831	 * @param selector a regular symbol denoting the accessor to select.
832	 * @return a closure wrapping the selected method or an accessor for a field.
833	 */
834	public ATClosure impl_selectAccessor(ATObject receiver, ATSymbol selector) throws InterpreterException;
835	
836	/**
837	 * Interprets <code>o.&m:=</code>.
838	 * Implements slot mutator selection. This method is an implementation-level method (not part of the MOP).
839	 * @param receiver the dynamic receiver of the slot selection.
840	 * @param selector an assignment symbol denoting the mutator to select.
841	 * @return a closure representing the mutator of a given slot.
842	 */
843	public ATClosure impl_selectMutator(ATObject receiver, ATAssignmentSymbol selector) throws InterpreterException;
844	
845	
846    /**
847     * Interprets <code>x := v</code> (equivalent to <code>x:=(v)</code>) or <code>f(v)</code>.
848     * Implements functions calls and lexical access to variables.
849     * This method is an implementation-level method (not part of the MOP).
850
851     * Variable lookup first queries the local fields of this object, then the local
852     * method dictionary. If the selector is not found, the search continues in
853     * this object's <i>lexical parent</i>. Hence, variable lookup follows
854     * <b>lexical scoping rules</b>.
855     * <p>
856     * Similar to the behaviour of <tt>invoke</tt>, if the selector is bound to a
857     * field rather than a method, <tt>call</tt> treats the field as an accessor
858     * or mutator method (depending on the selector).
859     * <p>
860     * Note that, unlike <tt>invoke</tt> and <tt>select</tt>, <tt>call</tt> does
861     * not give rise to the invocation of <tt>doesNotUnderstand</tt> if the selector
862     * was not found. The reason for this is that lexical lookup is a static process
863     * for which it makes less sense to provide dynamic interception facilities.
864     *
865     * @param selector a symbol denoting the name of the field or method to look up lexically.
866     * @param arguments the arguments to the lexically scoped function call.
867     * @return if selector is bound to a field, the value of the field; otherwise if selector
868     * is bound to a method, the return value of the method.
869     * @throws XUndefinedSlot if the selector could not be found in the lexical scope of this object.
870     */
871    public ATObject impl_call(ATSymbol selector, ATTable arguments) throws InterpreterException;
872    
873    /**
874     * Interprets <code>f(v)</code>.
875     * Implements the protocol to access lexical variables and methods. This operation (which is not exposed
876     * as part of the MOP) locates the lexically visible binding with the given selector and will return 
877     * the value of the slot.
878     * <p>
879     * When this object has a local slot corresponding to the selector:
880     * <ul>
881     * <li> and the slot contains a method or closure, it will be applied with the given arguments (within a context
882     *   where self is bound to this object)
883     * <li> and the slot contains a value and the argumentlist is empty, the value is returned
884     * <li> and the slot contains a value and the argumentlist is not empty, an arity mismatch exception is raised
885     * </ul>
886     * <p>
887     * When no local slot is found, lookup continues along the lexical parent chain. When the lexical chain is 
888     * completely traversed, an undefined slot exception is raised.
889     */
890    public ATObject impl_callAccessor(ATSymbol selector, ATTable arguments) throws InterpreterException;
891    
892    /**
893     * Interprets <code>x := v</code> (which is equivalent to <code>x:=(v)</code>.
894     * Implements the protocol to assign lexical variables. This operation (which is not exposed as part of the MOP) 
895     * locates slots to assign corresponding to a specific assignment symbol (selector + ":=") and looks for:
896     * <ol>
897     * <li> a mutator method with the specified assignment symbol (i.e. including the ":=") which can then be 
898     * invoked with the provided arguments.
899     * <li> a field with a corresponding selector (i.e. without the ":=") which is then treated as if it were
900     * a unary mutator method.
901     * </ol>
902     * 
903     * If the slot is a method slot, an {@link XUnassignableField} exception is raised, otherwise the arity of the 
904     * arguments is verified (should be precisely 1) and the first argument is used as the new value of the slot.
905     * <p>
906     * When no local slot is found, lookup continues along the lexical parent chain. When the lexical chain is 
907     * completely traversed, a selector not found exception is reported.
908     */
909    public ATObject impl_callMutator(ATAssignmentSymbol selector, ATTable arguments) throws InterpreterException;
910    
911    /**
912     * Interprets <code>x</code>.
913     * This method is an implementation-level method (not part of the MOP).
914     * 
915     * This method is equivalent to {@link #impl_call(ATSymbol, ATTable)} where
916     * the arguments equal <tt>[]</tt>, except for one case: when the selector
917     * resolves to a field containing a closure, the closure is not auto-applied
918     * with zero arguments, but is instead returned. For all other purposes,
919     * evaluating <tt>m</tt> is equivalent to evaluating <tt>m()</tt> such that
920     * fields and nullary methods can be uniformly accessed.
921     * 
922     * @param selector the name of a lexically visible field or method.
923     * @return the value of a field or the return value of an accessor method.
924     */
925    public ATObject impl_callField(ATSymbol selector) throws InterpreterException;
926    
927    /**
928     * Interprets <code>&x</code> or <code>&x:=</code>.
929     * This method is an implementation-level method (not part of the MOP).
930     * 
931     * This operation is the lexical counterpart of {@link #meta_select(ATObject, ATSymbol)}.
932     * @param selector the name of a lexically visible field or method.
933     * @return a closure wrapping a method, or an accessor or mutator linked to a lexically visible field.
934     */
935    public ATClosure impl_lookup(ATSymbol selector) throws InterpreterException;
936
937    /**
938     * Interprets <code>&x</code>.
939     * This method is an implementation-level method (not part of the MOP).
940     * 
941     * @param selector the name of a lexically visible field or method.
942     * @return a closure wrapping a method, or an accessor linked to a lexically visible field.
943     */
944    public ATClosure impl_lookupAccessor(ATSymbol selector) throws InterpreterException;
945
946    /**
947     * Interprets <code>&x:=</code>.
948     * This method is an implementation-level method (not part of the MOP).
949     * 
950     * If the selector minus <tt>:=</tt> is bound to a field, the field is assigned
951     * i
952     * @param selector the name of a lexically visible method or of a field (whose name does not have the <tt>:=</tt> prefix).
953     * @return a closure wrapping a method, or a mutator linked to a lexically visible field.
954     */
955    public ATClosure impl_lookupMutator(ATAssignmentSymbol selector) throws InterpreterException;
956    
957    /**
958     * This is a callback method used in AmbientTalk's native equality protocol.
959     * When evaluating <tt>o1 == o2</tt> in AmbientTalk, <tt>o1</tt>'s <tt>==</tt>
960     * method will invoke <tt>o2.identityEquals(o1)</tt>.
961     * 
962     * The native implementation can make use of Java's <tt>==</tt> operator where
963     * appropriate.
964     */
965    public ATBoolean impl_identityEquals(ATObject other) throws InterpreterException;
966    
967}