/**
 * AmbientTalk/2 Project
 * ATObject.java created on 13-jul-2006 at 15:33:42
 * (c) Programming Technology Lab, 2006 - 2007
 * Authors: Tom Van Cutsem & Stijn Mostinckx
 * 
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use,
 * copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following
 * conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */
package edu.vub.at.objects;

import edu.vub.at.actors.ATAsyncMessage;
import edu.vub.at.exceptions.InterpreterException;
import edu.vub.at.exceptions.XArityMismatch;
import edu.vub.at.exceptions.XDuplicateSlot;
import edu.vub.at.exceptions.XIllegalQuote;
import edu.vub.at.exceptions.XIllegalUnquote;
import edu.vub.at.exceptions.XObjectOffline;
import edu.vub.at.exceptions.XSelectorNotFound;
import edu.vub.at.exceptions.XUnassignableField;
import edu.vub.at.exceptions.XUndefinedSlot;
import edu.vub.at.objects.coercion.ATConversions;
import edu.vub.at.objects.grammar.ATAssignmentSymbol;
import edu.vub.at.objects.grammar.ATSymbol;
import edu.vub.at.objects.mirrors.NATMirage;
import edu.vub.at.objects.mirrors.OBJMirrorRoot;
import edu.vub.at.objects.natives.NATCallframe;
import edu.vub.at.objects.natives.NATObject;
import edu.vub.at.objects.natives.NATText;
import edu.vub.at.objects.natives.NativeATObject;
import edu.vub.at.objects.symbiosis.JavaClass;
import edu.vub.at.objects.symbiosis.JavaObject;

/**
 * ATObject represents the public interface common to any AmbientTalk/2 object.
 * Any value representing an ambienttalk object should implement this interface.
 * 
 * More specifically, this interface actually defines two interfaces all at once:
 * <ul>
 *  <li>A <b>base-level</b> interface to AmbientTalk objects, describing all
 *      methods and fields that a regular AmbientTalk object understands.
 *  <li>A <b>meta-level</b> interface to AmbientTalk objects, describing all
 *      methods and fields that the <b>mirror</b> on any AmbientTalk object
 *      understands.
 * </ul>
 * 
 * In the AmbientTalk/2 Interpreter implementation, there are only a few classes
 * that (almost) fully implement this interface. The principal implementors are:
 * 
 * <ul>
 *   <li>{@link NativeATObject}: provides a default implementation for all <i>native</i> data types.
 *   For example, native methods, closures, abstract grammar nodes, booleans, numbers, etc.
 *   are all represented as AmbientTalk objects with 'native' behaviour.
 *   <li>{@link NATCallframe}: overrides most of the default behaviour of {@link NativeATObject} to
 *   implement the behaviour of call frames, also known as <i>activation records</i>. In
 *   AmbientTalk, call frames are the objects that together define the runtime stack.
 *   They are objects with support for fields but without support for actual methods.
 *   <li>{@link NATObject}: extends the behaviour of call frames to include support for
 *   full-fledged, programmer-defined objects with support for methods and delegation.
 *   <li>{@link JavaClass} and {@link JavaObject}: adapt the behaviour of native AmbientTalk
 *   objects to engage in symbiosis with either a Java class or a Java object.
 *   This implementation makes use of the Java Reflection API to regard Java objects
 *   as though it were AmbientTalk objects.
 *   <li>{@link NATMirage} and {@link OBJMirrorRoot}: these two classes work in tandem to
 *   enable reflection on AmbientTalk objects. That is, because of these two classes, an
 *   AmbientTalk programmer can himself invoke the methods provided in this interface.
 *   {@link NATMirage} implements each operation in this interface by forwarding a
 *   downed invocation to a custom so-called <i>mirror</i> object. This mirror object
 *   can delegate to {@link OBJMirrorRoot}, which is a special object that implements
 *   each meta-level operation of this interface as a base-level operation. Hence, in
 *   a sense, {@link OBJMirrorRoot} also 'implements' this interface, but at the
 *   AmbientTalk level, rather than at the Java level.
 * </ul>
 * 
 * @author tvcutsem
 */
public interface ATObject extends ATConversions {

    /* ------------------------------
      * -- Message Sending Protocol --
      * ------------------------------ */

    /**
     * This behavioural meta-level operation reifies the act of sending
     * an asynchronous message.
     * 
     * When the base-level AmbientTalk code <code>rcv<-m()</code> is
     * evaluated in the context of an object <tt>o</tt>, an asynchronous message
     * <code><-m()</code> is first created by the current actor mirror.
     * Subsequently, this message needs to be sent to the receiver. This
     * meta-level operation is reified by this method, as if by invoking:
     * <pre>(reflect: o).send(message)</pre>
     * The default behaviour is to access the current actor's mirror and to
     * ask the actor to send the message in this object's stead by invoking
     * <pre>actor.send(message)</pre>
     * @param receiver the object designated to receive the asynchronous message
     * @param message the asynchronous message to be sent by this object
     * 
     * @return the result of message sending, which will be the value of an
     * asynchronous message send expression.
     */
    public ATObject meta_send(ATObject receiver, ATAsyncMessage message) throws InterpreterException;
    
    /**
     * This behavioural meta-level operation reifies the act of receiving
     * an asynchronous message.
     * 
     * When an AmbientTalk object receives a message that was sent to it
     * asynchronously, the message is delivered to the object's mirror by
     * means of this meta-level operation.
     * <p>
     * The default behaviour of a mirror in response to the reception of a
     * message <tt>msg</tt> is to invoke:
     * <pre>msg.process(self)</pre>
     * In turn, the default message processing behaviour is to invoke
     * the method corresponding to the message's selector on this object.
     * Hence, usually a <tt>receive</tt> operation is simply translated into
     * a <tt>invoke</tt> operation. The reason for having a separate <tt>receive</tt>
     * operation is that this enables the AmbientTalk meta-level programmer to
     * distinguish between synchronously and asynchronously received messages.
     * @param message the message that was asynchronously sent to this object
     * @return by default, the value of invoking the method corresponding to the message
     */
    public ATObject meta_receive(ATAsyncMessage message) throws InterpreterException;
   
    /**
     * This meta-level operation reifies synchronous message sending ("method invocation").
     * Hence, the meta-level equivalent
     * of the base-level code <code>o.m()</code> is:
     * <pre>(reflect: o).invoke(o,`m,[])</pre>.
     * 
     * Method invocation comprises selector lookup and the application of the value
     * bound to the selector. Selector lookup first queries an object's local
     * fields, then the method dictionary:
     * <ul>
     *  <li>If the selector ends with <tt>:=</tt> and matches a field, the field
     *  is assigned if a unary argument list is specified (i.e. the field is treated
     *  as a mutator method).
     *  <li>Otherwise, if the selector is bound to a field containing
     * a closure, that closure is applied to the given arguments.
     *  <li>If the field is not bound to a closure, the field value is returned provided no arguments were
     * specified (i.e. the field is treated like an accessor method).
     *  <li>If the selector is bound to a method, the method is applied.
     *  <li>If the selector is not found, the search continues in the objects <i>dynamic parent</i>.
     * </ul>
     * <p>
     * Note also that the first argument to <tt>invoke</tt> denotes the
     * so-called "receiver" of the invocation. It is this object to which
     * the <tt>self</tt> pseudo-variable should be bound during method execution.
     * 
     * @see #meta_doesNotUnderstand(ATSymbol) for what happens if the selector
     * is not found.
     *
     * @param receiver the object to which <tt>self</tt> is bound during execution
     * of the method
     * @param selector a symbol denoting the name of the method, accessor or mutator to be invoked
     * @param arguments the table of actual arguments to be passed to the method
     * @return by default, the object returned from the invoked method
     */
    public ATObject meta_invoke(ATObject receiver, ATSymbol selector, ATTable arguments) throws InterpreterException;

    /**
     * This meta-level operation reifies "field selection".
     * In other words, the base-level code
     * <code>o.m</code>
     * is interpreted at the meta-level as:
     * <code>(reflect: o).invokeField(o, `m)</code>
     * 
     * This meta-level operation is nearly identical to {@link #meta_invoke(ATObject, ATSymbol, ATTable)} with one
     * important difference. When the selector is bound to a field storing a closure, this meta-level operation
     * does <b>not</b> auto-apply the closure, but returns the closure instead.
     * 
     * For all other cases, the following equality holds:
     * <code>o.m == o.m()</code>
     * or, at the meta-level:
     * <code>(reflect: o).invokeField(o, `m) == (reflect: o).invoke(o, `m, [])</code>
     * 
     * This effectively means that for client objects, it should not matter whether
     * a property is implemented as a field or as a pair of accessor/mutator methods.
     * 
     * @param receiver the base-level object from which the 'field' should be selected.
     * @param selector a symbol denoting the name of the method, accessor or mutator to be invoked
     * @return the value of a field, or the return value of a nullary method.
     */
    public ATObject meta_invokeField(ATObject receiver, ATSymbol selector) throws InterpreterException;
    
    /**
     * This meta-level method is used to determine whether an object has a
     * field or method corresponding to the given selector, without actually invoking
     * or selecting any value associated with that selector.
     * <p>
     * The lookup process is the same as that for the <tt>invoke</tt> operation (i.e.
     * not only the object's own fields and methods are searched, but also those of
     * its dynamic parents).
     * 
     * @param selector a symbol denoting the name of a field (accessor or mutator) or method
     * @return a boolean denoting whether the object responds to <tt>o.selector</tt>
     */
    public ATBoolean meta_respondsTo(ATSymbol selector) throws InterpreterException;

    /**
     * This behavioural meta-level operation reifies a failed dynamic method or field lookup.
     * 
     * When method invocation or field selection fails to find the selector in
     * the dynamic parent chain of an object, rather than immediately raising an
     * {@link XSelectorNotFound} exception, the mirror of the original receiver
     * of the method invocation or field selection is asked to handle failed lookup.
     * <p>
     * The default behaviour of <tt>doesNotUnderstand</tt> is to raise an
     * {@link XSelectorNotFound} exception.
     * <p>
     * This method is very reminiscent of Smalltalk's well-known
     * <tt>doesNotUnderstand:</tt> and of Ruby's <tt>method_missing</tt> methods.
     * There are, however, two important differences:
     * <ul>
     *  <li> <tt>doesNotUnderstand</tt> is a <b>meta</b>-level operation in AmbientTalk.
     *  It is an operation defined on mirrors, not on regular objects.
     *  <li> <tt>doesNotUnderstand</tt> in AmbientTalk relates to <i>attribute
     *  selection</i>, not to <i>method invocation</i>. Hence, this operation is
     *  more general in AmbientTalk than in Smalltalk: it intercepts both failed
     *  method invocations as well as failed field selections. Hence, it can be used
     *  to model "virtual" fields. This shows in the interface: this operation
     *  does not consume the actual arguments of a failed method invocation. Moreover,
     *  a closure should be returned which can subsequently be applied for failed invocations.
     *  Failed selections can simply return this closure without application. Hence, arguments
     *  should be consumed by means of currying, e.g. by making <tt>doesNotUnderstand</tt>
     *  return a block which can then take the arguments table as its sole parameter.
     * </ul>
     *
     * @param selector a symbol denoting the name of a method or field that could not be found
     * @return by default, this operation does not return a value, but raises an exception instead.
     * @throws edu.vub.at.exceptions.XSelectorNotFound the default reaction to a failed selection
     */
    public ATClosure meta_doesNotUnderstand(ATSymbol selector) throws InterpreterException;

    /* -----------------------------
     * -- Object Passing protocol --
     * ----------------------------- */

    /**
     * This behavioural meta-level operation reifies object serialization.
     * 
     * When an AmbientTalk object crosses actor boundaries, e.g. by means of
     * parameter passing, as a return value or because it was explicitly
     * exported, this meta-level operation is invoked on the object's mirror.
     * <p>
     * This operation allows objects to specify themselves how they should
     * be parameter-passed during inter-actor communication. The interpreter
     * will never pass an object to another actor directly, but instead always
     * parameter-passes the <i>return value</i> of invoing <tt>pass()</tt> on
     * the object's mirror.
     * <p>
     * Mirrors on by-copy objects implement <tt>pass</tt> as follows:
     * <pre>def pass() { base }</pre>
     * Mirrors on by-reference objects implement <tt>pass</tt> by returning
     * a far reference to their base-level object.
     * 
     * @return the object to be parameter-passed instead of this object. For objects,
     * the default is a far reference to themselves. For isolates, the default is
     * to return themselves.
     */
    public ATObject meta_pass() throws InterpreterException;

    /**
     * This behavioural meta-level operation reifies object deserialization.
     * 
     * When an AmbientTalk object has just crossed an actor boundary (e.g.
     * because of inter-actor message sending) this meta-level operation
     * is invoked on the object's mirror.
     * <p>
     * This meta-level operation gives objects a chance to tell the interpreter
     * which object they actually represent, because the object retained
     * after parameter passing is the return value of the <tt>resolve</tt>
     * operation.
     * <p>
     * Mirrors on by-copy objects, like isolates, implement <tt>resolve</tt> as follows:
     * <pre>def resolve() { base }</pre>
     * In other words, by-copy objects represent themselves. By-reference objects
     * are paremeter passed as far references. Mirrors on far references implement
     * <tt>resolve</tt> by trying to resolve the far reference into a local, regular
     * object reference (which is possible if the object they point to is located
     * in the actor in which they just arrived). If it is not possible to resolve
     * a far reference into a local object, the far reference remains a far reference.
     * <p>
     * Note that for isolates, this operation also ensures that the isolate's
     * lexical scope is rebound to the lexical root of the recipient actor.
     *  
     * @return the object represented by this object
     * @throws XObjectOffline if a far reference to a local object can no longer be resolved
     * because the object has been taken offline 
     */
    public ATObject meta_resolve() throws InterpreterException;
    
    /* ------------------------------------------
     * -- Slot accessing and mutating protocol --
     * ------------------------------------------ */
    
    /**
     * This meta-level operation reifies first-class field or method selection. Hence, the
     * base-level evaluation of <code>o.&x</code> is interpreted at the meta-level as:
     * <pre>(reflect: o).select(o, `x)</pre>
     * 
     * The selector lookup follows the same search rules as those for <tt>invoke</tt>.
     * That is: first an object's local fields and method dictionary are searched,
     * and only then the object's <i>dynamic parent</i>.
     * <p>
     * The <tt>select</tt> operation can be used to both select fields or methods from
     * an object. When the selector is bound to a method, the return value of
     * <tt>select</tt> is a closure that wraps the found method in the object in which
     * the method was found. This ensures that the method retains its context information,
     * such as the lexical scope in which it was defined and the value of <tt>self</tt>, which
     * will be bound to the original receiver, i.e. the first argument of <tt>select</tt>.
     * <p>
     * If the selector matches a field, an accessor is returned. If the selector ends with
     * <tt>:=</tt>, a mutator is returned instead. An accessor is a nullary closure which upon
     * application yields the field's value. A mutator is a unary closure which upon
     * application assigns the field to the specified value.
	 * Even for fields already bound to a closure, selecting the field returns an accessor
	 * closure, not the bound closure itself.
     *
     * @see #meta_doesNotUnderstand(ATSymbol) for what happens if the selector is not found.
     *
     * @param receiver the dynamic receiver of the selection. If the result of the selection is
     * a method, the closure wrapping the method will bind <tt>self</tt> to this object.
     * @param selector a symbol denoting the name of the field or method to select.
     * @return if selector is bound to a field, an accessor or mutator for the field; otherwise if
     * the selector is bound to a method, a closure wrapping the method.
     */
    public ATClosure meta_select(ATObject receiver, ATSymbol selector) throws InterpreterException;

    /**
     * This meta-level operation reifies field definition. Hence, the base-level
     * code <code>def x := v</code> evaluated in a lexical scope <tt>lex</tt>
     * is interpreted at the meta-level as:
     * <pre>(reflect: lex).defineField(`x, v)</pre>
     * 
     * Invoking this meta-level operation on an object's mirror adds a new field
     * to that object. An object cannot contain two or more fields with the
     * same name.
     *
     * @param name a symbol denoting the name of the new field
     * @param value the value of the new field
     * @return nil
     * @throws edu.vub.at.exceptions.XDuplicateSlot if the object already has a
     * local field with the given name
     */
    public ATNil meta_defineField(ATSymbol name, ATObject value) throws InterpreterException;

    /* -----------------------------------------
      * -- Cloning and instantiation protocol --
      * ---------------------------------------- */

    /**
     * This meta-level operation reifies the act of cloning the base-level object.
     * Hence, the code <code>clone: o</code> is interpreted at the meta-level as
     * <pre>(reflect: o).clone()</pre>
     * 
     * AmbientTalk's default cloning semantics are based on shallow copying.
     * A cloned object has copies of the original object's fields, but the values
     * of the fields are shared between the clones. A clone has the same methods
     * as the original object. Methods added at a later stage to the original
     * will not affect the clone's methods and vice versa. This means that each
     * objects has its own independent fields and methods.
     * <p>
     * If the cloned AmbientTalk object contains programmer-defined field objects,
     * each of these fields is re-instantiated with the clone as a parameter. The
     * clone is intialized with the re-instantiated fields rather than with the
     * fields of the original object. This property helps to ensure that each
     * object has its own independent fields.
     * <p>
     * If the object has a <i>shares-a</i> relationship with its parent, the object
     * and its clone will <b>share</b> the same parent object. Shares-a relationships
     * are the default in AmbientTalk, and they match with the semantics of
     * shallow copying: the dynamic parent of an object is a regular field, hence
     * its contents is shallow-copied.
     * <p>
     * If the object has an <i>is-a</i> relationship with its parent object, a
     * clone of the object will receive a clone of the parent object as its parent.
     * Hence, is-a relationships "override" the default shallow copying semantics
     * and recursively clone the parent of an object up to a shares-a relationship.
     * <p>
     * If a mirage is cloned, its mirror is automatically re-instantiated with
     * the new mirage, to ensure that each mirage has its independent mirror.
     * @return a clone of the mirror's <tt>base</tt> object
     */
    public ATObject meta_clone() throws InterpreterException;

    /**
     * This meta-level operation reifies instance creation. The default
     * implementation of an AmbientTalk object's <tt>new</tt> method is:
     * <pre>def new(@initargs) { (reflect: self).newInstance(initargs) }</pre>
     * 
     * Creating a new instance of an object is a combination of:
     * <ul>
     *  <li>creating a clone of the object
     *  <li>initializing the clone by invoking its <tt>init</tt> method
     * </ul>
     * 
     * The default implementation is:
     * <pre>def newInstance(initargs) {
     *  def instance := self.clone();
     *  instance.init(@initargs);
     *  instance;
     *}
     * </pre>
     * 
     * Instance creation in AmbientTalk is designed to mimick class instantiation
     * in a class-based language. Instantiating a class <tt>c</tt> requires <i>allocating</i>
     * a new instance <tt>i</tt> and then invoking the <i>constructor</i> on that
     * new instance. In AmbientTalk, class allocation is replaced by object
     * <i>cloning</i>. The benefit is that an instantiated object its variables are
     * already initialized to useful values, being those of the object from which
     * it is instantiated. The <tt>init</tt> method plays the role of "constructor"
     * in AmbientTalk.
     *
     * @param initargs a table denoting the actual arguments to be passed to
     * the <tt>init</tt> method
     * @return the new instance
     */
    public ATObject meta_newInstance(ATTable initargs) throws InterpreterException;

    /* ---------------------------------
      * -- Structural Access Protocol  --
      * --------------------------------- */

    /**
     * This structural meta-level operation adds a field object to the receiver mirror's
     * base object. An object cannot contain two or more fields with the same name.
     * 
     * Note that the field object passed as an argument serves as a <i>prototype</i>
     * object: the actual field object added is an <i>instance</i> of the passed field object.
     * A field object should always have an <tt>init</tt> method that takes as an argument
     * the new host object to which it is added. This is often useful, as the behaviour
     * of a field may depend on the object in which it resides. Because <tt>addField</tt>
     * creates a new instance of the field, this gives the field object a chance to
     * properly refer to its new host. 
     * <p>
     * As an example, here is how to add a read-only field <tt>foo</tt> initialized
     * to <tt>5</tt> to an object <tt>obj</tt>:
     * <pre>def makeConstantField(nam, val) {
     *   object: {
     *     def new(newHost) { self }; // singleton pattern
     *     def name := nam;
     *     def readField() { val };
     *     def writeField(newVal) { nil };
     *   }
     * };
     * (reflect: obj).addField(makeConstantField(`foo, 5));
     * </pre>
     * 
     * @param field the prototype field object whose instance should be added
     * to the receiver's base object
     * @return nil
     * @throws XDuplicateSlot if the base object already has a field with the
     * same name as the new field
     */
    public ATNil meta_addField(ATField field) throws InterpreterException;

    /**
     * This structural meta-level operation adds a method to the receiver
     * mirror's base object. An object cannot contain two or more methods
     * with the same name.
     * 
     * @param method a method object to add to the receiver's base object's
     * method dictionary.
     * @return nil
     * @throws XDuplicateSlot if a method with the new method's selector already
     * exists in the base object.
     */
    public ATNil meta_addMethod(ATMethod method) throws InterpreterException;

    /**
     * This structural meta-level operation allows the metaprogrammer to reify a
     * field of the receiver mirror's base object. Hence, unlike <tt>select</tt>
     * and <tt>lookup</tt>, <tt>grabField</tt> returns a <i>field object</i> rather
     * than the <i>value</i> bound to the field. For example: one could express
     * <code>obj.super := val</code> at the meta-level as:
     * 
     * <pre>
     * def superField := (reflect: obj).grabField(`super);
     * superField.writeField(val);
     * </pre>
     *
     * Another important difference between <tt>select</tt>, <tt>lookup</tt> and
     * <tt>grabField</tt> is that <tt>grabField</tt> only considers the fields
     * <i>local</i> to the receiver's base object. Fields of lexical or dynamic
     * parent objects are <i>not</i> considered.
     *
     * @param selector a symbol representing the name of the field to select.
     * @return a mirror on this object's field slot.
     * @throws XUndefinedSlot if the field cannot be found within the receiver's
     * base object.
     */
    public ATField meta_grabField(ATSymbol selector) throws InterpreterException;

    /**
     * This structural meta-level operation allows the metaprogrammer to
     * reify a method defined on the receiver mirror's base object. Note that,
     * unlike the <tt>select</tt> or <tt>lookup</tt> operations, <tt>grabMethod</tt>
     * returns the bare method object, i.e. <i>not</i> a closure wrapping the method.
     * <p>
     * Also, unlike <tt>select</tt> and <tt>lookup</tt>, <tt>grabField</tt> only
     * considers the locally defined methods of an object, methods of lexical or
     * dynamic parent objects are <i>not</i> considered.
     *
     * @param selector a symbol representing the name of the method to grab from
     * the receiver's base object.
     * @return the bare method object bound to the given selector.
     * @throws XSelectorNotFound if the method object cannot be found within the
     * receiver's base object.
     */
    public ATMethod meta_grabMethod(ATSymbol selector) throws InterpreterException;

    /**
     * This structural meta-level operation allows access to all of the
     * fields defined on the receiver mirror's base object. Note that
     * this method only returns the base object's <i>locally</i> defined
     * fields. Fields from parent objects are not returned.
     * 
     * @see ATObject#meta_grabField(ATSymbol) for details about the returned
     * field objects. 
     * @return a table of field objects (of type {@link ATField}).
     */
    public ATTable meta_listFields() throws InterpreterException;

    /**
     * This structural meta-level operation allows access to all of the
     * methods defined on the receiver mirror's base object. Note that
     * this method only returns the base object's <i>locally</i> defined
     * methods. Methods from parent objects are not returned.
     * 
     * @see ATObject#meta_grabMethod(ATSymbol) for details about the returned
     * method objects.
     * @return a table of method objects (of type {@link ATMethod}).
     */
    public ATTable meta_listMethods() throws InterpreterException;

    /**
     * This structural meta-level operation returns whether or not
     * the receiver mirror's base object is an <i>extension</i> of its
     * parent object.
     * <p>
     * In AmbientTalk, all objects are part of a dynamic parent delegation chain:
     * each object has a <tt>super</tt> field that denotes the object to which to
     * delegate messages the object cannot understand itself. There are, however,
     * two kinds of delegation links:
     * <ul>
     *  <li><b>IS-A</b> links: this kind of link denotes that the child object is
     *  a true extension of its parent, and cannot meaningfully exist without the
     *  parent's state. When the child is cloned, its parent will be cloned as well.
     *  <li><b>SHARES-A</b> links: this kind of link denotes that the child object
     *  simply delegates to its parent for purposes of sharing or code reuse. The
     *  child can meaningfully exist without the parent's state. When the child is
     *  cloned, the clone will delegate to the same parent.
     * </ul>
     *
     * Examples:
     * <pre>(reflect: (extend: parent with: code)).isExtensionOfParent() => true
     *(reflect: (share: parent with: code)).isExtensionOfParent() => false
     * </pre>
     * 
     * Note that accessing the dynamic parent itself is not a meta-level operation,
     * the dynamic parent can simply be accessed from the base level by performing
     * <code>obj.super</code>.
     * 
     * @return whether the base object extends its parent object via an
     * <b>IS-A</b> link or not.
     */
    public ATBoolean meta_isExtensionOfParent() throws InterpreterException;

    /* ------------------------------------------
      * -- Abstract Grammar evaluation protocol --
      * ------------------------------------------ */

    /**
     * This behavioural meta-level operation reifies the evaluation of
     * abstract grammar objects into values. For objects, this operation
     * returns the base object itself, signifying that the evaluation
     * function defined on objects is the identity function. In other words,
     * objects are <i>self-evaluating</i>. Parse tree objects (first-class
     * abstract grammar elements), however, have dedicated evaluation
     * functions. For example, evaluating <code>x</code> is equivalent to
     * evaluating <code>(reflect: `x).eval(ctx)</code> where <tt>ctx</tt>
     * is a reification of the current evaluation context.
     * 
     * @param ctx a context object that stores the current lexical scope and
     * the current value of <tt>self</tt>
     * @return the value of the abstract grammar element denoted by this mirror's
     * base object.
     * @throws XIllegalUnquote if an unquote abstract grammar element is evaluated. Such
     * abstract grammar elements should only be encountered in a quoted parse tree.
     */
    public ATObject meta_eval(ATContext ctx) throws InterpreterException;

    /**
     * This behavioural meta-level operation reifies the quotation of
     * abstract grammar elements. Regular objects simply return themselves
     * upon quotation. When an abstract grammar element is quoted, rather
     * than tree-recursively invoking <tt>eval</tt> on the parse trees,
     * <tt>quote</tt> is tree-recursively invoked. When encountering
     * an unquote, <tt>eval</tt> is again invoked on the unquoted subtree,
     * with the context passed as an argument to <tt>quote</tt>.
     * 
     * @param ctx a context object passed on to be used in subsequent evaluations.
     * @throws XIllegalQuote exception whenever an unquote-splice unquotation is discovered
     * in an Abstract Grammar node where the resulting table cannot be spliced.
     */
    public ATObject meta_quote(ATContext ctx) throws InterpreterException;

    /**
     * This behavioural meta-level operation reifies the act of printing
     * the base object in the read-eval-print loop. This operation may be
     * overridden by mirrors to customise the printed representation of
     * their base object.
     * 
     * @return a text value denoting a human-readable representation of the object.
     */
    public NATText meta_print() throws InterpreterException;

    /* ----------------------------------
     * -- Object Relational Comparison --
     * ---------------------------------- */
    
    /**
     * This meta-level operation determines whether this mirror's base object
     * is related to the parameter object by a combination of cloning and
     * extension operators. The default implementation is:
     * 
     * <pre>def isRelatedTo(object) {
     *  self.isCloneOf(object).or: { (reflect: base.super).isRelatedTo(object) }
     *}</pre>
     * 
     * @param object the object to compare this mirror's base object to
     * @return true if the given object is a clone of the base object or a clone
     * of the base object's parents.
     */
    public ATBoolean meta_isRelatedTo(ATObject object) throws InterpreterException;
    
    /**
     * This meta-level operation determines whether this mirror's base object
     * is a clone of the parameter object. The <i>is-clone-of</i> relation is transitive,
     * so if <tt>martin</tt> is a clone of <tt>sally</tt> and <tt>sally</tt> is a clone of
     * <tt>dolly</tt>, then <tt>martin</tt> is a clone of <tt>dolly</tt> as well.
     * The relation is reflexive: <tt>dolly</tt> is a clone of itself.
     * The relation is symmetric: <tt>dolly</tt> is also a clone of <tt>sally</tt>.
     * 
     * @param other the object to check the is-clone-of relationship with.
     * @return true if the base object and the parameter object are clones (i.e. one
     * was created by cloning the other), false otherwise.
     */
    public ATBoolean meta_isCloneOf(ATObject other) throws InterpreterException;

    /* ---------------------------------
     * -- Type Testing and Querying --
     * --------------------------------- */
    
    /**
     * Tests whether the receiver mirror's base object is tagged as a particular type.
     * 
     * The default implementation first compares the object's local type tags to the given type
     * by means of the {@link ATTypeTag#base_isSubtypeOf(ATTypeTag)} method. If no local type
     * is found, the test is applied recursively on this object's dynamic parent. In code:
     * <pre>def isTaggedAs(type) {
     *  (nil != (self.tagsOf: object).find: { |localType|
	 *    localType.isSubtypeOf(type)
	 *  }).or: { (reflect: base.super).isTaggedAs(type) }
	 * };
     * </pre>
     * 
     * The primitive method <tt>is: obj taggedAs: type</tt> is defined in terms of this
     * method:
     * <pre>
     * def is: obj taggedAs: type {
     *  (reflect: obj).isTaggedAs(type)
     *};
     * </pre>
     * 
     * @param type the type tag object to check for
     * @return true if this mirror's base object or one of its parent objects is tagged
     * with a subtype of the given type, false otherwise.
     */
    public ATBoolean meta_isTaggedAs(ATTypeTag type) throws InterpreterException;
    
    /**
     * Returns all of the local type tags of this object. The primitive method
     * <tt>tagsOf: obj</tt> is defined in terms of this method:
     * 
     * <pre>
     * def tagsOf: obj {
     *  (reflect: obj).typeTags
     *};
     * </pre>
     * 
     * @return a table of the type tags that were attached directly to this mirror's base
     * object. The type tags of its parent objects are not returned.
     */
    public ATTable meta_typeTags() throws InterpreterException;
    
     /* -------------------------------
      * - Base Level Object interface -
      * ------------------------------- */

    /**
     * Bound to the dynamic parent of this object.
     * 
     * The dynamic parent of an object is the object to which failed
     * selection or invocation requests or type tests are delegated to.
     * 
     * @return the current dynamic parent of this object.
     */
    public ATObject base_super() throws InterpreterException;
    
    /**
     * The identity operator. In AmbientTalk, equality of objects
     * is by default pointer-equality (i.e. objects are equal only
     * if they are identical).
     * 
     * This is a primitive method, present by default in every AmbientTalk
     * object but redefinable by the programmer.
     * 
     * @return by default, true if the parameter object and this object are identical,
     * false otherwise.
     */
    public ATBoolean base__opeql__opeql_(ATObject other) throws InterpreterException;

    /**
     * The object instantiation method. Note that in class-based OO languages,
     * this method is usually at the level of the <i>class</i>. In AmbientTalk,
     * this method is situated at the object-level directly. It can be overridden
     * to e.g. enforce the singleton pattern or to return instances of other
     * objects.
     * 
     * The default implementation of this method is:
     * <pre>def new(@args) {
     *  (reflect: self).newInstance(@args)
     *};
     * </pre>
     * 
     * This is a primitive method, present by default in every AmbientTalk
     * object but redefinable by the programmer.
     * 
     * @see ATObject#meta_newInstance(ATTable) for a description of object instantiation.
     * @param initargs the variable argument list to pass to the <tt>init</tt> method.
     * @return by default, the new instance of this mirror's base object.
     */
    public ATObject base_new(ATObject[] initargs) throws InterpreterException;

    /**
     * The object initialisation method. In class-based languages, this method
     * is often called the constructor. AmbientTalk only supports one constructor
     * per object, but thanks to variable argument lists and optional parameters,
     * the same flexibility as defining multiple constructors can often be achieved.
     * Also, by overriding <tt>new</tt>, the developer may invoke additional methods
     * on newly created objects if this is desirable.
     * 
     * The default implementation of this method is:
     * <pre>def init(@args) {
     *  super^init(@args)
     *};
     * </pre>
     * 
     * This is a primitive method, present by default in every AmbientTalk
     * object but redefinable by the programmer.
     * 
     * @see ATObject#meta_newInstance(ATTable) for a description of object initialisation.
     * @param initargs the arguments to the <tt>init</tt> constructor method.
     * @return the return value of invoking the <tt>init</tt> method. Note that
     * this value is <i>discarded</i> when <tt>init</tt> is invoked from the
     * <tt>newInstance</tt> meta-level operation.
     */
    public ATObject base_init(ATObject[] initargs) throws InterpreterException;
    
    /* -----------------------------------------
     * - Implementation-Level Object interface -
     * ----------------------------------------- */
    
    /**
     * The <tt>lexicalParent</tt> field of a mirror denotes the lexical parent
     * pointer of the mirror's base object. The lexical parent is the enclosing
     * <i>lexical scope</i> in which the object was defined.
     * 
     * @return the object denoting this mirror's base object's lexically
     * enclosing scope.
     */
    public ATObject impl_lexicalParent() throws InterpreterException;
    
	/**
	 * Interprets <code>o.x()</code> or <code>o.m(arg)</code>.
	 * Implements slot access. This method is an implementation-level method (not part of the MOP).
	 * @param receiver the dynamic receiver of the slot invocation.
	 * @param selector a regular symbol denoting the slot accessor.
	 * @param arguments the actual arguments to the slot invocation.
	 * @return the result of applying the accessor.
	 * @throws XArityMismatch if a field accessor is not invoked with exactly zero arguments.
	 */
	public ATObject impl_invokeAccessor(ATObject receiver, ATSymbol selector, ATTable arguments) throws InterpreterException;
	
    /**
     * Interprets <code>o.x := v</code>.
     * Implements slot mutation. This method is an implementation-level method (not part of the MOP).
     * @param receiver the dynamic receiver of the slot invocation.
	 * @param selector an assignment symbol denoting which slot to invoke.
	 * @param arguments the actual arguments to the slot invocation.
	 * @return the result of applying the mutator.
	 * @throws XArityMismatch if a field mutator is not invoked with exactly one argument.
     */
	public ATObject impl_invokeMutator(ATObject receiver, ATAssignmentSymbol selector, ATTable arguments) throws InterpreterException;
	
	/**
	 * Interprets <code>o.&m</code>.
	 * Implements slot accessor selection. This method is an implementation-level method (not part of the MOP).
	 * @param receiver the dynamic receiver of the slot selection.
	 * @param selector a regular symbol denoting the accessor to select.
	 * @return a closure wrapping the selected method or an accessor for a field.
	 */
	public ATClosure impl_selectAccessor(ATObject receiver, ATSymbol selector) throws InterpreterException;
	
	/**
	 * Interprets <code>o.&m:=</code>.
	 * Implements slot mutator selection. This method is an implementation-level method (not part of the MOP).
	 * @param receiver the dynamic receiver of the slot selection.
	 * @param selector an assignment symbol denoting the mutator to select.
	 * @return a closure representing the mutator of a given slot.
	 */
	public ATClosure impl_selectMutator(ATObject receiver, ATAssignmentSymbol selector) throws InterpreterException;
	
	
    /**
     * Interprets <code>x := v</code> (equivalent to <code>x:=(v)</code>) or <code>f(v)</code>.
     * Implements functions calls and lexical access to variables.
     * This method is an implementation-level method (not part of the MOP).

     * Variable lookup first queries the local fields of this object, then the local
     * method dictionary. If the selector is not found, the search continues in
     * this object's <i>lexical parent</i>. Hence, variable lookup follows
     * <b>lexical scoping rules</b>.
     * <p>
     * Similar to the behaviour of <tt>invoke</tt>, if the selector is bound to a
     * field rather than a method, <tt>call</tt> treats the field as an accessor
     * or mutator method (depending on the selector).
     * <p>
     * Note that, unlike <tt>invoke</tt> and <tt>select</tt>, <tt>call</tt> does
     * not give rise to the invocation of <tt>doesNotUnderstand</tt> if the selector
     * was not found. The reason for this is that lexical lookup is a static process
     * for which it makes less sense to provide dynamic interception facilities.
     *
     * @param selector a symbol denoting the name of the field or method to look up lexically.
     * @param arguments the arguments to the lexically scoped function call.
     * @return if selector is bound to a field, the value of the field; otherwise if selector
     * is bound to a method, the return value of the method.
     * @throws XUndefinedSlot if the selector could not be found in the lexical scope of this object.
     */
    public ATObject impl_call(ATSymbol selector, ATTable arguments) throws InterpreterException;
    
    /**
     * Interprets <code>f(v)</code>.
     * Implements the protocol to access lexical variables and methods. This operation (which is not exposed
     * as part of the MOP) locates the lexically visible binding with the given selector and will return 
     * the value of the slot.
     * <p>
     * When this object has a local slot corresponding to the selector:
     * <ul>
     * <li> and the slot contains a method or closure, it will be applied with the given arguments (within a context
     *   where self is bound to this object)
     * <li> and the slot contains a value and the argumentlist is empty, the value is returned
     * <li> and the slot contains a value and the argumentlist is not empty, an arity mismatch exception is raised
     * </ul>
     * <p>
     * When no local slot is found, lookup continues along the lexical parent chain. When the lexical chain is 
     * completely traversed, an undefined slot exception is raised.
     */
    public ATObject impl_callAccessor(ATSymbol selector, ATTable arguments) throws InterpreterException;
    
    /**
     * Interprets <code>x := v</code> (which is equivalent to <code>x:=(v)</code>.
     * Implements the protocol to assign lexical variables. This operation (which is not exposed as part of the MOP) 
     * locates slots to assign corresponding to a specific assignment symbol (selector + ":=") and looks for:
     * <ol>
     * <li> a mutator method with the specified assignment symbol (i.e. including the ":=") which can then be 
     * invoked with the provided arguments.
     * <li> a field with a corresponding selector (i.e. without the ":=") which is then treated as if it were
     * a unary mutator method.
     * </ol>
     * 
     * If the slot is a method slot, an {@link XUnassignableField} exception is raised, otherwise the arity of the 
     * arguments is verified (should be precisely 1) and the first argument is used as the new value of the slot.
     * <p>
     * When no local slot is found, lookup continues along the lexical parent chain. When the lexical chain is 
     * completely traversed, a selector not found exception is reported.
     */
    public ATObject impl_callMutator(ATAssignmentSymbol selector, ATTable arguments) throws InterpreterException;
    
    /**
     * Interprets <code>x</code>.
     * This method is an implementation-level method (not part of the MOP).
     * 
     * This method is equivalent to {@link #impl_call(ATSymbol, ATTable)} where
     * the arguments equal <tt>[]</tt>, except for one case: when the selector
     * resolves to a field containing a closure, the closure is not auto-applied
     * with zero arguments, but is instead returned. For all other purposes,
     * evaluating <tt>m</tt> is equivalent to evaluating <tt>m()</tt> such that
     * fields and nullary methods can be uniformly accessed.
     * 
     * @param selector the name of a lexically visible field or method.
     * @return the value of a field or the return value of an accessor method.
     */
    public ATObject impl_callField(ATSymbol selector) throws InterpreterException;
    
    /**
     * Interprets <code>&x</code> or <code>&x:=</code>.
     * This method is an implementation-level method (not part of the MOP).
     * 
     * This operation is the lexical counterpart of {@link #meta_select(ATObject, ATSymbol)}.
     * @param selector the name of a lexically visible field or method.
     * @return a closure wrapping a method, or an accessor or mutator linked to a lexically visible field.
     */
    public ATClosure impl_lookup(ATSymbol selector) throws InterpreterException;

    /**
     * Interprets <code>&x</code>.
     * This method is an implementation-level method (not part of the MOP).
     * 
     * @param selector the name of a lexically visible field or method.
     * @return a closure wrapping a method, or an accessor linked to a lexically visible field.
     */
    public ATClosure impl_lookupAccessor(ATSymbol selector) throws InterpreterException;

    /**
     * Interprets <code>&x:=</code>.
     * This method is an implementation-level method (not part of the MOP).
     * 
     * If the selector minus <tt>:=</tt> is bound to a field, the field is assigned
     * i
     * @param selector the name of a lexically visible method or of a field (whose name does not have the <tt>:=</tt> prefix).
     * @return a closure wrapping a method, or a mutator linked to a lexically visible field.
     */
    public ATClosure impl_lookupMutator(ATAssignmentSymbol selector) throws InterpreterException;
    
    /**
     * This is a callback method used in AmbientTalk's native equality protocol.
     * When evaluating <tt>o1 == o2</tt> in AmbientTalk, <tt>o1</tt>'s <tt>==</tt>
     * method will invoke <tt>o2.identityEquals(o1)</tt>.
     * 
     * The native implementation can make use of Java's <tt>==</tt> operator where
     * appropriate.
     */
    public ATBoolean impl_identityEquals(ATObject other) throws InterpreterException;
    
}