# Meta-Object Protocols

## Overview

XMF is an environment for language design and deployment. Languages control the structure and behaviour of the values that they denote. In this sense, language design is a meta-activity and requires a meta-language that represents the structure and behaviour of the language components. XMF provides a meta-circular object-oriented kernel language called XOCL. The meta-circular property means that XOCL is defined completely in itself. This property validates XOCL as a meta-language. Object-orientation provides a basis for application extension and reuse through inheritance and modularity through encapsulation.

XOCL is both meta-circular and object-oriented, it is suitable for language definition where  languages can be easily constructed as modular extensions of the basic XOCL language. Instantiations of XOCL are languages and extensions of XOCL are meta-languages.

All languages have key semantic features that can be represented as an interface in the definition of the language. Consider a language with an operational semantics. In this case programs written in the language may be viewed as controlling a machine that contains the state of the execution at any given snapshot in time. The key semantic features of the language form the API of the machine.

Given a language L, we would like to construct a new language that is L-like. If L is defined using object-oriented principles then it is attractive to construct the new language as an extension of {\em L} using inheritance. Syntax structures and values of the new language can be  defined by extending the appropriate features of L. We would like to construct the semantics  of the new language using the same approach. If we have constructed the semantics of L by encapsulating the key features as an implementation of the API as described above, then the new language semantics can be defined by inheriting and extending these operations as appropriate.

Where the semantics of a language has been constructed using object-oriented principles, the resulting collection of classes and operations is referred to as a meta-object-protocol. This document describes the XOCL MOP.

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## Message Passing

XOCL performs computation in terms of messages between elements. A message consists of a name andsome data. A message is sent from a source element to a target element. The target element receives the message, performs appropriate computation and returns a result. Messages between elements are synchronous: the source element halts computation and waits the return value from the target element.

Message passing occurs when the source element performs an expression of the form: o.m(x,y,z,...) where o is the target element, m is the message name and x, y, z etc. is the data, or parameters, of the message.

Message passing is defined by the MOP component referred to as the message passing protocol. The protocol is defined by the meta-class of the target element and is called sendInstance:
context Element  @Operation send(message,args):Element    self.of().sendInstance(self,message,args)  end
A default protocol is provided by the Kernel meta-class Classifier:
context Classifier  @Operation sendInstance(element,name,args)    // Get all the operations of element with the     // correct name and arity. Select the most     // specific and invoke it.    let arity = args->size then        operations = element.of().allOperations()          ->asSeq          ->select(o | o.name = name and o.arity() = arity)    in if operations->isEmpty       then element.error("Cannot handle " + message + "/" + arity)       else operations->head.invoke(element,args)       end    end  end
Since Class is a sub-class of Classifier, any sub-class of Class that defines a new sendInstance operation will provide a specialized message passing protocol for the instances of its instances. This can be used to implement specialized operation lookup mechanisms, to  facilitiate debugging information and to change the basic message passing mechanisms (for example by defining a class of objects with message queues).

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## Object Creation

Objects are created by sending a new message to a class together with some initialization data. The preferred way of invoking the new operation of a class is to apply the class as an operator to the initialization arguments. This is preferred because it is succinct and because the compiler and XMF VM can handle class instantiation more efficiently in this form:
context Classifier  @Operation invoke(target:Element,args:Seq(Element)):Element    self.new(args)  end
The new operation is defined by the meta-class of the receiving class. It constitutes the instantiation procotol for a collection of classes. The class Classifier defines the default instantiation protocol:
context Classifier  @Operation new(args:Seq(Element)):Element    self.new().init(args)  end
where the operation new creates an empty new instance of the receiver and init initializes the new instance.

Sub-classes of Classifier can define their own instantiation protocol. Typically this will use {\tt super} to create an instance using the default protocol and then perform some extra computation to initialize the new instance; however, in principle the instantiation protocol can by-pass the default protocol altogether. To create a raw instance of a class C and add a single slot named "x" with initial value 10 you can do the following:
let o = Kernel_mkObj()in Kernel_setOf(o,C);   Kernel_addAtt(o,Symbol("x",100));   oend
Using the kernel-level operations, you can create a completely bespoke instantiation protocol.

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## Slot Access

Objects have internal storage in the form of named slots. Access to a slot value is via the object and the name of the slot. Slots are named using symbols. Access is defined by the object's slot access protocol. The slot access protocol is used when an expression of the form o.a is performed. Access involves checking that the slot exists and then accessing the value of the slot.

The existence of a slot can be checked using the hasSlot operation defined by Object:
context Object   @Operation hasSlot(name):Boolean    self.of().hasInstanceSlot(self,name)  end
The hasSlot operation invokes the hasInstanceSlot operation of the object's class. hasInstanceSlot forms part of the slot access protocol for the object; the operation is defined by the object's meta-class. The default definition is provided by Class and uses the kernel operation Kernel_hasSlot to directly check whether there is a machine-level slot:
context Class  @Operation hasInstanceSlot(object,name)    Kernel_hasSlot(object,name)  end
Access to a slot's value is provided by the operation get defined by Object:
context Object  @Operation get(name:String):Element    self.of().getInstanceSlot(self,name)  end
The operation getInstanceSlot is defined by an object's meta-class and describes how to access the storage associated with an object and a slot name. The default protocol is provided by Class and uses the kernel operation Kernel_getSlotValue to access the machine-level slot (as added using Kernel_addAtt):
context Class  @Operation getInstanceSlot(object,name)    Kernel_getSlotValue(object,name)  end
Typically a new slot access protocol is required because a collection of classes implement object storage in a non-standard way (for example using a table, in a data base or distributed over a network).

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## Slot Update

The value of an object's slot can be updated using the object's slot update protocol. A slot is updated when an expression of the form o.a := e is performed. The class Object provides an operation used to set the value of a slot:
context Object  @Operation set(name:String,value:Element):Element    self.of().setInstanceSlot(self,name,value);    self  end
The object's meta-class defines an operation setInstanceSlot that forms the update protocol. The default update protocol is defined by Class and uses the kernel-level operation Kernel_setSlotValue to update the machine-level slot and to invoke any daemons that are defined on the object:
context Class  @Operation setInstanceSlot(object,name,value)    Kernel_setSlotValue(object,name,value)  end
A new slot update protocol is used to circumvent the default storage. For example the storage for a slot may be in a database or accessed over a network.

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## Default Parents

A class is created as an instance of a meta-class. When a class is created it must have some parents. The meta-class defines an operation defaultParents that produces a set of classes that are the default parents for its instances. The basic definition for defaultParents is provided by Classifier:
context Classifier  @Operation defaultParents():Set(Classifier)    Set{Element}  end
Most classes are instances of the class Class, that overrides the definition as follows:
context Class  @Operation defaultParents():Set(Classifier)    Set{Object}  end
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## Example

Suppose that we want to define a class of objects that can have standard attribute defined  slots in addition to dynamic slots. Attribute defined slots are defined at the class level. Dynamic slots are defined at the object level and can be added and removed dynamically. Both types of slot can be accessed and updated via the standard protocols using o.a and o.a := e expressions.

In order to implement these objects we require a new slot access and update protocol. The protocol is defined at the meta-level and is to be called the Elastic protocol. We require two new classes: ElasticObject that is the super-class of all user-defined elastic classes; and, ElasticClass that defines the elastic protocol.

The class ElasticObject uses a table to contain the dynamic slots:
context Root  @Class ElasticObject    @Attribute slots : Table = Table(100) end  end
An elastic object provides operations to add and remove the dynamic slots:
context ElasticObject  @Operation addSlot(name:String,value)    slots.put(Symbol(name),value)  endcontext ElasticObject  @Operation removeSlot(name)    slots.remove(Symbol(name))  end
An elastic object can remove all the dynamic slots:
context ElasticObject  @Operation removeAll()    @For key inTableKeys slots do      self.removeSlot(key.toString())    end  end
An elastic object can increment the values of all the dynamic slots. Note that incAll uses the slot access and update protocol for the object to change the value of the dynamic slots:
context ElasticObject  @Operation incAll()    @For key inTableKeys slots do      self.set(key,self.get(key) + 1)    end  end
The class ElasticClass defines the elastic MOP:
context Root  @Class ElasticClass extends Class  end
ElasticObject must be a parent of any elastic class:
context ElasticClass  @Operation defaultParents()    Set{ElasticObject}  end
The elastic slot access protocol inspects the slots table to see if the required slot is defined there. If not then the protocol uses super to revert to the default protocol inherited from Class:
context ElasticClass  @Operation getInstanceSlot(object,name)    if Kernel_getSlotValue(object,Symbol("slots")).hasKey(name)    then Kernel_getSlotValue(object,Symbol("slots")).get(name)    else super(object,name)    end  end context ElasticClass  @Operation setInstanceSlot(object,name,value)    if Kernel_getSlotValue(object,Symbol("slots")).hasKey(name)    then Kernel_getSlotValue(object,Symbol("slots")).put(name,value)    else super(object,name,value)    end  end context ElasticClass  @Operation hasInstanceSlot(object,name)    if Kernel_getSlotValue(object,Symbol("slots")).hasKey(name)    then true    else super(object,name)    end  end
There is no specific need for sendInstance in the elastic protocol, however it is defined for completeness and simply prints a message before reverting to the default protocol:
context ElasticClass  @Operation sendInstance(element,message,args)    format(stdout,"Sending message ~S(~{,~;~S~})~%",Seq{message,args});    super(element,message,args)  end
There is no specific need for new in the elastic protocol, however it is defined for completeness and simply prints a message before reverting to the default protocol:
context ElasticClass  @Operation new(args)    format(stdout,"Creating a new instance of an elastic class~%");    super(args)  end
The following is an example class definition that specifies its meta-class as ElasticClass:
context Root  @Class C metaclass ElasticClass    @Attribute s : Element end    @Operation test()      self.addSlot("x",100);      self.addSlot("y",200);      self.addSlot("z",300);      self.x := self.x + 1;      self.y := self.y + 1;      self.z := self.z + 1;      self.incAll();      self.s := self.x + self.y + self.z;      self.removeAll();      s    end  end
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