IOS Terminology

There are some terms to be familiar with:

metaobject protocol (MOP)

the mechanism by which the object system implements its behaviour – see Metaobject protocol

class

data describing an instance such as super-classes and slot names

A class has a name which is commonly surrounded by angle brackets, eg. <name> giving rise to <class> etc., but not required to be so.

If you use define-class then the same symbol is bound to the class instance. Hence the symbol <class> is bound to the class instance whose name is <class>.

super-class

classes whose slots are inherited

There will be direct super-classes, given when the class is defined, and a total set of super-classes derived from those inherited classes. The total set is called the class precedence list.

slot

a named field to contain any value

There is a distinction between the direct slots of this class and the slots accrued from inheritance through super-classes. The actual slots of the class are a merge of direct and inherited slots. If there are multiple instances of the same named slot across the direct and inherited slots there will only be one such actual slot.

class precedence list (CPL)

the CPL is an ordered list of the class and all inherited super-classes – omitting duplicates

IOS uses C3 linearization for which its example would be:

define-class O
define-class A O
define-class B O
define-class C O
define-class D O
define-class E O

define-class K1 (C B A)
define-class K3 (A D)
define-class K2 (B D E)

define-class Z (K1 K3 K2)

class-cpl Z      ; (Z K1 C K3 K2 B A D E O <object> <top>)

Note

object> and <top> appear because O is implicitly a sub-class of <object> as no super-classes are given for it and <object>’s super-class is <top> which has no super-class itself.

method resolution order (MRO)

the ordering of applicable methods from most specific to least specific, usually derived from the CPL

It is preferable that the ordering is deterministic.

instance proc

if an instance is in functional position and is a generic function then the VM will invoke the associated instance proc with the same arguments

generic function

the declaration of a function call in the object system

The instance proc of a generic function will determine an MRO and invoke the most specific method.

Therefore the actual behaviour of a generic function is implemented using methods associated with specific argument classes.

In normal usage a user would not explicitly define a generic function but, rather, the generic function will be created as a side-effect of defining methods.

methods

the behaviour of a generic function for specific argument classes

In normal usage this will implicitly define the associated generic function.

specializer

the class of an argument to a method

applicable method

a method is applicable if its specializers are in the CPLs of the classes of the arguments to the generic function

instance

an instance of a class in the usual way

What is unusual with this type of object system is that, technically, everything is an instance. Which isn’t helpful so think of instances in the usual way.

Last built at 2024-12-21T07:10:36Z+0000 from 62cca4c (dev) for Idio 0.3.b.6