FRAMES

Frames provide a way of representing and manipulating complex objects. There are two main features of frame systems:
 

Procedural Attachment	Inference in a frame system is performed on an ad hoc basis by attaching procedures to the attributes of an object. These procedures are triggered depending on how the attribute is accessed.
Inheritance	Like all object-oriented systems, instance frames can inherit properties from generic frames in a hierarchy. iProlog supports multiple inheritance.

In iProlog, frames are stored in Prolog's database and their contents can be queried and matched in the same way as other data. This section describes a front-end to iProlog that provides a convenient syntax for manipulating frames.

• Generic Frames
• Instance Frames
• Frame Commands
• Dæmons
• Auxiliary functions and predicates

• A Complete Example

Generic Frames

The structure of a generic frame is shown below

 frame_name ako inheritance-list
 slot1:
 facet11 demon11
 facet12 demon12;
 slot2:
 facet21 demon21
 facet22 demon22!

Example

cylinder ako object with

 height:
 range number(new value) and new value > 0
 help print("Height must be a positive number")
 if_needed ask
 if_removed remove volume from this cylinder
 cache yes;
 radius:
 range number(new value) and new value > 0
 help print("Radius must be a positive number")
 if_needed ask
 if_removed remove cross_section from this cylinder
 cache yes;
 cross_section:
 if_needed pi * radius of this cylinder ** 2
 if_removed remove volume from this cylinder
 cache yes;
 volume:
 if_needed cross_section of this cylinder * height of this cylinder
 cache yes!

Slots

Generic frames describe generic objects, e.g. person, physical object, etc. A frame's attributes (slots) may be explicitly listed within the frame definition or they may be inherited from a generic frame. The value of "ako" contains a list of generic frame names from which slots can be inherited. In the example above, the "cylinder" frame can inherit attributes from the "object" frame.

Facets and Dæmons

A slot in a generic frame contains a list of facets and demons. Facets are labels which identify the type of demon. The allowable facets are described in the Reference Manual. Demons are procedures that are attached to slots and are triggered automatically by different kinds of accesses to the frame. They are written in Prolog.

Multiple Inheritance

The inheritance specification may be a single name such as object in the example above, or it may be a list. e.g.. [cylinder, container]. This means that the object would inherit all the properties of cylinder and container. The inheritance lists are searched left-to-right and depth-first. If there is more the one parent frame with the same slot name, the first one encountered is used.

Instance Frames

The structure of an instance frame

 frame-name isa inheritance-list with
 slot1: value1;
 slot2: value2;
 ................
 slotN: valueN!

Example

 c isa cylinder with
 height: 2;
 radius: 4!

Instance frames describe instances of generic frames, i.e. cylinders, etc. The inheritance list, following the isa keyword contains a list of the names of the generic frame names from which the frame can inherit slots. Whenever a slot cannot be found within an instance frame, inheritance is applied. iProlog searches the inheritance hierarchy looking for a particular slot. However, instead of containing an actual value, the slot will contain a procedure for computing the corresponding value.

iProlog allows multiple inheritance, i.e. a frame may inherit attributes from more than one frame. The inheritance hierarchy is then searched in a depth-first, left-to-right manner.

Frame Commands

Dæmons

Auxiliary Functions and Predicates for Frames