Ordered facts encode information positionally. To access that information, a user must know not only what data is stored in a fact but which field contains the data. N (or deftemplate) facts provide the user with the ability to abstract the structure of a fact by assigning names to each field in the fact. The deftemplate construct (see section 3) is used to create a template which can then be used to access fields by name. The deftemplate construct is analogous to a record or structure definition in programming languages such as Pascal and C.
The deftemplate construct allows the name of a template to be defined along with zero or more definitions of named fields or slots. Unlike ordered facts, the slots of a deftemplate fact may be constrained by type, value, and numeric range. In addition, default values can be specified for a slot. A slot consists of an opening parenthesis followed by the name of the slot, zero or more fields, and a closing parenthesis. Note that slots may not be used in an ordered fact and that positional fields may not be used in a deftemplate fact.
Deftemplate facts are distinguished from ordered facts by the first field within the fact. The first field of all facts must be a symbol, however, if that symbol corresponds to the name of a deftemplate, then the fact is a deftemplate fact. The first field of a deftemplate fact is followed by a list of zero or more slots. As with ordered facts, deftemplate facts are enclosed by an opening parenthesis on the left and a closing parenthesis on the right.
Some examples of deftemplate facts are shown following.
(client (name "Joe Brown") (id X9345A)) (point-mass (x-velocity 100) (y-velocity -200)) (class (teacher "Martha Jones") (#-students 30) (Room "37A")) (grocery-list (#-of-items 3) (items bread milk eggs))
Note that the order of slots in a deftemplate fact is not important. For example the following facts are all identical:
(class (teacher "Martha Jones") (#-students 30) (Room "37A")) (class (#-students 30) (teacher "Martha Jones") (Room "37A")) (class (Room "37A") (#-students 30) (teacher "Martha Jones"))
In contrast, note that the following ordered fact are not identical.
(class "Martha Jones" 30 "37A") (class 30 "Martha Jones" "37A") (class "37A" 30 "Martha Jones")
The immediate advantages of clarity and slot order independence for deftemplate facts should be readily apparent.
In addition to being asserted and retracted, deftemplate facts
can also be modified and duplicated (using the modify
and duplicate commands). Modifying a fact
changes a set of specified slots within that fact. Duplicating
a fact creates a new fact identical to the original fact and then
changes a set of specified slots within the new fact. The benefit
of using the modify and duplicate commands is that slots which
donít change, donít have to be specified.