CLIPS Fact Commands in AI

CLIPS shell

The CLIPS shell provides the basic elements of an expert system:
1. fact-list, and instance-list: Global memory for data
2. Knowledge-base: Contains all the rules, the rule-base
3. inference engine: Controls overall execution of rules

Making a List

As with other programming languages, CLIPS recognizes certain keywords. For example, if you want to put data in the fact-list, you can use the assert command.

         CLIPS> (assert (duck))
         <Fact-1>
         CLIPS> (facts)
         f-0 (initial-fact)
         f-1 (duck)
        For a total of 2 facts.
       CLIPS>

What happens if you try to put a second duck into the fact-list?
Let's try it and see. Assert a new (duck), then issue a (facts) command as follows

Fact Command

The keyboard command to see facts is with the facts command. Enter (facts) in response to the CLIPS prompt and CLIPS will respond with a list of facts in the fact-list.
Be sure to put parentheses around the command or CLIPS will not accept it. The result of the (facts) command in this example should be

CLIPS> (facts)
f-0 (initial-fact)
f-1 (duck)
For a total of 2 facts
CLIPS>

Clearing Up the Facts

The (clear) commend actually does more than just remove facts. Besides removing all the facts, (clear) also removes all the rules.

CLIPS> (facts)
f-0 (initial-fact)
f-1 (duck)
f-2 (quack)
For a total of 3 facts.
CLIPS> (clear)
CLIPS>

CLIPS> (clear)
CLIPS> (assert (a) (b) (c))
<Fact-3>
CLIPS> (facts)
f-0 (initial-fact)
f-1 (a)
f-2 (b)
f-3 (c)
For a total of 4 facts.

Retract that Fact

Removing facts from the fact-list is called retraction and is done with the retract command.
To retract a fact, you must specify the fact-index.

CLIPS> (retract 3)
CLIPS>

You can also retract multiple facts at once, as shown by the following.

       CLIPS> (retract 1   3)
       CLIPS> (facts)
       f-0 (initial-fact)
       f-2 (animal-sound quack)
        For a total of 2 facts
You can just use (retract*) to retract all the facts, where the "*" indicates all.

Watch that Fact

One command allows you to continuously watch facts being asserted and retracted.

      CLIPS> (assert (animal-is duck) )
      ==> f-1 (animal-is duck)
<Fact - 1>
      CLIPS> (retract 1)
      <== f-1 (animal-is duck)
     CLIPS> (facts)
     f-0 (initial-fact)
     For a total of 1 fact.
     CLIPS>

To turn off watching facts, enter (unwatch facts).

White space

Multiple fields normally are separated by white space consisting of one or more spaces, tabs, carriage returns, or line feeds.

(assert ( The
duck
says
"Quack") )
FALSE
be careful if you insert a carriage return inside of a string
CLIPS> (assert ( The
duck
says
"Quack
" ))
<Fact-6>

CLIPS is said to be case-sensitive because it distinguishes between uppercase and lowercase letters.
For example, assert the facts (duck) and (Duck) and then issue a facts) command. You'll see that CLIPS allows you to assert (duck) and (Duck) as different facts because CLIPS is case-sensitive.

Making Good Rules

To accomplish useful work, an expert system must have rules as well as facts.
The pseudocode for a rule about duck sounds might be

        IF the animal is a duck
        THEN the sound made is quack

                CLIPS> (assert (animal-is duck) )
                <Fact-1>
                CLIPS> (defrule duck
                 (animal-is duck)
                 =>
                 (assert (sound-is quack))))
                CLIPS>

(defrule duck "Here comes the quack"       ; Rule header
(animal-is duck)                                           ; Pattern
=>                                                                ; THEN arrow
( assert (sound-is quack)))                           ; Action