CLIPS Shell: A Beginner’s Guide to Rule-Based Programming

CLIPS Shell: A Beginner’s Guide to Rule-Based Programming

What CLIPS Shell is

CLIPS Shell is an interactive command-line environment for CLIPS, a forward-chaining rule-based programming language and expert system tool developed by NASA. It lets you create, test, and run production rules, facts, and expert-system constructs in real time.

Core concepts

• Facts: Data items stored in the working memory (e.g., (person (name John) (age 30))).
• Rules: Condition-action pairs (if conditions match facts, then actions run). Rules use pattern matching on facts.
• Agenda: The list of activated rules waiting to fire, ordered by salience and conflict resolution strategy.
• Templates (deftemplate): Structured fact definitions for complex data.
• Deffacts / Defglobal / Definstances: Ways to declare initial facts, global variables, and object instances.
• Modules: Namespaces to organize rules and facts.

Basic workflow in the shell

1. Start CLIPS — run the CLIPS executable to open the shell prompt.
2. Define facts and templates — use (deftemplate …) and (assert …) or (deffacts …).
3. Write rules — using (defrule name (conditions) => (actions)).
4. Run the engine — use (reset) to initialize and (run) to execute the agenda.
5. Inspect state — (facts), (rules), (agenda), (watch) help debug and view runtime state.
6. Iterate — modify rules/templates and rerun.

Common commands

• (load “file.clp”) — load CLIPS source file.
• (clear) — remove all constructs and facts.
• (reset) — assert deffacts and prepare working memory.
• (run [n]) — execute rules (optionally n steps).
• (assert ) — add a fact to working memory.
• (retract ) — remove a fact.
• (facts) — list current facts.
• (rules) — list defined rules.
• (agenda) — show activated rules.
• (watch facts rules activations) — enable detailed tracing.

Simple example

1. Define a template and a rule:

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(deftemplate person (slot name) (slot age)) (defrule adult (person (name ?n) (age ?a&:(>= ?a 18))) =>(printout t ?n “ is an adult.” crlf)) 

2. Assert a fact and run:

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(assert (person (name Alice) (age 25))) (reset) (run) 

Output: “Alice is an adult.”

Tips for beginners

• Use (watch) to trace rule firings and fact assertions.
• Keep rules small and focused to ease debugging.
• Use salience to prioritize rules when needed.
• Modularize with deftemplate and modules for larger systems.
• Save and load sessions via files to preserve work.

Resources to learn more

• Official CLIPS documentation and reference manual (searchable online).
• Example CLIPS projects and tutorials (community repositories).