Forgoing Implicity and Using Abstractions: CLIPS


In this article, I'll attempt to describe how the "higher level" programming language CLIPS compares to Ruby, another "higher level" language.

Higher Level Programming Languages

When I started professionally programming, I was hired to write web applications in PHP. PHP is an example of what some programmers refer to as a "higher level" programming language.

The term "High Level" can take on a few different meanings, but I'll refer to them as the following for the sake of this article: languages that provide frameworks on top of complex functionality normally delegated to the developer to implement in lower level languages.

In other words: developers use these languages to prevent re-inventing the wheel.

Ruby as a High Level Language

After my time writing PHP, I got a job writing Ruby. Ruby is also a "higher level programming language," meaning "you can write a-cool-feature in Ruby in x% less code."

For Ruby, one might argue those cool features could include:

Rules Engines: whenever

Eventually I got a job writing Rules Engines in Ruby. This was an eye-opening experience: Not only did we have the power of Ruby, the developers also had the power of a rules-based "language" implemented on top of Ruby.

In addition to traditional if, else, or for keywords, a rules-based programming language provides the concept of whenever. How? Rules engines are "run," and thus exposes the ability to write code that runs "when conditions are met while our rules engine is running."

You might think of a rules engine as a bunch of nested if and goto statements all wrapped up in a while(should_run) loop. In other words: rules engines provide a framework for making data-driven run loops.

Comparing Ruby to CLIPS

This brings me to CLIPS. Working directly with home-grown rules engines taught me a lot. Rules Engines are complex, and, like Ruby's elaborate standard library, provide the developer an extremely powerful tool set containing "things you're going to try to write yourself anyway."

CLIPS provides the same abstractions that Ruby does plus the added functionality of a rules engine in C, so it implements those same "higher level" functionalities in the "lower level" speedy C.

CLIPS Features

Some of those functionalities include things like:

  1. automatic caching for commonly evaluated conditions in code
  2. Built-in query language DSLs for Instances and Facts
  3. Ability to fine-tune, optimize and compile your Rules Engine into a single binary

Working with applications that expose relational databases like MySQL, PostgreSQL, and SQLite means that one must translate tabular data into business logic using a non-rules-based imperative language. This translation can be expensive, so developers often implement a cache of some kind to "pre-fetch" the data and index it. If you have to "bootstrap" your application, this might be part of what is happening during that process.

CLIPS provides a caching mechanism due to its underlying use of the Rete Algorithm, interweaving your data into your running application. It creates a directional acyclic graph of nodes representing conditions in the rules in your rules engine. The moment you put data into the system, the Rete Algorithm feeds the data into the graph it created from the Rules you defined. The data traverses these nodes, continuing on to a node's children if the data evaluates true for a condition. The leaves of the graph are your rules, and the data that makes it to a leaf of the graph triggers the rule represented by that leaf. In this way, the data is "passed in" to the rule like one might pass an argument into a function, and thus business rules are executed.


CLIPS is a programming language that can provide a lot of value to your developers in the same way that moving to a "higher level" language does. It abstracts common and complex functionality so that the developer does not need to write these things over again, allowing them to focus their efforts on writing business logic. I hope you were able to relate to this article somehow, and that maybe it's sparked at least a tiny bit of interest in checking out CLIPS. If it did, you might find the Basic Programming Guide and the Advanced Programming Guide interesting to read. Afterall, it's the next layer of abstraction, the next "higher level" language.

- ryjo