**Models of Computation**

Some examples of models of computation include General Recursive Functions, Turing Machines, Lambda Calculus, Finite State Machines, and Production Systems. These are only some models of computation.

Martin Fowler has a pretty good post on rules engines (production system).* “… providing an alternative computational model.”*

Wikipedia’s definition is *“A model of computation is the definition of the set of allowable operations used in computation and their respective costs.”* I myself like the definition provided by “The Computational Beauty of Nature” [p27]. *“… a model of computation describes how to build recipes or, if you like , a recipe for recipes.”*

Both definitions seem to define two different things. The first definition describes programming languages. The second describes meta programming. These two definitions are not mutually exclusive. All programming languages have some form of meta programming. Just some languages have more powerful meta programming facilities.

In Common Lisp the most common way for the usage of meta programming is through the use of **defmacro**. Bill Clementson’s Blog post covers Joswig’s use of an embedded DSL in Lisp. Guess what allows him to make such a compact DSL? **defmacro **of course. Another nice Joswig post on comp.lang.lisp goes through an iterative cycle of design using defmacro.