Meta-circular interpreter

In computing, a meta-circular evaluator or meta-circular interpreter is an interpreter which defines each feature of the interpreted language using a similar facility of the interpreter's host language. For example, interpreting a lambda application may be implemented using function application. Meta-circular evaluation is most prominent in the context of Lisp. A self-interpreter is a meta-circular interpreter where the interpreted language is nearly identical to the host language; the two terms are often used synonymously.

The dissertation of Corrado Böhm describes the design of a self-hosting compiler. Due to the difficulty of compiling higher-order functions, many languages were instead defined via interpreters, most prominently Lisp. The term itself was coined by John C. Reynolds, and popularized through its use in the book Structure and Interpretation of Computer Programs.

A self-interpreter is a meta-circular interpreter where the host language is also the language being interpreted. A self-interpreter displays a universal function for the language in question, and can be helpful in learning certain aspects of the language. A self-interpreter will provide a circular, vacuous definition of most language constructs and thus provides little insight into the interpreted language's semantics, for example evaluation strategy. Addressing these issues produces the more general notion of a "definitional interpreter".

In combination with an existing language implementation, meta-circular interpreters provide a baseline system from which to extend a language, either upwards by adding more features or downwards by compiling away features rather than interpreting them. They are also useful for writing tools that are tightly integrated with the programming language, such as sophisticated debuggers. A language designed with a meta-circular implementation in mind is often more suited for building languages in general, even ones completely different from the host language.

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