In computing, a meta-circular evaluator is a special case of a self-interpreter in which the existing facilities of the parent interpreter are directly applied to the source code being interpreted, without any need for additional implementation. Meta-circular evaluation is most common in the context of homoiconic languages.
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The first appearance of the idea is in the dissertation of Corrado Böhm (1951). The definition of Lisp 1.5 (1961) by John McCarthy, where the evaluation rules of Lisp are described as a Lisp program, had additional impact.
The difference between self-interpreters and meta-circular interpreters is that the latter restate language features in terms of the features themselves, instead of actually implementing them. (Circular definitions, in other words; hence the name). They depend on their host environment to give the features meaning.
Meta-circular evaluation is discussed at length in section 4.1, titled The Metacircular Evaluator, of the MIT university textbook Structure and Interpretation of Computer Programs (SICP). The core idea they present is two functions:
The two functions then call each other in circular fashion to fully evaluate a program.
Ramifications
Meta-circular implementations are suited to extending the language they are written in. 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.
Examples
Many languages have one or more meta-circular implementations. Here below is a partial list.
Some languages with a meta-circular implementation designed from the bottom up, in grouped chronological order:
Some languages with a meta-circular implementation via third parties: