In computing, a meta-circular evaluator (MCE) or meta-circular interpreter (MCI) 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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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:
- Lisp, 1958
- Forth, 1968
- PostScript, 1982
- Prolog, 1972
- TeX, based on virgin TeX, 1978
- Smalltalk, 1980
- Factor, 2003
Some languages with a meta-circular implementation via third parties:
- Reynolds, John C. (August 1972). "Definitional Interpreters for Higher-Order Programming Languages" (PDF). Higher-Order and Symbolic Computation. 11 (4): 363–397. doi:10.1023/A:1010027404223. Retrieved 14 April 2017.
- "The Metacircular Evaluator". Structure and Interpretation of Computer Programs. MIT.
- C. Böhm, Calculatrices digitales. Du déchiffrage des formules logico-mathématiques par la machine même dans la conception du programme, Ann. Mat. Pura Appl. (4) 37 (1954) 1-51
- Knuth, Donald E.; Pardo, Luis Trabb (August 1976). The early development of programming languages. p. 36.
- McCarthy, John (1961). "A Universal LISP Function" (PDF). Lisp 1.5 Programmer's Manual. p. 10.
- Harvey, Brian. "Why Structure and Interpretation of Computer Programs matters". people.eecs.berkeley.edu. Retrieved 14 April 2017.
- Braithwaite, Reginald (2006-11-22). "The significance of the meta-circular interpreter". Retrieved 2011-01-22.
- Reynolds, John C. (1998). "Definitional Interpreters Revisited" (PDF). Higher-Order and Symbolic Computation. 11 (4): 356–7. doi:10.1023/A:1010075320153. Retrieved 14 April 2017.
- Oriol, Manuel; Meyer, Bertrand (2009-06-29). Objects, Components, Models and Patterns: 47th International Conference, TOOLS EUROPE 2009, Zurich, Switzerland, June 29-July 3, 2009, Proceedings. Springer Science & Business Media. p. 330. ISBN 9783642025716. Retrieved 14 April 2017.
- Meta-circular implementation of the Pico programming language
- Structure and Interpretation of Computer Programs (SICP), online version of full book, accessed 2009-01-18.