Editing Interpreter (computing) (section)

===Self-interpreter===
{{main|Meta-circular evaluator}}
A self-interpreter is a [[programming language]] interpreter written in a programming language which can interpret itself; an example is a [[BASIC programming language|BASIC]] interpreter written in BASIC. Self-interpreters are related to [[Self-hosting (compilers)|self-hosting compiler]]s.

If no [[compiler]] exists for the language to be interpreted, creating a self-interpreter requires the implementation of the language in a host language (which may be another programming language or [[Assembler (computing)|assembler]]). By having a first interpreter such as this, the system is [[Bootstrapping (compilers)|bootstrapped]] and new versions of the interpreter can be developed in the language itself. It was in this way that [[Donald Knuth]] developed the TANGLE interpreter for the language [[WEB]] of the de-facto standard [[TeX]] [[typesetting|typesetting system]].

Defining a computer language is usually done in relation to an abstract machine (so-called [[operational semantics]]) or as a mathematical function ([[denotational semantics]]). A language may also be defined by an interpreter in which the semantics of the host language is given. The definition of a language by a self-interpreter is not well-founded (it cannot define a language), but a self-interpreter tells a reader about the expressiveness and elegance of a language. It also enables the interpreter to interpret its source code, the first step towards reflective interpreting.

An important design dimension in the implementation of a self-interpreter is whether a feature of the interpreted language is implemented with the same feature in the interpreter's host language. An example is whether a [[closure (computer science)|closure]] in a [[Lisp programming language|Lisp]]-like language is implemented using closures in the interpreter language or implemented "manually" with a data structure explicitly storing the environment. The more features implemented by the same feature in the host language, the less control the programmer of the interpreter has; for example, a different behavior for dealing with number overflows cannot be realized if the arithmetic operations are delegated to corresponding operations in the host language.

Some languages such as [[Lisp programming language|Lisp]] and [[Prolog]] have elegant self-interpreters.<ref>Bondorf, Anders. "[https://web.archive.org/web/20181112101324/https://pdfs.semanticscholar.org/a089/c5ae66c3311b45de0aaddfa457e4eb821316.pdf Logimix: A self-applicable partial evaluator for Prolog]." Logic Program Synthesis and Transformation. Springer, London, 1993. 214-227.</ref> Much research on self-interpreters (particularly reflective interpreters) has been conducted in the [[Scheme (programming language)|Scheme programming language]], a dialect of Lisp. In general, however, any [[Turing completeness|Turing-complete]] language allows writing of its own interpreter. Lisp is such a language, because Lisp programs are lists of symbols and other lists. XSLT is such a language, because XSLT programs are written in XML. A sub-domain of [[metaprogramming]] is the writing of [[domain-specific language]]s (DSLs).

Clive Gifford introduced<ref>{{cite web |last1=Gifford |first1=Clive |title=Eigenratios of Self-Interpreters |url=http://eigenratios.blogspot.com/2006/11/wanted-eigenratios-of-brainfck-self.html |website=Blogger |access-date=10 November 2019}}</ref> a measure quality of self-interpreter (the eigenratio), the limit of the ratio between computer time spent running a stack of ''N'' self-interpreters and time spent to run a stack of {{nowrap|''N'' − 1}} self-interpreters as ''N'' goes to infinity. This value does not depend on the program being run.

The book ''[[Structure and Interpretation of Computer Programs]]'' presents examples of [[meta-circular evaluator|meta-circular interpretation]] for Scheme and its dialects. Other examples of languages with a self-interpreter are [[Forth (programming language)|Forth]] and [[Pascal (programming language)|Pascal]].