In computer programming, homoiconicity is a property of some programming languages, in which the primary representation of programs is also a data structure in a primitive type of the language itself, from homo meaning the same and icon meaning representation.

History

The original source is the paper Macro Instruction Extensions of Compiler Languages, according to the early and influential paper TRAC, A Text-Handling Language:

Alan Kay used and possibly popularized the term "homoiconic" through his use of the term in his 1969 PhD thesis:

Use and Advantages

One advantage of homoiconicity is that extending the language with new concepts typically becomes simpler, as data representing code can be passed between the meta and base layer of the program. A typical demonstration of homoiconicity is the metacircular interpreter.

Examples

Languages which are considered to be homoiconic include most members of the Lisp family, Prolog, Smalltalk, Curl, REBOL, SNOBOL, XSLT, TRAC, Tcl, Io, Joy, Factor, Clojure, PostScript and V.

In Von Neumann architecture systems (including the vast majority of systems today), raw machine code also has this property, the data type being bytes in memory.

References

See also

• Cognitive dimensions, design principles for programming languages.

External links

• Definition of Homoiconic at the C2 Wiki Note that the discussion about the exact definition is still ongoing at this site.