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An esoteric programming language (sometimes shortened to esolang) is a programming language designed to test the boundaries of computer programming language design, as a proof of concept, as software art, as a hacking interface to another language (particularly functional programming or procedural programming languages), or as a joke. The use of distinguishes these languages from programming languages that working developers use to write software. Usually, an esolang's creators do not intend the language to be used for mainstream programming, although some esoteric features, such as visuospatial syntax, have inspired practical applications in the arts. Such languages are often popular among hackers and hobbyists.

Usability is rarely a goal for esoteric programming language designers—often it is quite the opposite. Their usual aim is to remove or replace conventional language features while still maintaining a language that is Turing-complete, or even one for which the computational class is unknown.

The earliest, and still the canonical example of an esoteric language was INTERCAL, designed in 1972 by Don Woods and James M. Lyon, with the stated intention of being unlike any other programming language the authors were familiar with. It parodied elements of established programming languages of the day, such as Fortran, COBOL, and assembly language.

For many years INTERCAL was represented only by paper copies of the INTERCAL manual. The language's revival in 1990 as an implementation in C under Unix stimulated a wave of interest in the intentional design of esoteric computer languages.

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