Scheme is a programming language that supports multiple paradigms, including functional and imperative programming. It is one of the three main dialects of Lisp, alongside Common Lisp and Clojure. Unlike Common Lisp, Scheme follows a minimalist design philosophy, specifying a small standard core with powerful tools for language extension. Scheme is a programming language that supports multiple paradigms, including functional and imperative programming. It is one of the three main dialects of Lisp, alongside Common Lisp and Clojure. Unlike Common Lisp, Scheme follows a minimalist design philosophy, specifying a small standard core with powerful tools for language extension. Scheme was created during the 1970s at the MIT AI Lab and released by its developers, Guy L. Steele and Gerald Jay Sussman, via a series of memos now known as the Lambda Papers. It was the first dialect of Lisp to choose lexical scope and the first to require implementations to perform tail-call optimization, giving stronger support for functional programming and associated techniques such as recursive algorithms. It was also one of the first programming languages to support first-class continuations. It had a significant influence on the effort that led to the development of Common Lisp. The Scheme language is standardized in the official IEEE standard and a de facto standard called the Revisedn Report on the Algorithmic Language Scheme (RnRS). The most widely implemented standard is R5RS (1998); a new standard, R6RS, was ratified in 2007. Scheme has a diverse user base due to its compactness and elegance, but its minimalist philosophy has also caused wide divergence between practical implementations, so much that the Scheme Steering Committee calls it 'the world's most unportable programming language' and 'a family of dialects' rather than a single language. Scheme started in the 1970s as an attempt to understand Carl Hewitt's Actor model, for which purpose Steele and Sussman wrote a 'tiny Lisp interpreter' using Maclisp and then 'added mechanisms for creating actors and sending messages'. Scheme was originally called 'Schemer', in the tradition of other Lisp-derived languages such as Planner or Conniver. The current name resulted from the authors' use of the ITS operating system, which limited filenames to two components of at most six characters each. Currently, 'Schemer' is commonly used to refer to a Scheme programmer. A new language standardization process began at the 2003 Scheme workshop, with the goal of producing an R6RS standard in 2006. This process broke with the earlier RnRS approach of unanimity. R6RS features a standard module system, allowing a split between the core language and libraries. A number of drafts of the R6RS specification were released, the final version being R5.97RS. A successful vote resulted in the ratification of the new standard, announced on August 28, 2007. Currently the newest releases of various Scheme implementations support the R6RS standard. There is a portable reference implementation of the proposed implicitly phased libraries for R6RS, called psyntax, which loads and bootstraps itself properly on various older Scheme implementations. A feature of R6RS is the record-type descriptor (RTD). When an RTD is created and used, the record type representation can show the memory layout. It also calculated object field bit mask and mutable Scheme object field bit masks, and helped the garbage collector know what to do with the fields without traversing the whole fields list that are saved in the RTD. RTD allows users to expand the basic RTD to create a new record system. R6RS introduces numerous significant changes to the language. The source code is now specified in Unicode, and a large subset of Unicode characters may now appear in Scheme symbols and identifiers, and there are other minor changes to the lexical rules. Character data is also now specified in Unicode. Many standard procedures have been moved to the new standard libraries, which themselves form a large expansion of the standard, containing procedures and syntactic forms that were formerly not part of the standard. A new module system has been introduced, and systems for exception handling are now standardized. Syntax-rules has been replaced with a more expressive syntactic abstraction facility (syntax-case) which allows the use of all of Scheme at macro expansion time. Compliant implementations are now required to support Scheme's full numeric tower, and the semantics of numbers have been expanded, mainly in the direction of support for the IEEE 754 standard for floating point numerical representation.