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.[2]

Scheme is widely used by a number[37] of schools; in particular, a number of introductory Computer Science courses use Scheme in conjunction with the textbook Structure and Interpretation of Computer Programs (SICP).[38] For the past 12 years, PLT has run the ProgramByDesign (formerly TeachScheme!) project, which has exposed close to 600 high school teachers and thousands of high school students to rudimentary Scheme programming. MIT's old introductory programming class 6.001 was taught in Scheme,[39] Although 6.001 has been replaced by more modern courses, SICP continues to be taught at MIT.[40] The textbook How to Design Programs by Matthias Felleisen, currently at Northeastern University, is used by some institutes of higher education for their introductory computer science courses. Both Northeastern University and Worcester Polytechnic Institute use Scheme exclusively for their introductory courses Fundamentals of Computer Science (CS2500) and Introduction to Program Design (CS1101), respectively.[41][42] Rose-Hulman uses Scheme in its more advanced Programming Language Concepts course.[43] Indiana University's introductory class, C211, is taught entirely in Scheme. The introductory class at UC Berkeley, CS 61A, was until 2015 taught entirely in Scheme, save minor diversions into Logo to demonstrate dynamic scope; all course materials, including lecture webcasts, are available online free of charge.[44] A self-paced version of the course, CS 61AS, continues to use Scheme.[45] The introductory computer science courses at Yale and Grinnell College are also taught in Scheme.[46] Programming Design Paradigms,[47] a mandatory course for the Computer science Graduate Students at Northeastern University, also extensively uses Scheme. The introductory Computer Science course at the University of Minnesota - Twin Cities, CSCI 1901, also uses Scheme as its primary language, followed by a course that introduces students to the Java programming language.[48] In the software industry, Tata Consultancy Services, Asia's largest software consultancy firm, uses Scheme in their month-long training program for fresh college graduates.[citation needed]

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The introduction of lexical scope resolved the problem by making an equivalence between some forms of lambda notation and their practical expression in a working programming language. Sussman and Steele showed that the new language could be used to elegantly derive all the imperative and declarative semantics of other programming languages including ALGOL and Fortran, and the dynamic scope of other Lisps, by using lambda expressions not as simple procedure instantiations but as "control structures and environment modifiers".[23] They introduced continuation-passing style along with their first description of Scheme in the first of the Lambda Papers, and in subsequent papers, they proceeded to demonstrate the raw power of this practical use of lambda calculus.

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Scheme is primarily a functional programming language. It shares many characteristics with other members of the Lisp programming language family. Scheme's very simple syntax is based on s-expressions, parenthesized lists in which a prefix operator is followed by its arguments. Scheme programs thus consist of sequences of nested lists. Lists are also the main data structure in Scheme, leading to a close equivalence between source code and data formats (homoiconicity). Scheme programs can easily create and evaluate pieces of Scheme code dynamically.

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The R6RS standard has caused controversy because it is seen to have departed from the minimalist philosophy.[13][14] In August 2009, the Scheme Steering Committee, which oversees the standardization process, announced its intention to recommend splitting Scheme into two languages: a large modern programming language for programmers; and a small version, a subset of the large version retaining the minimalism praised by educators and casual implementors.[7] Two working groups were created to work on these two new versions of Scheme. The Scheme Reports Process site has links to the working groups' charters, public discussions and issue tracking system.

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Up to the R5RS standard, the standard comment in Scheme was a semicolon, which makes the rest of the line invisible to Scheme. Numerous implementations have supported alternative conventions permitting comments to extend for more than a single line, and the R6RS standard permits two of them: an entire s-expression may be turned into a comment (or "commented out") by preceding it with #; (introduced in SRFI 62[29]) and a multiline comment or "block comment" may be produced by surrounding text with #| and |#.

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These meditations were entirely employed on Mr Allworthy's fortune; for, first, he exercised much thought in calculating, as well as he could, the exact value of the whole: which calculations he often saw occasion to alter in his own favour: and, secondly and chiefly, he pleased himself with intended alterations in the house and gardens, and in projecting many other schemes, as well for the improvement of the estate as of the grandeur of the place: for this purpose he applied himself to the studies of architecture and gardening, and read over many books on both these subjects; for these sciences, indeed, employed his whole time, and formed his only amusement. 

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In contrast to Common Lisp, all data and procedures in Scheme share a common namespace, whereas in Common Lisp functions and data have separate namespaces making it possible for a function and a variable to have the same name, and requiring special notation for referring to a function as a value. This is sometimes known as the "Lisp-1 vs. Lisp-2" distinction, referring to the unified namespace of Scheme and the separate namespaces of Common Lisp.[24]

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These meditations were entirely employed on Mr Allworthy's fortune; for, first, he exercised much thought in calculating, as well as he could, the exact value of the whole: which calculations he often saw occasion to alter in his own favour: and, secondly and chiefly, he pleased himself with intended alterations in the house and gardens, and in projecting many other schemes, as well for the improvement of the estate as of the grandeur of the place: for this purpose he applied himself to the studies of architecture and gardening, and read over many books on both these subjects; for these sciences, indeed, employed his whole time, and formed his only amusement.

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Invocations of macros and procedures bear a close resemblance—both are s-expressions—but they are treated differently. When the compiler encounters an s-expression in the program, it first checks to see if the symbol is defined as a syntactic keyword within the current lexical scope. If so, it then attempts to expand the macro, treating the items in the tail of the s-expression as arguments without compiling code to evaluate them, and this process is repeated recursively until no macro invocations remain. If it is not a syntactic keyword, the compiler compiles code to evaluate the arguments in the tail of the s-expression and then to evaluate the variable represented by the symbol at the head of the s-expression and call it as a procedure with the evaluated tail expressions passed as actual arguments to it.

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Guile has been adopted by GNU project as its official scripting language, and that implementation of Scheme is embedded in such applications as GNU LilyPond and GnuCash as a scripting language for extensions. Likewise, Guile used to be the scripting language for the desktop environment GNOME,[51] and GNOME still has a project that provides Guile bindings to its library stack.[52] There is a project to incorporate Guile into GNU Emacs, GNU's flagship program, replacing the current Emacs Lisp interpreter.[citation needed]

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Alonzo Church's mathematical notation, the lambda calculus, has inspired Lisp's use of "lambda" as a keyword for introducing a procedure, as well as influencing the development of functional programming techniques involving the use of higher-order functions in Lisp. But early Lisps were not suitable expressions of the lambda calculus because of their treatment of free variables.[8]
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".[8] 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.

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The R6RS standard has caused controversy because it is seen to have departed from the minimalist philosophy.[13][14] In August 2009, the Scheme Steering Committee, which oversees the standardization process, announced its intention to recommend splitting Scheme into two languages: a large modern programming language for programmers; and a small version, a subset of the large version retaining the minimalism praised by educators and casual implementors.[7] Two working groups were created to work on these two new versions of Scheme. The Scheme Reports Process site has links to the working groups' charters, public discussions and issue tracking system.

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Up to the R5RS standard, the standard comment in Scheme was a semicolon, which makes the rest of the line invisible to Scheme. Numerous implementations have supported alternative conventions permitting comments to extend for more than a single line, and the R6RS standard permits two of them: an entire s-expression may be turned into a comment (or "commented out") by preceding it with #; (introduced in SRFI 62[29]) and a multiline comment or "block comment" may be produced by surrounding text with #| and |#.

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Guile has been adopted by GNU project as its official scripting language, and that implementation of Scheme is embedded in such applications as GNU LilyPond and GnuCash as a scripting language for extensions. Likewise, Guile used to be the scripting language for the desktop environment GNOME,[51] and GNOME still has a project that provides Guile bindings to its library stack.[52] There is a project to incorporate Guile into GNU Emacs, GNU's flagship program, replacing the current Emacs Lisp interpreter.[citation needed]

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Crowdfunding exists in mainly two types, reward-based crowdfunding and equity-based crowdfunding. In the former, small firms could pre-sell a product or service to start a business whereas in the latter, backers buys certain amount of shares of a firm in exchange of money.[8] As for reward-based crowdfunding, project creators would set a funding target and deadline. Anyone who is interested can pledge on the projects. Projects must reach its targeted amount in order for it to be carried out. Once the projects ended with enough funds, projects creators would have to make sure that they fulfil their promises by the intended timeline and delivery their products or services.[9]
Sources of funding include credit, venture capital, donations, grants, savings, subsidies, and taxes. Fundings such as donations, subsidies, and grants that have no direct requirement for return of investment are described as "soft funding" or "crowdfunding". Funding that facilitates the exchange of equity ownership in a company for capital investment via an online funding portal as per the Jumpstart Our Business Startups Act (alternately, the "JOBS Act of 2012") (U.S.) is known as equity crowdfunding.

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Prior to R5RS, Scheme had no standard equivalent of the eval procedure which is ubiquitous in other Lisps, although the first Lambda Paper had described evaluate as "similar to the LISP function EVAL"[19] and the first Revised Report in 1978 replaced this with enclose, which took two arguments. The second, third and fourth revised reports omitted any equivalent of eval.

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Similar procedures are provided for input. R5RS Scheme provides the predicates input-port? and output-port?. For character input and output, write-char, read-char, peek-char and char-ready? are provided. For writing and reading Scheme expressions, Scheme provides read and write. On a read operation, the result returned is the end-of-file object if the input port has reached the end of the file, and this can be tested using the predicate eof-object?.

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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.[11]

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The information on this computer and network is the property of a private corporation and is protected by intellectual property rights. You must be assigned an account on this computer to access information and are only allowed to access information as defined by the system administrators. Use of company resources should be for business purposes only and should be in compliance with the Code of Conduct and company policies and procedures. By accessing the property of a private corporation, your activities may be monitored and/or recorded and you should have no expectation of privacy on this computer and its networks. Use of this computer implies consent to monitoring and recording.

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Crowdfunding exists in mainly two types, reward-based crowdfunding and equity-based crowdfunding. In the former, small firms could pre-sell a product or service to start a business whereas in the latter, backers buys certain amount of shares of a firm in exchange of money.[8] As for reward-based crowdfunding, project creators would set a funding target and deadline. Anyone who is interested can pledge on the projects. Projects must reach its targeted amount in order for it to be carried out. Once the projects ended with enough funds, projects creators would have to make sure that they fulfil their promises by the intended timeline and delivery their products or services.[9]

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These meditations were entirely employed on Mr Allworthy's fortune; for, first, he exercised much thought in calculating, as well as he could, the exact value of the whole: which calculations he often saw occasion to alter in his own favour: and, secondly and chiefly, he pleased himself with intended alterations in the house and gardens, and in projecting many other schemes, as well for the improvement of the estate as of the grandeur of the place: for this purpose he applied himself to the studies of architecture and gardening, and read over many books on both these subjects; for these sciences, indeed, employed his whole time, and formed his only amusement.

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The elegant, minimalist design has made Scheme a popular target for language designers, hobbyists, and educators, and because of its small size, that of a typical interpreter, it is also a popular choice for embedded systems and scripting. This has resulted in scores of implementations,[35] most of which differ from each other so much that porting programs from one implementation to another is quite difficult, and the small size of the standard language means that writing a useful program of any great complexity in standard, portable Scheme is almost impossible.[7] The R6RS standard specifies a much broader language, in an attempt to broaden its appeal to programmers. 

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The elegant, minimalist design has made Scheme a popular target for language designers, hobbyists, and educators, and because of its small size, that of a typical interpreter, it is also a popular choice for embedded systems and scripting. This has resulted in scores of implementations,[35] most of which differ from each other so much that porting programs from one implementation to another is quite difficult, and the small size of the standard language means that writing a useful program of any great complexity in standard, portable Scheme is almost impossible.[7] The R6RS standard specifies a much broader language, in an attempt to broaden its appeal to programmers.

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The information on this computer and network is the property of a private corporation and is protected by intellectual property rights. You must be assigned an account on this computer to access information and are only allowed to access information as defined by the system administrators. Use of company resources should be for business purposes only and should be in compliance with the Code of Conduct and company policies and procedures. By accessing the property of a private corporation, your activities may be monitored and/or recorded and you should have no expectation of privacy on this computer and its networks. Use of this computer implies consent to monitoring and recording.
Kickstarter uses an all-or-nothing funding model. If your project doesn’t reach its goal, then funds don’t get collected, and no money changes hands. This minimizes risk for creators — imagine ending up with only $5,000 and a bunch of people expecting a $50,000 film! All-or-nothing funding makes it easier for backers to pledge to your project with confidence that you’ll be able to get the job done.

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Up to the R5RS standard, the standard comment in Scheme was a semicolon, which makes the rest of the line invisible to Scheme. Numerous implementations have supported alternative conventions permitting comments to extend for more than a single line, and the R6RS standard permits two of them: an entire s-expression may be turned into a comment (or "commented out") by preceding it with #; (introduced in SRFI 62[29]) and a multiline comment or "block comment" may be produced by surrounding text with #| and |#.
^ Jump up to: a b 1178-1990 (Reaff 2008) IEEE Standard for the Scheme Programming Language. IEEE part number STDPD14209, unanimously reaffirmed at a meeting of the IEEE-SA Standards Board Standards Review Committee (RevCom), March 26, 2008 (item 6.3 on minutes), reaffirmation minutes accessed October 2009. NOTE: this document is only available for purchase from IEEE and is not available online at the time of writing (2009).

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