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89-460: ALGOL ( / ˈ æ l ɡ ɒ l , - ɡ ɔː l / ; short for " Algorithmic Language ") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the Association for Computing Machinery (ACM) in textbooks and academic sources for more than thirty years. In

178-444: A class is only a definition; no memory is allocated. When memory is allocated to a class, it's called an object . Object-oriented imperative languages developed by combining the need for classes and the need for safe functional programming . A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a method , member function , or operation . Object-oriented programming

267-592: A character array, similar to C. The language allows the array identifier to be used as a pointer to the array, and hence in a REPLACE statement. A simpler program using an inline format: An even simpler program using the Display statement. Note that its output would end up at the system console ('SPO'): An alternative example, using Elliott Algol I/O is as follows. Elliott Algol used different characters for "open-string-quote" and "close-string-quote", represented here by   ‘   and   ’   . Below

356-587: A departure from the minimalist philosophy. 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. Two working groups were created to work on these two new versions of Scheme. The Scheme Reports Process site has links to

445-526: 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. In Scheme, the same primitives that are used to manipulate and bind data can be used to bind procedures. There is no equivalent of Common Lisp's defun and #' primitives. This subsection documents design decisions that have been taken over

534-651: 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

623-509: A major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal. COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like object-oriented programming . ALGOL (1960) stands for "ALGOrithmic Language." It had

712-485: 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. The R6RS standard has caused controversy because some see it as

801-455: A particular ALGOL 68 program; notably, they are able to express the kind of requirements that in many other programming language standards are labelled "semantics" and have to be expressed in ambiguity-prone natural language prose, and then implemented in compilers as ad hoc code attached to the formal language parser. (The way the bold text has to be written depends on the implementation, e.g. 'INTEGER'—quotation marks included—for integer. This

890-469: A pointer to swap(i, A[i]) in to a function. Now that every time swap is referenced, it is reevaluated. Say i := 1 and A[i] := 2, so every time swap is referenced it will return the other combination of the values ([1,2], [2,1], [1,2] and so on). A similar situation occurs with a random function passed as actual argument. Call-by-name is known by many compiler designers for the interesting " thunks " that are used to implement it. Donald Knuth devised

979-428: A powerful and expressive programming language." Like most modern programming languages and unlike earlier Lisps such as Maclisp , Scheme is lexically scoped: all possible variable bindings in a program unit can be analyzed by reading the text of the program unit without consideration of the contexts in which it may be called. This contrasts with dynamic scoping which was characteristic of early Lisp dialects, because of

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1068-635: A profound influence on programming language design. Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable structured design. Algol was first to define its syntax using the Backus–Naur form . This led to syntax-directed compilers. It added features like: Algol's direct descendants include Pascal , Modula-2 , Ada , Delphi and Oberon on one branch. On another branch there's C , C++ and Java . BASIC (1964) stands for "Beginner's All Purpose Symbolic Instruction Code." It

1157-441: A program. These include the jump (called goto in many languages), switch , and the subprogram, subroutine , or procedure call (which usually returns to the next statement after the call). Early in the development of high-level programming languages , the introduction of the block enabled the construction of programs in which a group of statements and declarations could be treated as if they were one statement. This, alongside

1246-532: A reference syntax, a publication syntax, and an implementation syntax, syntaxes that permitted it to use different keyword names and conventions for decimal points (commas vs periods) for different languages. ALGOL was used mostly by research computer scientists in the United States and in Europe; commercial applications were hindered by the absence of standard input/output facilities in its description, and

1335-417: A starting point of powerful mathematical logic. Second, it can reduce the requirement of programmers to consider the implementation details, because it can be used to imitate machine evaluation. Finally, the lambda calculation created a substantial meta-theory. The introduction of lexical scope resolved the problem by making an equivalence between some forms of lambda notation and their practical expression in

1424-441: A symbol called var is bound to the number 10: Blocks can be nested to create arbitrarily complex block structures according to the need of the programmer. The use of block structuring to create local bindings alleviates the risk of namespace collision that can otherwise occur. One variant of let , let* , permits bindings to refer to variables defined earlier in the same construct, thus: The other variant, letrec ,

1513-492: A syntax that would likely fail IBM's compiler. The American National Standards Institute (ANSI) developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports: COBOL (1959) stands for "COmmon Business Oriented Language." Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced. The US Department of Defense influenced COBOL's development, with Grace Hopper being

1602-522: A traditional programmer's puzzle, shows that Scheme can handle continuations as first-class objects, binding them to variables and passing them as arguments to procedures. When executed this code displays a counting sequence: @*@**@***@****@*****@******@*******@********... 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

1691-474: 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". They introduced continuation-passing style along with their first description of Scheme in

1780-504: Is R5RS (1998). The most recently ratified standard of Scheme is "R7RS-small" (2013). The more expansive and modular R6RS was ratified in 2007. Both trace their descent from R5RS; the timeline below reflects the chronological order of ratification. 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

1869-513: Is a C++ header file for the STUDENT class in a simple school application: Here is a C++ source file for the STUDENT class in a simple school application: Here is a driver program for demonstration: Here is a makefile to compile everything: Scheme (programming language) Scheme is a dialect of the Lisp family of programming languages . Scheme was created during the 1970s at

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1958-534: Is a function with the same name as the class name. It is executed when the calling operation executes the new statement. A module's other file is the source file . Here is a C++ source file for the GRADE class in a simple school application: Here is a C++ header file for the PERSON class in a simple school application: Here is a C++ source file for the PERSON class in a simple school application: Here

2047-769: Is a language so far ahead of its time that it was not only an improvement on its predecessors but also on nearly all its successors." The Scheme programming language, a variant of Lisp that adopted the block structure and lexical scope of ALGOL, also adopted the wording "Revised Report on the Algorithmic Language Scheme" for its standards documents in homage to ALGOL. ALGOL 60 as officially defined had no I/O facilities; implementations defined their own in ways that were rarely compatible with each other. In contrast, ALGOL 68 offered an extensive library of transput (input/output) facilities. ALGOL 60 allowed for two evaluation strategies for parameter passing:

2136-399: Is a relatively small language -- making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language , but uses a high-level syntax . It added advanced features like: C allows the programmer to control in which region of memory data is to be stored. Global variables and static variables require

2225-493: Is a version from Elliott 803 Algol (A104). The standard Elliott 803 used five-hole paper tape and thus only had upper case. The code lacked any quote characters so £ (UK Pound Sign) was used for open quote and ? (Question Mark) for close quote. Special sequences were placed in double quotes (e.g. ££L?? produced a new line on the teleprinter). The ICT 1900 series Algol I/O version allowed input from paper tape or punched card. Paper tape 'full' mode allowed lower case. Output

2314-486: Is attributable to the use of lambda calculus to derive much of the syntax of the language from more primitive forms. For instance of the 23 s-expression-based syntactic constructs defined in the R5RS Scheme standard, 14 are classed as derived or library forms, which can be written as macros involving more fundamental forms, principally lambda. As R5RS (§3.1) says: "The most fundamental of the variable binding constructs

2403-423: Is designed to enable mutually recursive procedures to be bound to one another. (See Hofstadter's male and female sequences for the definitions used in this example.) All procedures bound in a single letrec may refer to one another by name, as well as to values of variables defined earlier in the same letrec , but they may not refer to values defined later in the same letrec . A variant of let ,

2492-602: Is executing operations on objects . Object-oriented languages support a syntax to model subset/superset relationships. In set theory , an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other persons don't have. Object-oriented languages model subset/superset relationships using inheritance . Object-oriented programming became

2581-493: Is imperative at its core, as are its main target languages, VB.NET and C# that run on it; however Microsoft's F# , a functional language, also runs on it. FORTRAN (1958) was unveiled as "The IBM Mathematical FORmula TRANslating system." It was designed for scientific calculations, without string handling facilities. Along with declarations , expressions , and statements , it supported: It succeeded because: However, non IBM vendors also wrote Fortran compilers, but with

2670-450: Is known as stropping .) Here is an example of how to produce a table using Elliott 803 ALGOL. The following code samples are ALGOL 68 versions of the above ALGOL 60 code samples. ALGOL 68 implementations used ALGOL 60's approaches to stropping . In ALGOL 68's case tokens with the bold typeface are reserved words, types (modes) or operators. Note: lower (⌊) and upper (⌈) bounds of an array, and array slicing, are directly available to

2759-646: 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. The reliance on lists as data structures

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2848-408: Is shared by all Lisp dialects. Scheme inherits a rich set of list-processing primitives such as cons , car and cdr from its Lisp progenitors. Scheme uses strictly but dynamically typed variables and supports first class procedures . Thus, procedures can be assigned as values to variables or passed as arguments to procedures. This section concentrates mainly on innovative features of

2937-488: Is still used for Unisys MCP system software. Imperative programming The term is often used in contrast to declarative programming , which focuses on what the program should accomplish without specifying all the details of how the program should achieve the result. Procedural programming is a type of imperative programming in which the program is built from one or more procedures (also termed subroutines or functions). The terms are often used as synonyms, but

3026-502: Is the lambda expression, because all other variable binding constructs can be explained in terms of lambda expressions." Example: a macro to implement let as an expression using lambda to perform the variable bindings. Thus using let as defined above a Scheme implementation would rewrite " (let ((a 1)(b 2)) (+ b a)) " as " ((lambda (a b) (+ b a)) 1 2) ", which reduces implementation's task to that of coding procedure instantiations. In 1998, Sussman and Steele remarked that

3115-436: Is usually written in the imperative style, although low-level compilers and interpreters using other paradigms exist for some architectures such as lisp machines . From this low-level perspective, the program state is defined by the contents of memory, and the statements are instructions in the native machine language of the computer. Higher-level imperative languages use variables and more complex statements, but still follow

3204-483: The begin ... end pairs for delimiting them. It was also the first language implementing nested function definitions with lexical scope . Moreover, it was the first programming language which gave detailed attention to formal language definition and through the Algol 60 Report introduced Backus–Naur form , a principal formal grammar notation for language design. There were three major specifications, named after

3293-569: The IBM 2741 keyboard with typeball (or golf ball ) print heads inserted (such as the APL golf ball ). These became available in the mid-1960s while ALGOL 68 was being drafted. The report was translated into Russian, German, French, and Bulgarian, and allowed programming in languages with larger character sets, e.g., Cyrillic alphabet of the Soviet BESM -4. All ALGOL's characters are also part of

3382-587: The MIT Computer Science and Artificial Intelligence Laboratory (MIT CSAIL) 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

3471-539: The Unicode standard and most of them are available in several popular fonts . 2009 October: Unicode – The ⏨ (Decimal Exponent Symbol) for floating point notation was added to Unicode 5.2 for backward compatibility with historic Buran programme ALGOL software. To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60. The Burroughs dialects included special Bootstrapping dialects such as ESPOL and NEWP . The latter

3560-606: The \ (Back slash) character added to it in order to support ALGOL's Boolean operators /\ and \/ . 1962: ALCOR – This character set included the unusual "᛭" runic cross character for multiplication and the "⏨" Decimal Exponent Symbol for floating point notation. 1964: GOST – The 1964 Soviet standard GOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL. 1968: The "Algol 68 Report" – used extant ALGOL characters, and further adopted →, ↓, ↑, □, ⌊, ⌈, ⎩, ⎧, ○, ⊥, and ¢ characters which can be found on

3649-437: The maintainability and overall quality of imperative programs. The concepts behind object-oriented programming attempt to extend this approach. Procedural programming could be considered a step toward declarative programming. A programmer can often tell, simply by looking at the names, arguments, and return types of procedures (and related comments), what a particular procedure is supposed to do, without necessarily looking at

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3738-420: The named let form provide support for iteration using tail recursion. Continuations in Scheme are first-class objects . Scheme provides the procedure call-with-current-continuation (also known as call/cc ) to capture the current continuation by packing it up as an escape procedure bound to a formal argument in a procedure provided by the programmer. (R5RS sec. 6.4) First-class continuations enable

3827-401: The " man or boy test " to separate compilers that correctly implemented " recursion and non-local references." This test contains an example of call-by-name. ALGOL 68 was defined using a two-level grammar formalism invented by Adriaan van Wijngaarden and which bears his name. Van Wijngaarden grammars use a context-free grammar to generate an infinite set of productions that will recognize

3916-405: The "named let" form, has an identifier after the let keyword. This binds the let variables to the argument of a procedure whose name is the given identifier and whose body is the body of the let form. The body may be repeated as desired by calling the procedure. The named let is widely used to implement iteration. Example: a simple counter Like any procedure in Scheme, the procedure created in

4005-560: The 20th century saw the development of many such languages. Smalltalk -80, originally conceived by Alan Kay in 1969, was released in 1980, by the Xerox Palo Alto Research Center ( PARC ). Drawing from concepts in another object-oriented language— Simula (which is considered the world's first object-oriented programming language , developed in the 1960s)— Bjarne Stroustrup designed C++ , an object-oriented language based on C . Design of C++ began in 1979 and

4094-702: The ALGOL Bulletin I was drawn into the international discussions of the language and was selected to be member of the European language design group in November 1959. In this capacity I was the editor of the ALGOL 60 report, produced as the result of the ALGOL 60 meeting in Paris in January 1960." The following people attended the meeting in Paris (from 11 to 16 January): Alan Perlis gave a vivid description of

4183-500: The ALGOLs were defined so that only uppercase letters were required. 1960: IFIP – The Algol 60 language and report included several mathematical symbols which are available on modern computers and operating systems, but, unfortunately, were unsupported on most computing systems at the time. For instance: ×, ÷, ≤, ≥, ≠, ¬, ∨, ∧, ⊂, ≡, ␣ and ⏨. 1961 September: ASCII – The ASCII character set, then in an early stage of development, had

4272-523: The Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions. Microsoft's Visual Basic is still widely used and produces a graphical user interface . C programming language (1973) got its name because the language BCPL was replaced with B , and AT&T Bell Labs called the next version "C." Its purpose was to write the UNIX operating system . C

4361-419: The common call-by-value , and call-by-name . Call-by-name has certain effects in contrast to call-by-reference . For example, without specifying the parameters as value or reference , it is impossible to develop a procedure that will swap the values of two parameters if the actual parameters that are passed in are an integer variable and an array that is indexed by that same integer variable. Think of passing

4450-487: The concept of the lexical closure (on page 21), which had been described in an AI Memo in 1970 by Joel Moses , who attributed the idea to Peter J. Landin . 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

4539-443: The creation of complex programs. FORTRAN , developed by John Backus at International Business Machines (IBM) starting in 1954, was the first major programming language to remove the obstacles presented by machine code in the creation of complex programs. FORTRAN was a compiled language that allowed named variables, complex expressions, subprograms, and many other features now common in imperative languages. The next two decades saw

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4628-410: The details of how it achieves its result. At the same time, a complete program is still imperative since it fixes the statements to be executed and their order of execution to a large extent. The programming paradigm used to build programs for almost all computers typically follows an imperative model. Digital computer hardware is designed to execute machine code , which is native to the computer and

4717-423: The development of many other major high-level imperative programming languages. In the late 1950s and 1960s, ALGOL was developed in order to allow mathematical algorithms to be more easily expressed and even served as the operating system 's target language for some computers. MUMPS (1966) carried the imperative paradigm to a logical extreme, by not having any statements at all, relying purely on commands, even to

4806-433: The dominant language paradigm by the late 1990s. C++ (1985) was originally called "C with Classes." It was designed to expand C's capabilities by adding the object-oriented facilities of the language Simula . An object-oriented module is composed of two files. The definitions file is called the header file . Here is a C++ header file for the GRADE class in a simple school application: A constructor operation

4895-503: The exactness of a number. inexact->exact produces "the exact number that is numerically closest to the argument". exact->inexact produces "the inexact number that is numerically closest to the argument". The R6RS standard omits these procedures from the main report, but specifies them as R5RS compatibility procedures in the standard library (rnrs r5rs (6)). In the R5RS standard, Scheme implementations are not required to implement

4984-472: The extent of making the IF and ELSE commands independent of each other, connected only by an intrinsic variable named $ TEST. COBOL (1960) and BASIC (1964) were both attempts to make programming syntax look more like English. In the 1970s, Pascal was developed by Niklaus Wirth , and C was created by Dennis Ritchie while he was working at Bell Laboratories . Wirth went on to design Modula-2 and Oberon . For

5073-413: The fewest clock cycles to store. The stack is automatically used for the standard variable declarations . Heap memory is returned to a pointer variable from the malloc() function. In the 1970s, software engineers needed language support to break large projects down into modules . One obvious feature was to decompose large projects physically into separate files . A less obvious feature

5162-643: The first implementation was completed in 1983. In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were Perl , released by Larry Wall in 1987; Python , released by Guido van Rossum in 1990; Visual Basic and Visual C++ (which included Microsoft Foundation Class Library (MFC) 2.0), released by Microsoft in 1991 and 1993 respectively; PHP , released by Rasmus Lerdorf in 1994; Java , by James Gosling ( Sun Microsystems ) in 1995, JavaScript , by Brendan Eich ( Netscape ), and Ruby , by Yukihiro "Matz" Matsumoto, both released in 1995. Microsoft's .NET Framework (2002)

5251-473: The first of the Lambda Papers, and in subsequent papers, they proceeded to demonstrate the raw power of this practical use of lambda calculus. Scheme inherits its block structure from earlier block structured languages, particularly ALGOL . In Scheme, blocks are implemented by three binding constructs : let , let* and letrec . For instance, the following construct creates a block in which

5340-456: The goal of producing an R6RS standard in 2006. This process broke with the earlier R n RS approach of unanimity. R6RS features a standard module system, allowing a split between the core language and libraries . Several drafts of the R6RS specification were released, the final version being R5.97RS. A successful vote resulted in ratifying the new standard, announced on August 28, 2007. Currently

5429-461: The introduction of subroutines , enabled complex structures to be expressed by hierarchical decomposition into simpler procedural structures. Many imperative programming languages (such as Fortran , BASIC , and C ) are abstractions of assembly language . The earliest imperative languages were the machine languages of the original computers. In these languages, instructions were very simple, which made hardware implementation easier but hindered

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5518-555: The lack of interest in the language by large computer vendors (other than Burroughs Corporation ). ALGOL 60 did however become the standard for the publication of algorithms and had a profound effect on future language development. John Backus developed the Backus normal form method of describing programming languages specifically for ALGOL 58. It was revised and expanded by Peter Naur for ALGOL 60, and at Donald Knuth 's suggestion renamed Backus–Naur form . Peter Naur: "As editor of

5607-466: The lambda calculus because of their treatment of free variables . A formal lambda system has axioms and a complete calculation rule. It is helpful for the analysis using mathematical logic and tools. In this system, calculation can be seen as a directional deduction. The syntax of lambda calculus follows the recursive expressions from x, y, z, ...,parentheses, spaces, the period and the symbol λ. The function of lambda calculation includes: First, serve as

5696-497: The language, including those features that distinguish Scheme from other Lisps. Unless stated otherwise, descriptions of features relate to the R5RS standard. In examples provided in this section, the notation "===> result" is used to indicate the result of evaluating the expression on the immediately preceding line. This is the same convention used in R5RS. Scheme is a very simple language, much easier to implement than many other languages of comparable expressive power . This ease

5785-524: The meeting: "The meetings were exhausting, interminable, and exhilarating. One became aggravated when one's good ideas were discarded along with the bad ones of others. Nevertheless, diligence persisted during the entire period. The chemistry of the 13 was excellent." A significant contribution of the ALGOL 58 Report was to provide standard terms for programming concepts: statement, declaration, type, label, primary, block, and others. ALGOL 60 inspired many languages that followed it. Tony Hoare remarked: "Here

5874-474: 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

5963-405: The minimalism of Scheme was not a conscious design goal, but rather the unintended outcome of the design process. "We were actually trying to build something complicated and discovered, serendipitously, that we had accidentally designed something that met all our goals but was much simpler than we had intended....we realized that the lambda calculus—a small, simple formalism—could serve as the core of

6052-550: The named let is a first-class object. Scheme has an iteration construct, do , but it is more idiomatic in Scheme to use tail recursion to express iteration . Standard-conforming Scheme implementations are required to optimize tail calls so as to support an unbounded number of active tail calls (R5RS sec. 3.5) —a property the Scheme report describes as proper tail recursion —making it safe for Scheme programmers to write iterative algorithms using recursive structures, which are sometimes more intuitive. Tail recursive procedures and

6141-542: The needs of the United States Department of Defense , Jean Ichbiah and a team at Honeywell began designing Ada in 1978, after a 4-year project to define the requirements for the language. The specification was first published in 1983, with revisions in 1995, 2005, and 2012. The 1980s saw a rapid growth in interest in object-oriented programming . These languages were imperative in style, but added features to support objects . The last two decades of

6230-411: 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

6319-417: The processing costs associated with the primitive textual substitution methods used to implement lexical scoping algorithms in compilers and interpreters of the day. In those Lisps, it was perfectly possible for a reference to a free variable inside a procedure to refer to quite distinct bindings external to the procedure, depending on the context of the call. The impetus to incorporate lexical scoping, which

6408-408: The programmer to create non-local control constructs such as iterators , coroutines , and backtracking . Continuations can be used to emulate the behavior of return statements in imperative programming languages. The following function find-first , given function func and list lst , returns the first element x in lst such that (func x) returns true. The following example,

6497-517: The programmer. The variations and lack of portability of the programs from one implementation to another is easily demonstrated by the classic hello world program . ALGOL 58 had no I/O facilities. Since ALGOL 60 had no I/O facilities, there is no portable hello world program in ALGOL. The next three examples are in Burroughs Extended Algol. The first two direct output at the interactive terminal they are run on. The first uses

6586-406: The quality of exactness. An exact number can only be produced by a sequence of exact operations involving other exact numbers—inexactness is thus contagious. The standard specifies that any two implementations must produce equivalent results for all operations resulting in exact numbers. The R5RS standard specifies procedures exact->inexact and inexact->exact which can be used to change

6675-438: The results in memory for later use. High-level imperative languages, in addition, permit the evaluation of complex expressions , which may consist of a combination of arithmetic operations and function evaluations, and the assignment of the resulting value to memory. Looping statements (as in while loops , do while loops , and for loops ) allow a sequence of statements to be executed multiple times. Loops can either execute

6764-520: The same paradigm. Recipes and process checklists , while not computer programs , are also familiar concepts that are similar in style to imperative programming; each step is an instruction, and the physical world holds the state. Since the basic ideas of imperative programming are both conceptually familiar and directly embodied in the hardware, most computer languages are in the imperative style. Assignment statements , in imperative paradigm, perform an operation on information located in memory and store

6853-460: The sense that the syntax of most modern languages is "Algol-like", it was arguably more influential than three other high-level programming languages among which it was roughly contemporary: FORTRAN , Lisp , and COBOL . It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages, including PL/I , Simula , BCPL , B , Pascal , Ada , and C . ALGOL introduced code blocks and

6942-446: The statements they contain a predefined number of times, or they can execute them repeatedly until some condition is met. Conditional branching statements allow a sequence of statements to be executed only if some condition is met. Otherwise, the statements are skipped and the execution sequence continues from the statement following them. Unconditional branching statements allow an execution sequence to be transferred to another part of

7031-415: The use of procedures has a dramatic effect on how imperative programs appear and how they are constructed. Heavy procedural programming, in which state changes are localized to procedures or restricted to explicit arguments and returns from procedures, is a form of structured programming . Since the 1960s, structured programming and modular programming in general have been promoted as techniques to improve

7120-793: The whole numerical tower, but they must implement "a coherent subset consistent with both the purposes of the implementation and the spirit of the Scheme language" (R5RS sec. 6.2.3). The new R6RS standard does require implementation of the whole tower, and "exact integer objects and exact rational number objects of practically unlimited size and precision, and to implement certain procedures...so they always return exact results when given exact arguments" (R6RS sec. 3.4, sec. 11.7.1). Example 1: exact arithmetic in an implementation that supports exact rational complex numbers. Example 2: Same arithmetic in an implementation that supports neither exact rational numbers nor complex numbers but does accept real numbers in rational notation. Both implementations conform to

7209-405: The working groups' charters, public discussions and issue tracking system. The ninth draft of R7RS (small language) was made available on April 15, 2013. A vote ratifying this draft closed on May 20, 2013, and the final report has been available since August 6, 2013, describing "the 'small' language of that effort: therefore it cannot be considered in isolation as the successor to R6RS". Scheme

7298-551: The years they were first published: ALGOL 68 is substantially different from ALGOL 60 and was not well received, so reference to "Algol" is generally understood to mean ALGOL 60 and its dialects. ALGOL was developed jointly by a committee of European and American computer scientists in a meeting in 1958 at the Swiss Federal Institute of Technology in Zurich (cf. ALGOL 58 ). It specified three different syntaxes:

7387-430: The years which have given Scheme a particular character, but are not the direct outcomes of the original design. Scheme specifies a comparatively full set of numerical datatypes including complex and rational types, which is known in Scheme as the numerical tower (R5RS sec. 6.2 ). The standard treats these as abstractions, and does not commit the implementor to any particular internal representations. Numbers may have

7476-496: 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 Institute of Electrical and Electronics Engineers (IEEE) standard and a de facto standard called the Revised Report on the Algorithmic Language Scheme (R n RS). A widely implemented standard

7565-533: Was an unusual scoping model in the early 1970s, into their new version of Lisp, came from Sussman's studies of ALGOL . He suggested that ALGOL-like lexical scoping mechanisms would help to realize their initial goal of implementing Hewitt's Actor model in Lisp. The key insights on how to introduce lexical scoping into a Lisp dialect were popularized in Sussman and Steele's 1975 Lambda Paper, "Scheme: An Interpreter for Extended Lambda Calculus", where they adopted

7654-438: Was developed at Dartmouth College for all of their students to learn. If a student did not go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the microcomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language. Basic pioneered the interactive session . It offered operating system commands within its environment: However,

7743-469: 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

7832-430: Was to a line printer. The open and close quote characters were represented using '(' and ')' and spaces by %. ALGOL 68 code was published with reserved words typically in lowercase, but bolded or underlined. In the language of the "Algol 68 Report" the input/output facilities were collectively called the "Transput". The ALGOLs were conceived at a time when character sets were diverse and evolving rapidly; also,

7921-519: Was to decompose large projects logically into abstract datatypes . At the time, languages supported concrete ( scalar ) datatypes like integer numbers, floating-point numbers, and strings of characters . Concrete datatypes have their representation as part of their name. Abstract datatypes are structures of concrete datatypes — with a new name assigned. For example, a list of integers could be called integer_list . In object-oriented jargon, abstract datatypes are called classes . However,

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