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Report on the Algorithmic Language ALGOL 60

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Report on the Algorithmic Language ALGOL 60 Reprinted from the COMMUNICATIONS OF THE ASSOCIATION FOR COMPUTING MACHINERY Vol. 3, No.5, May 1960 Made in U.S.A. With tYPographical corrections as of June ~8, 1980 Report on the Algorithmic Language ALGOL 60 PETER NAUR (Editor) J. W. BACKUS C. KATZ H. RUTISHAUSER J. H. WEGSTEIN F.L.BAUER J. MCCARTHY K. SAMELSON A. VAN WIJNGAARDEN J. GREEN A. J. PERLIS B. VAUQUOIS M. WOODGER Dedicated to the Memory of WILLIAM TURANSKI INTRODUCTION Background Meanwhile, in the United States, anyone who wished to After the publication of a preliminary report on the suggest changes or corrections to ALGOL was requested to send his comments to the ACM Communications where algorithmic language ALGOL,!' 2 as prepared at a conference in Zurich in 1958, much interest in the ALGOL language they were published. These comments then became the developed. basis of consideration for changes in the ALGOL language. As a result of an informal meeting held at Mainz in Both the SHARE and USE organizations established November 1958, about forty interested persons from ALGOL working groups, and both organizations were several European countries held an ALGOL implementation represented on the ACM Committee on Programming conference in Copenhagen in February 1959. A "hardware Languages. The ACM Committee met in Washington in group" was formed for working cooperatively right down November 1959 and considered all comments on ALGOL to the level of the paper tape code. This conference also that had been sent to the ACM Communications. Also, led to the publication by Regnecentralen, Copenhagen, of seven representatives were selected to attend the January an ALGOL Bulletin, edited by Peter Naur, which served 1960 international conference. These seven representa­ as a forum for further discussion. During the June 1959 tives held a final preparatory meeting in Boston in Decem­ ICIP Conference in Paris several meetings, both formal ber 1959. and informal ones, were held. These meetings revealed January 1960 Conference some misunderstandings as to the intent of the group The thirteen representatives," from Denmark, England, which was primarily responsible for the formulation of the France, Germany, Holland, Switzerland, and the United language, but at the same time made it clear that there States, conferred in Paris from January 11 to 16, 1960. exists a wide appreciation of the effort involved. As a re­ Prior to this meeting a completely new draft report was sult of the discussions it was decided to hold an inter­ worked out from the preliminary report and the recom­ national meeting in January 1960 for improving the mendations of the preparatory meetings by Peter Naur ALGOL language and preparing a final report. At a Euro­ and the conference adopted this new form as the basis pean ALGOL Conference in Paris in November 1959 which for its report. The Conference then proceeded to work for was attended by about fifty people, seven European agreement on each item of the report. The present report representatives were selected-to attend the January 1960 represents the union of the Committee's concepts and the Conference, and they represent the following organiza­ intersection of its agreements. tions: Association Francaise de Calcul, British Computer Society, Gesellschaft fur Angewandte Mathematik und As with the preliminary ALGOL report, three different . Mechanik, and Nederlands Rekenmachine Genootschap. levels of language are recognized, namely a Reference The seven representatives held a final preparatory meeting Language, a Publication Language and several Hardware at Mainz in December 1959. Representations. I Preliminary report-International Algebraic Language, REFERENCE LANGUAGE Comm. A8soc. Camp. Mach. 1, No. 12 (1958),8. 1. It is the working language of the committee. 2 Report on the Algorithmic Language ALGOL by the ACM 2. It is the defining language. Committee on Programming Languages and the GAMM Com­ mittee on Programming, edited by A. J. Perlis and K. Samelson, S William Turanski of the American group was killed by an Numerische Mathematik Bd. 1, S. 41-60 (1959). automobile just prior to the January 1960 Conference. Communications of the ACM 299 3. The characters are determined by ease of mutual HARDWARE REPRESENTATIONS understanding and not by any computer limitations, coders 1. Each one of these is a condensation of the reference notation, or pure mathematical notation. language enforced by the limited number of characters on 4. It is the basic reference and guide for compiler standard input equipment. builders. 2. Each one of these uses the character set of a particu­ 5. It is the guide for all hardware representations. lar computer and is the language accepted by a translator 6. It is the guide for transliterating from publication for that computer. language to any locally appropriate hardware representa­ 3. Each one of these must be accompanied by a special tions. set of rules for transliterating from Publication or Refer­ 7. The main publications of the ALGOL language itself ence language. will use the reference representation. For transliteration between the reference language and PUBLICATION LANGUAGE a language suitable for publications, among others, the following rules are recommended. 1. The publication language admits variations of the reference language according to usage of printing and Reference Language Publication Language Subscript bracket [] Lowering of the line between the handwriting (e.g., subscripts, spaces, exponents, Greek brackets and removal of the brackets letters). Exponentation i Raising of the exponent 2. It is used for stating and communicating processes. Parentheses () Any form of parentheses, brackets, 3. The characters to be used may be different in differ­ braces Basis of ten 10 Raising of the ten and of the following ent countries, but univocal correspondence. with reference integral number, inserting of the representation must be secured. intended multiplication sign .. DESCRIPTION OF THE REFERENCE LANGUAGE WaB sieh liberhaupt sagen lasst, IIlsBt sich klar sagen; und wovon man nicht reden kann, daruber mUBB man sehweigen. LUDWIG WITTGENBTEIN. 1. Structure of the Language Sequences of statements may be combined into compound As stated in the introduction, the algorithmic language statements by insertion of statement brackets. has three different kinds of representations-reference, Statements are supported by declarations which are not hardware, and publication-and the development de­ themselves computing instructions, but inform the trans­ scribed in the sequel is in terms of the reference representa­ lator of the existence and of certain properties of objects tion. This means that all objects defined within the appearing in statements, such as the class of numbers language are represented by a given set of symbols-and taken on as values by a variable, the dimension of an it is only in the choice of symbols that the other two array of numbers, or even the set of rules defining a func­ representations may differ. Structure and content must tion. Each declaration is attached to and valid for one be the same for all representations. compound statement. A compound statement which in­ The purpose of the algorithmic language is to describe cludes declarations is called a block. computational processes. The basic concept used for the A program is a self-contained compound statement, i.e, description of calculating rules is the well-known arithmetic a compound statement which is not contained within expression containing as constituents numbers, variables, another compound statement and which makes no use of and functions. From such expressions are compounded, other compoundstatements not contained within it. by applying rules of arithmetic composition, self-con­ In the sequel the syntax and semantics of the language 4 tained units of the language-explicit formulae-called will be given. assignment statements. To show the flow of computational processes, certain 4 Whenever the precision of arithmetic is stated as being in nonarithmetic statements and statement clauses are general not specified, or the outcome of a certain process is said to added which may describe, e.g., alternatives, or iterative be undefined, this is to be interpreted in the sense that a program only fully defines a computational process if the accompanying repetitions of computing statements. Since it is necessary information specifies the precision assumed, the kind of arithmetic for the function of these statements that one statement assumed, and the course of action to be taken in all such cases as refer to another, statements may be provided with labels. may occur during the execution of the computation. 300 Communications of the ACM 1.1. FORMALISM FOR SYNTACTIC DESCRIPTION Letters do not have individual meaning. They are used The syntax will be described with the aid of metalinguis­ for forming identifiers and strings" (cf. sections 2.4. tic formulae." Their interpretation is best explained by an Identifiers, 2.6. Strings). example 2.2.1. DIGITS (ab) ::= (I [I (ab) (I (ab)(d) (digit) ::= 0111213141516171819 Sequences of characters enclosed in the brackets ( ) repre­ Digits are used for forming numbers, identifiers, and sent metalinguistic variables whose values are sequences strings. of symbols. The marks :: = and I (the latter with the meaning of or) are metalinguistic connectives. Any mark 2.2.2. LOGICAL VALUES in a formula, which is not a variable or a connective, (logical value) :: = truelfalse denotes itself (or the class of marks which are similar
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