ALGOL 60

This article is about the programming language. For other uses, see Algol (disambiguation).

ALGOL 60

Paradigm	procedural, imperative, structured
Designed by	Backus, Bauer, Green, Katz, McCarthy, Naur, Perlis, Rutishauser, Samelson, van Wijngaarden, Vauquois, Wegstein, Woodger
First appeared	1960
Website	{{#property:P856}}
Influenced by
ALGOL 58
Influenced
Most subsequent imperative languages (so-called ALGOL-like languages), e.g. Simula, CPL, Pascal, Ada, C

ALGOL 60 (short for ALGOrithmic Language 1960) is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them. ALGOL 60 was the first language implementing nested function definitions with lexical scope. It gave rise to many other programming languages, including CPL, Simula, BCPL, B, Pascal and C.

Niklaus Wirth based his own ALGOL W on ALGOL 60 before moving to develop Pascal. Algol-W was intended to be the next generation ALGOL but the ALGOL 68 committee decided on a design that was more complex and advanced rather than a cleaned simplified ALGOL 60. The official ALGOL versions are named after the year they were first published. Algol 68 is substantially different from Algol 60 and was criticised partially for being so, so that in general "Algol" refers to dialects of Algol 60.

Standardization

ALGOL 60 — with COBOL — were the first languages to seek standardization.

• ISO 1538:1984 Programming languages — Algol 60 (stabilized)
• ISO/TR 1672:1977 Hardware representation of ALGOL basic symbols ... (now withdrawn)

History

ALGOL 60 was used mostly by research computer scientists in the United States and in Europe. Its use in commercial applications was hindered by the absence of standard input/output facilities in its description and the lack of interest in the language by large computer vendors. 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 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 January 11 to 16):

• Friedrich L. Bauer, Peter Naur, Heinz Rutishauser, Klaus Samelson, Bernard Vauquois, Adriaan van Wijngaarden, and Michael Woodger (from Europe)
• John W. Backus, Julien Green, Charles Katz, John McCarthy, Alan J. Perlis, and Joseph Henry Wegstein (from the USA).

Alan Perlis gave a vivid description of 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."

ALGOL 60 inspired many languages that followed it. Tony Hoare remarked: "Here 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 implementations timeline

To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.

Name	Year	Author	State	Description	Target CPU
X1 ALGOL 60	August 1960	Edsger W. Dijkstra and Jaap A. Zonneveld	Netherlands	First implementation of ALGOL 60	Electrologica X1
Algol	1960	Edgar T. Irons	USA	Algol 60	CDC 1604
Burroughs Algol (Several variants)	1961	Burroughs Corporation (with participation by Hoare, Dijkstra, and others)	USA	Basis of the Burroughs (and now Unisys MCP based) computers	Burroughs large systems and their midrange as well.
Case ALGOL	1961		USA	Simula was originally contracted as a simulation extension of the Case ALGOL	UNIVAC 1107
GOGOL	1961	William McKeeman	USA	For ODIN time-sharing system	PDP-1
DASK ALGOL	1961	Peter Naur, Jørn Jensen	Denmark	Algol 60	DASK at Regnecentralen
SMIL ALGOL	1962	Torgil Ekman, Carl-Erik Fröberg	Sweden	Algol 60	SMIL at Lund University
GIER ALGOL	1962	Peter Naur, Jørn Jensen	Denmark	Algol 60	GIER at Regnecentralen
Dartmouth ALGOL 30	1962	Thomas Eugene Kurtz et al.	USA		LGP-30
USS 90 Algol	1962	L. Petrone	Italy
Elliott ALGOL	1962	C. A. R. Hoare	UK	Discussed in his 1980 Turing Award lecture	Elliott 803 & the Elliott 503
Algol Translator	1962	G. van der Mey and W.L. van der Poel	Netherlands	Staatsbedrijf der Posterijen, Telegrafie en Telefonie	ZEBRA
Kidsgrove Algol	1963	F. G. Duncan	UK		English Electric Company KDF9
VALGOL	1963	Val Schorre	USA	A test of the META II compiler compiler
FP6000 Algol	1963	Roger Moore	Canada	written for Saskatchewan Power Corp	FP6000
Whetstone	1964	Brian Randell and L J Russell	UK	Atomic Power Division of English Electric Company. Precursor to Ferranti Pegasus, National Physical Laboratories ACE and English Electric DEUCE implementations.	English Electric Company KDF9
NU ALGOL	1965		Norway		UNIVAC
ALGEK	1965		USSR	Minsk-22	АЛГЭК, based on ALGOL-60 and COBOL support, for economical tasks
MALGOL	1966	publ. A. Viil, M Kotli & M. Rakhendi,	Estonian SSR	Minsk-22
ALGAMS	1967	GAMS group (ГАМС, группа автоматизации программирования для машин среднего класса), cooperation of Comecon Academies of Science	Comecon	Minsk-22, later ES EVM, BESM
ALGOL/ZAM	1967		Poland		Polish ZAM computer
Chinese Algol	1972		China	Chinese characters, expressed via the Symbol system
DG/L	1972		USA		DG Eclipse family of Computers
NASE	1990	Erik Schoenfelder	Germany	Interpreter	Linux and MS Windows
MARST	2000	Andrew Makhorin	Russia	Algol-60 to C translator	All CPUs supported by the GNU Compiler Collection; MARST is part of the GNU project

The Burroughs dialects included special Bootstrapping dialects such as ESPOL and NEWP.

Properties

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 (ALGOL 68 parlance for Input/Output) facilities.

ALGOL 60 allowed for two evaluation strategies for parameter passing: 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 a pointer to swap(i, A[i]) in to a function. Now that every time swap is referenced, it's reevaluated. Say i := 1 and A[i] := 2, so every time swap is referenced it'll 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 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 60 Reserved words and restricted identifiers

There are 35 such reserved words in the standard Burroughs large systems sub-language:

ALPHA ARRAY BEGIN BOOLEAN COMMENT CONTINUE DIRECT

DO DOUBLE ELSE END EVENT FALSE FILE

FOR FORMAT GO IF INTEGER LABEL LIST

LONG OWN POINTER PROCEDURE REAL STEP SWITCH

TASK THEN TRUE UNTIL VALUE WHILE ZIP

There are 71 such restricted identifiers in the standard Burroughs large systems sub-language:

ACCEPT AND ATTACH BY CALL CASE CAUSE CLOSE DEALLOCATE DEFINE DETACH DISABLE DISPLAY DIV DUMP

ENABLE EQL EQV EXCHANGE EXTERNAL FILL FORWARD GEQ GTR IMP IN INTERRUPT IS LB

LEQ LIBERATE LINE LOCK LSS MERGE MOD MONITOR MUX NEQ NO NOT ON OPEN

OR OUT PICTURE PROCESS PROCURE PROGRAMDUMP RB READ RELEASE REPLACE RESET RESIZE REWIND RUN

SCAN SEEK SET SKIP SORT SPACE SWAP THRU TIMES TO WAIT WHEN WITH WRITE

and also the names of all the intrinsic functions.

Standard Operators

Priority		Operator
first arithmetic	first	↑ (power)
	second	×, / (real), ÷ (integer)
	third	+, -
second		<, ≤, =, ≥, >, ≠
third		¬ (not)
fourth		∧ (and)
fifth		∨ (or)
sixth		⊃ (implication)
seventh		≡ (equivalence)

Examples and portability issues

Code sample comparisons

ALGOL 60

procedure Absmax(a) Size:(n, m) Result:(y) Subscripts:(i, k);
    value n, m; array a; integer n, m, i, k; real y;
comment The absolute greatest element of the matrix a, of size n by m,
    is transferred to y, and the subscripts of this element to i and k;
begin
    integer p, q;
    y := 0; i := k := 1;
    for p := 1 step 1 until n do
        for q := 1 step 1 until m do
            if abs(a[p, q]) > y then
                begin y := abs(a[p, q]);
                    i := p; k := q
                end
end Absmax

Implementations differ in how the text in bold must be written. The word 'INTEGER', including the quotation marks, must be used in some implementations in place of integer, above, thereby designating it as a special keyword.

Following is an example of how to produce a table using Elliott 803 ALGOL:

 FLOATING POINT ALGOL TEST'
 BEGIN REAL A,B,C,D'

 READ D'

 FOR A:= 0.0 STEP D UNTIL 6.3 DO
 BEGIN
   PRINT PUNCH(3),££L??'
   B := SIN(A)'
   C := COS(A)'
   PRINT PUNCH(3),SAMELINE,ALIGNED(1,6),A,B,C'
 END'
 END'

ALGOL 60 family

Since ALGOL 60 had no I/O facilities, there is no portable hello world program in ALGOL. The following program could (and still will) compile and run on an ALGOL implementation for a Unisys A-Series mainframe, and is a straightforward simplification of code taken from The Language Guide at the University of Michigan-Dearborn Computer and Information Science Department Hello world! ALGOL Example Program page.

BEGIN
  FILE F(KIND=REMOTE);
  EBCDIC ARRAY E[0:11];
  REPLACE E BY "HELLO WORLD!";
  WRITE(F, *, E);
END.

A simpler program using an inline format:

BEGIN
  FILE F(KIND=REMOTE);
  WRITE(F, <"HELLO WORLD!">);
END.

An even simpler program using the Display statement:

BEGIN DISPLAY("HELLO WORLD!") END.

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 ’.

 program HiFolks;
 begin
    print ‘Hello world’
 end;

Here's a version for the Elliott 803 Algol (A104) The standard Elliott 803 used 5 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).

  HIFOLKS'
  BEGIN
     PRINT £HELLO WORLD£L??'
  END'

The ICT 1900 series Algol I/O version allowed input from paper tape or punched card. Paper tape 'full' mode allowed lower case. Output was to a line printer. Note use of '(',')' and %.

  'PROGRAM' (HELLO)
  'BEGIN'
     'COMMENT' OPEN QUOTE IS '(', CLOSE IS ')', PRINTABLE SPACE HAS TO
               BE WRITTEN AS % BECAUSE SPACES ARE IGNORED;
     WRITE TEXT('('HELLO%WORLD')');
  'END'
  'FINISH'

See also

• ABC ALGOL
• ALGOL
• ALGOL 58
• ALGOL N
• ALGOL 68
• ALGOL W

• ALGOL X
• Atlas Autocode
• Coral 66
• Edinburgh IMP
• Jensen's Device
• ISWIM

• JOVIAL
• NELIAC
• Simula
• S-algol
• Scheme (programming language)

Further reading

• The design of the Whetstone Compiler. One of the early published descriptions of implementing a compiler. See the related papers: Whetstone Algol Revisited, and The Whetstone KDF9 Algol Translator by B. Randell

External links

• Revised Report on the Algorithmic Language Algol 60 by Peter Naur, et al. ALGOL definition
• A BNF syntax summary of ALGOL 60
• "The Emperor's Old Clothes" – Hoare's 1980 ACM Turing Award speech, which discusses ALGOL history and his involvement
• MARST, a free Algol-to-C translator
• AN IMPLEMENTATION OF ALGOL 60 FOR THE FP6000 Discussion of some implementation issues.
• Edinburgh University wrote compilers for Algol60 (later updated for Algol60M) based on their Atlas Autocode compilers initially bootstrapped from the Atlas to the KDF-9. The Edinburgh compilers generated code for the ICL1900, the ICL4/75 (an IBM360 clone), and the ICL2900. Here is the BNF for Algol60 and the ICL2900 compiler source, library documentation, and a considerable test suite including Brian Wichmann's tests. Also there is a rather superficial Algol60 to Atlas Autocode source-level translator.
• Eric S. Raymond's Retrocomputing Museum, among others a link to the NASE Algol-60 interpreter written in C.
• The NASE interpreter
• Stories of the B5000 and People Who Were There: a dedicated ALGOL computer [1], [2]
• Structure and Use of ALGOL 60, Hermann Bottenbruch [3]
• NUMAL A Library of Numerical Procedures in ALGOL 60 developed at The Stichting Centrum Wiskunde & Informatica (legal successor of Stichting Mathematisch Centrum) legal owner.
• Algol 60 resources: translators, documentation, programs