ALGOL 60

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, representing a key advance in the rise of structured programming. 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. Practically every computer of the era had a systems programming language based on ALGOL 60 concepts.

This article is about the programming language. For other uses, see Algol (disambiguation).
ALGOL 60
Paradigmsprocedural, imperative, structured
FamilyALGOL
Designed byBackus, Bauer, Green, Katz, McCarthy, Naur, Perlis, Rutishauser, Samelson, van Wijngaarden, Vauquois, Wegstein, Woodger
First appeared1960 (1960)
Typing disciplineStatic, strong
ScopeLexical
Influenced by
ALGOL 58
Influenced
Most subsequent imperative languages (so-called ALGOL-like languages), e.g. Simula, CPL, Pascal, Ada, 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 (now withdrawn)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.[1]

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."[2]

The following people attended the meeting in Paris (from January 11 to 16):

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."

The language originally did not include recursion. It was inserted into the specification at the last minute, against the wishes of some of the committee.[3]

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."[4][5]

ALGOL 60 implementations timeline

To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.[6]

Name Year Author State Description Target CPU
X1 ALGOL 60August 1960[7]Edsger W. Dijkstra and Jaap A. ZonneveldNetherlandsFirst implementation of ALGOL 60[8]Electrologica X1
Algol1960[9]Edgar T. IronsUSAAlgol 60CDC 1604
Burroughs Algol
(Several variants)		1961Burroughs Corporation (with participation by Hoare, Dijkstra, and others)USABasis of the Burroughs (and now Unisys MCP based) computersBurroughs large systems
and their midrange as well.
Case ALGOL1961USASimula was originally contracted as a simulation extension of the Case ALGOLUNIVAC 1107
GOGOL1961William M. McKeemanUSAFor ODIN time-sharing systemPDP-1
DASK ALGOL1961Peter Naur, Jørn JensenDenmarkAlgol 60DASK at Regnecentralen
SMIL ALGOL1962Torgil Ekman, Carl-Erik FröbergSwedenAlgol 60SMIL at Lund University
GIER ALGOL1962Peter Naur, Jørn JensenDenmarkAlgol 60GIER at Regnecentralen
Dartmouth ALGOL 301962Thomas Eugene Kurtz et al.USALGP-30
Alcor Mainz 20021962Ursula Hill-Samelson, Hans LangmaackGermanySiemens 2002
USS 90 Algol1962L. PetroneItaly
Elliott ALGOL1962C. A. R. HoareUKDiscussed in his 1980 Turing Award lectureElliott 803 & the Elliott 503
Algol 601962Roland Strobel[10]East GermanyImplemented by the Institute for Applied Mathematics, German Academy of Sciences at BerlinZeiss-Rechenautomat ZRA 1
Algol Translator1962G. van der Mey and W.L. van der PoelNetherlandsStaatsbedrijf der Posterijen, Telegrafie en TelefonieZEBRA
Kidsgrove Algol1963F. G. DuncanUKEnglish Electric Company KDF9
VALGOL1963Val SchorreUSAA test of the META II compiler compiler
FP6000 Algol1963Roger MooreCanadawritten for Saskatchewan Power CorpFP6000
Whetstone1964Brian Randell and Lawford John RussellUKAtomic Power Division of English Electric Company. Precursor to Ferranti Pegasus, National Physical Laboratories ACE and English Electric DEUCE implementations.English Electric Company KDF9
NU ALGOL1965NorwayUNIVAC
Algol 601965[11]F.E.J. Kruseman AretzNetherlandsMC compiler for the EL-X8Electrologica X8
ALGEK1965USSRMinsk-22АЛГЭК, based on ALGOL-60 and COBOL support, for economical tasks
MALGOL1966publ. A. Viil, M Kotli & M. Rakhendi,Estonian SSRMinsk-22
ALGAMS1967GAMS group (ГАМС, группа автоматизации программирования для машин среднего класса), cooperation of Comecon Academies of ScienceComeconMinsk-22, later ES EVM, BESM
ALGOL/ZAM1967PolandPolish ZAM computer
Chinese Algol1972ChinaChinese characters, expressed via the Symbol system
DG/L1972USADG Eclipse family of Computers
NASE1990Erik SchoenfelderGermanyInterpreterLinux and MS Windows
MARST2000Andrew MakhorinRussiaAlgol-60 to C translatorAll CPUs supported by the GNU Compiler Collection; MARST is part of the GNU project

The Burroughs dialects included special system programming 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 provided two evaluation strategies for parameter passing: the common call-by-value, and call-by-name. The procedure declaration specified, for each formal parameter, which was to be used: value specified for call-by-value, and omitted for 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.[12] 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

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:[13]

 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[14] at the University of Michigan-Dearborn Computer and Information Science Department Hello world! ALGOL Example Program page.[15]

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 £ (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 %.[16]

  '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

References
  1. Knuth, Donald E. (December 1964). "Backus normal Form vs Backus Naur Form". Comm. ACM. 7 (12): 735–6. doi:10.1145/355588.365140. S2CID 47537431.
  2. ACM Award Citation / Peter Naur, 2005
  3. van Emden, Maarten (2014). "How recursion got into programming: a tale of intrigue, betrayal, and advanced programming-language semantics". A Programmer's Place.
  4. Hoare, C.A.R. (December 1973). "Hints on Programming Language Design" (PDF). p. 27. (This statement is sometimes erroneously attributed to Edsger W. Dijkstra, also involved in implementing the first ALGOL 60 compiler.)
  5. Abelson, Hal; Dybvig, R. K.; et al. Rees, Jonathan; Clinger, William (eds.). "Revised(3) Report on the Algorithmic Language Scheme, (Dedicated to the Memory of ALGOL 60)". Retrieved 2009-10-20.
  6. The Encyclopedia of Computer Languages Archived September 27, 2011, at the Wayback Machine
  7. Daylight, E. G. (2011). "Dijkstra's Rallying Cry for Generalization: the Advent of the Recursive Procedure, late 1950s – early 1960s". The Computer Journal. 54 (11): 1756–1772. doi:10.1093/comjnl/bxr002.
  8. Kruseman Aretz, F.E.J. (30 June 2003). "The Dijkstra-Zonneveld ALGOL 60 compiler for the Electrologica X1" (PDF). Software Engineering. History of Computer Science. Kruislaan 413, 1098 SJ Amsterdam: Centrum Wiskunde & Informatica.CS1 maint: location (link)
  9. Irons, Edgar T., A syntax directed compiler for ALGOL 60, Communications of the ACM, Vol. 4, p. 51. (Jan. 1961)
  10. Rechenautomaten mit Trommelspeicher, Förderverein der Technischen Sammlung Dresden
  11. Kruseman Aretz, F.E.J. (1973). An Algol 60 compiler in Algol 60. Mathematical Centre Tracts. Amsterdam: Mathematisch Centrum.
  12. Aho, Alfred V.; Ravi Sethi; Jeffrey D. Ullman (1986). Compilers: Principles, Techniques, and Tools (1st ed.). Addison-Wesley. ISBN 978-0-201-10194-2., Section 7.5, and references therein
  13. "803 ALGOL", the manual for Elliott 803 ALGOL
  14. https://web.archive.org/web/20100209040840/http://www.engin.umd.umich.edu/CIS/course.des/cis400/index.html
  15. https://web.archive.org/web/20100204112923/http://www.engin.umd.umich.edu/CIS/course.des/cis400/algol/hworld.html
  16. "ICL 1900 series: Algol Language". ICL Technical Publication 3340. 1965.

Further reading
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