ALGOL: Difference between revisions

For other uses of "Algol", see Algol (disambiguation).

ALGOL (short for ALGOrithmic Language) is a family of imperative computer programming languages originally developed in the mid 1950s which became the de facto standard way to report algorithms in print for almost the next 30 years. It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages (including Pascal). ALGOL uses bracketed statement blocks and was the first language to use begin end pairs for delimiting them. Fragments of ALGOL-like syntax are sometimes still used as a notation for algorithms, so-called Pidgin Algol.

There are three official main branches of ALGOL family:

• ALGOL 58 - originally known as the IAL (for International Algorithmic Language.)
• ALGOL 60 - revised 1963
• ALGOL 68 - revised 1973

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.

Note: Throughout its effective life, the name of the programming language ALGOL was always presented in all-uppercase letters, and this is the practice adopted here.

History

ALGOL was developed jointly by a committee of European and American computer scientists in a meeting in 1958 at ETH Zurich. It specified three different syntaxes: a reference syntax, a publication syntax, and an implementation syntax. The different syntaxes 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. 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 the suggestion by Donald Knuth renamed to 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:

• 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.”
Both John Backus and Peter Naur served on the committee which created ALGOL 60, as did Wally Feurzeig who later created Logo.

ALGOL 60 inspired many languages that followed it; C.A.R. Hoare’s original quote on this is recalled in the aphorism: “ALGOL 60 was a great improvement on its successors.” (This is sometimes erroneously attributed to Edsger Dijkstra, also known for his pointed comments, who helped to implement the first ALGOL 60 compiler.)

True ALGOL 60s specification and implementation timeline

There were about 70 augmented, extensions, derivations and sublanguages of Algol 60[1]

Name	Year	Author	State	Description	Target CPU	Licencing
Elliott ALGOL	1960	C. A. R. Hoare	UK	Subject of the famous Turing lecture	National-Elliott 803 & the Elliott 503
Case ALGOL	1961		US	Simula was originally contracted as a simulation extension of the Case ALGOL	UNIVAC 1107
EMIDEC Algol	1961		US		EMIDEC
GOGOL	1961	Bill McKeeman	US	For ODIN time-sharing system	PDP-1
X1 Algol 60	1961	Edsger Dijkstra and J.A. Zonneveld	Netherlands	Mathematical Centre, Amsterdam	X1
Dartmouth ALGOL 30	1962	Thomas Eugene Kurtz et al	US		LGP-30
USS 90 Algol	1962	L. Petrone	Italy
Algol Translator	1962	G. van der May and W.L. van der Poel	Netherlands	Staatsbedrijf der Postrijen, Telegrafie en Telefonie	ZEBRA
Kidsgrove Algol	1963	F. G. Duncan	UK		English Electric KDF9
VALGOL	1963	Val Schorre	US	A test of the META II compiler compiler
Whetstone	1964	Brian Randell and L J Russell	UK	Atomic Power Division of English Electric. Precursor to Ferranti Pegasus (computer), National Physical Laboratories ACE (computer) and English Electric DEUCE implementations.	English Electric 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
RegneCentralen ALGOL	1967	Peter Naur	Denmark
DG/L	1973		US		DG Eclipse family of computers
Chinese Algol	1974		China	Chinese characters, expressed via the Symbol system	?

The Burroughs large systems are stack machines designed to be programmed in an extended variant of ALGOL 60, known as Elliott ALGOL; indeed their operating system the MCP, was written in Elliott ALGOL as far back as 1961. The Unisys Corporation still markets machines in this family today, running the MCP and supporting a diverse set of Elliott ALGOL compilers.

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 had certain limitations in contrast to call-by-reference, making it an undesirable feature in imperative language design. For example, it is impossible in ALGOL 60 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. However, call-by-name is still beloved of ALGOL implementors for the interesting “thunks” that are used to implement it. Donald Knuth devised the “Man or boy test” to test was 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 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.

Code sample (ALGOL 60)

(The way the bolded text has to be written depends on the implementation, e.g. 'INTEGER' (including the quotation marks) for integer.)

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

Here’s 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'

PUNCH(3) sends output to the teleprinter rather than the tape punch.
SAMELINE suppresses the carriage return + line feed normally printed between arguments.
ALIGNED(1,6) controls the format of the output with 1 digit before and 6 after the decimal point.

Timeline: Hello world

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 (IAL)

ALGOL 58 had no I/O facilities

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 this site.

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

ALGOL 68

In the language of the "Algol 68 Report", Input/output facilities were collectively called the "Transput".

ALGOL 68 code was published reserved words were typically lowercase, but bolded or underlined.

begin
  print(("Hello, world!",newline))
end

OR using a specific transput channel:

begin
  putf((stand out,$gl$,"Hello, world!"))
end

For ease of programming on the 7-bit computers of the time there were "official" methods to "BOLD" reserved words, for example, by using uppercase:

BEGIN
  print(("Hello, world!",newline))
END

Programmers were sometimes required to totally "THINK IN UPPERCASE" on computers that only had 6-bit characters, eg the CDC "super computers". In this case the above code would be written:

'BEGIN'
  PRINT(("HELLO, WORLD!",NEWLINE))
'END'

The "Algol 68 Report" was translated into Russian, German, French and Bulgarian, and allowed programming in languages with larger character sets, eg Cyrillic alphabet. eg the Russian BESM-4.

BEGIN
  print(("Здравствуй, мир!",newline))
END

Note: The 1964 Russian standard GOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL .

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.

See also

• ALGOL 68
• Jensen's Device
• JOVIAL
• Simula
• Atlas Autocode
• Edinburgh IMP
• CORAL66

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
• “803 ALGOL” The manual for Elliott 803 ALGOL.
• AN IMPLEMENTATION OF ALGOL 60 FOR THE FP6000 Discussion of some implementation issues.
• “Hints on Programming Language Design”, C.A.R. Hoare, December 1973 (source of the quote stating that Algol 60 is better than its successors).
• “The European Side of the Last Phase of the Development of ALGOL 60” by Peter Naur
• 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.

References

• B. Randell and L.J. Russell, ALGOL 60 Implementation: The Translation and Use of ALGOL 60 Programs on a Computer. Academic Press, 1964. 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
• E. W, Dijkstra, Algol 60 translation: an algol 60 translator for the x1 and making a translator for algol 60, report MR 35/61. Mathematisch Centrum, Amsterdam, 1961. [2]