ALGOL

Related Topics:
Algol_60 :: Algol_68

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, ALGOL 60, and ALGOL 68. Of these, ALGOL 60 was the most widely known in the United States. 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 58 was originally known as the IAL (for International Algorithmic Language.)

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. 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.

John Backus developed the Backus normal form method of describing programming languages specifically for ALGOL 58. It was revised and expanded by Peter Naur to the Backus-Naur form for ALGOL 60. 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 an early ALGOL 60 compiler.)

True ALGOL 60s specification and implementation timeline

There were about 70 augmented, extensions, derivations and sublanguages of Algol 60^*

Name	Year	Author	State	Description	Target CPU
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	Dijkstra	Netherlands
Dartmouth ALGOL 30	1962	Thomas Eugene Kurtz et al	US		LGP-30
USS 90 Algol	1962	L. Petrone	Italy
Kidsgrove Algol	1963	F. G. Duncan			English Electric KDF9
VALGOL	1963	Val Schorre	US	A test of the META II compiler compiler
NU ALGOL	1965		Norway		UNIVAC
MALGOL	1966	A. Viil Kotli & M. Rakhendi	Russian Federation
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 Corporation's B5000 and its successors were 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 descended from the B5000 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 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.

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(aq) > y then begin y := abs(aq); 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.

Hello world

Since ALGOL 60 had no I/O facilities, there is no portable "Hello world" program in ALGOL. The following code could run on an ALGOL implementation for a Burroughs A-Series mainframe, and is taken from this site.

BEGIN FILE F (KIND=REMOTE); EBCDIC ARRAY E ^*; REPLACE E BY "HELLO WORLD!"; WHILE TRUE DO BEGIN WRITE (F, *, E); END; END.

An alternative example, using Elliott Algol I/O is as follows. In fact, Elliott Algol used different characters for 'open-string-quote' and 'close-string-quote', but ASCII does not allow these to be shown here.

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 sequencies were placed in double quotes e.g. ££L?? produced a new line on the teleprinter.

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

Reserved words and restricted identifiers

There are 35 such reserved words in the standard Burroughs B6700 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 B6700 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.

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
"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).
Some Algol 60 source code, both in Machine Syntax and Reference Syntax.

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