| Paradigm(s) | procedural, imperative, structured |
|---|---|
| Appeared in | 1958 |
| Designed by | Bauer, Bottenbruch, Rutishauser, Samelson, Backus, Katz, Perlis, Wegstein, Naur, Vauquois, van Wijngaarden, Woodger, Green, McCarthy |
| Influenced | Most subsequent imperative languages (so-called ALGOL-like languages) e.g. Simula, C, CPL, Pascal, Ada |
ALGOL (short for ALGOrithmic Language)[1] is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years.[2] It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages, including BCPL, B, Pascal, Simula, and C. ALGOL introduced code blocks and the begin and end pairs for delimiting them and it was also the first language implementing nested function definitions with lexical scope. Fragments of ALGOL-like syntax are sometimes still used as pseudocode.
There were three major specifications:
Niklaus Wirth based his own ALGOL W on ALGOL 60 before developing 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 but was not well received, so that in general "Algol" means Algol 60 and dialects thereof.
Contents |
The International Algorithmic Language (IAL) was extremely influential and generally considered the ancestor of most of the modern programming languages (the so-called Algol-like languages). Additionally, in computer science, ALGOL object code was a simple and compact and stack-based instruction set architecture mainly used in teaching compiler construction and other high order language (of which Algol is generally considered the first).
ALGOL was developed jointly by a committee of European and American computer scientists in a meeting in 1958 at ETH Zurich (cf. ALGOL 58). 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 Donald Knuth's suggestion renamed Backus–Naur Form.[6]
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."[7]
The following people attended the meeting in Paris (from January 1 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."
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 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."[8] 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.[9]
As Peter Landin noted, the language Algol was the first language to seamlessly combine imperative effects with the (call-by-name) lambda calculus. Perhaps the most elegant formulation of the language is due to John C. Reynolds, and it best exhibits its syntactic and semantic purity. Reynolds's "idealized" Algol also made a convincing methodological argument regarding the suitability of "local" effects in the context of call-by-name languages, to be contrasted with the "global" effects used by call-by-value languages such as ML. The conceptual integrity of the language made it one of the main objects of semantic research, along with PCF and ML.[10]
To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.[11]
A discussion of implementation issues of an ALGOL 60 implementations can be found in the article "The early days of Algol" by Nicholas Enticknap and Pat Woodroffe.
| Name | Year | Author | State | Description | Target CPU |
|---|---|---|---|---|---|
| ZMMD-implementation | 1958 | Friedrich L. Bauer, Heinz Rutishauser, Klaus Samelson, Hermann Bottenbruch | Germany | implementation of ALGOL 58 | Z22 (later Zuse's Z23 was delivered with an Algol 60 compiler) [12] |
| X1 ALGOL 60 | August 1960[13] | Edsger W. Dijkstra and Jaap A. Zonneveld | Netherlands | First implementation of ALGOL 60[14] | Electrologica X1 |
| Elliott ALGOL | 1960s | C. A. R. Hoare | UK | Subject of the famous Turing lecture | Elliott 803 & the Elliott 503 |
| JOVIAL | 1960 | Jules Schwarz | USA | Was the DOD HOL prior to Ada (programming language) | Various (see article) |
| 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 | Bill McKeeman | USA | For ODIN time-sharing system | PDP-1 |
| RegneCentralen ALGOL | 1961 | Peter Naur, Jørn Jensen | Denmark | Inplementation of full Algol 60 | DASK at Regnecentralen |
| Dartmouth ALGOL 30 | 1962 | Thomas Eugene Kurtz et al. | USA | LGP-30 | |
| USS 90 Algol | 1962 | L. Petrone | Italy | ||
| 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 | |
| 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 | ||
| Simula 67 | 1967 | Ole-Johan Dahl and Kristen Nygaard | Norway | Algol 60 with classes | UNIVAC 1107 |
| Chinese Algol | 1972 | China | Chinese characters, expressed via the Symbol system | ||
| DG/L | 1972 | USA | DG Eclipse family of Computers | ||
| S-algol | 1979 | Prof. Ron Morrison | UK | Addition of orthogonal datatypes with intended use as a teaching language | PDP-11 with a subsequent implementation on the Java VM |
The Burroughs dialects included special Bootstrapping dialects such as ESPOL and NEWP.
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.[15] 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). Another problematic situation is passing a random() function.
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 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.
(The way the bold text has to be written depends on the implementation, e.g. 'INTEGER' (including the quotation marks) for integer; this is known as stropping.)
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.[16]
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.
The following code samples are ALGOL 68 versions of the above ALGOL 60 code samples.
ALGOL 68 reuses ALGOL 60's stropping. In ALGOL 68's case tokens with the bold typeface are reserved words, types (modes) or operators.
proc abs max = ([,]real a, ref real y, ref int i, k)real:
comment The absolute greatest element of the matrix a, of size ⌈a by 2⌈a
is transferred to y, and the subscripts of this element to i and k; comment
begin
real y := 0; i := ⌊a; k := 2⌊a;
for p from ⌊a to ⌈a do
for q from 2⌊a to 2⌈a do
if abs a[p, q] > y then
y := abs a[p, q];
i := p; k := q
fi
od
od;
y
end # abs max #
Note: lower (⌊) and upper (⌈) bounds of an array, and array slicing, are directly available to the programmer.
floating point algol68 test:
(
real a,b,c,d;
printf(($pg$,"Enter d:"));
read(d);
for step from 0 while a:=step*d; a <= 2*pi do
printf($l$);
b := sin(a);
c := cos(a);
printf(($z-d.6d$,a,b,c))
od
)
printf - sends output to the file stand out.
printf($p$); - selects a new page.
printf($l$); - selects a new line.
printf(($z-d.6d$,a,b,c)) - formats output with 1 digit before and 6 after the decimal point.
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 had no I/O facilities.
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.
'BEGIN'
'WRITE TEXT'("HELLO WORLD");
'END'
ALGOL 68 code was published with reserved words typically in lowercase, but bolded or underlined.
begin printf(($gl$,"Hello, world!")) end
In the language of the "Algol 68 Report" the input/output facilities were collectively called the "Transput".
The ALGOLs were conceived at a time when character sets were diverse and evolving rapidly; also, the ALGOLs were defined so that only uppercase letters were required.
1960: IFIP - The Algol 60 language and report included several mathematical symbols which are available on modern computers and operating systems, but, unfortunately, were not supported on most computing systems at the time. For instance: ×, ÷, ≤, ≥, ≠, ¬, ∨, ∧, ⊂, ≡, ␣ and ⏨.
1961 September: ASCII – The ASCII character set, then in an early stage of development, had the \ (Back slash) character added to it in order to support ALGOL's boolean operators /\ and \/.[17]
1962: ALCOR - This character set included the unusual "᛭" (iron/runic cross) character and the "⏨" (Decimal Exponent Symbol) for floating point notation. [18]
1964: GOST - The 1964 Russian standard GOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL.[19]
1968: The "Algol 68 Report" - used existing ALGOL characters, and further adopted →, ↓, ↑, □, ⌊, ⌈, ⎩, ⎧, ○, ⊥ and ¢ characters which can be found on the IBM 2741 keyboard with "golf-ball" print heads inserted (such as the APL golfball), these became available in the mid 1960s while ALGOL 68 was being drafted. The report was translated into Russian, German, French and Bulgarian, and allowed programming in languages with larger character sets, e.g. Cyrillic alphabet of the Russian BESM-4. All ALGOL's characters are also part of the unicode standard and most of them are available in several popular fonts.
2009 October: Unicode - The "⏨" (Decimal Exponent Symbol) for floating point notation was added to Unicode 5.2 for backward compatibility with historic Buran (spacecraft) ALGOL software.