This article is about the programming language family.
For other uses, see Algol (disambiguation).
|Paradigm||Procedural, imperative, structured|
|Designed by||Bauer, Bottenbruch, Rutishauser, Samelson, Backus, Katz, Perlis, Wegstein, Naur, Vauquois, van Wijngaarden, Woodger, Green, McCarthy|
|First appeared||1958; 62 years ago (1958)|
|Typing discipline||Static, strong|
ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the Association for Computing Machinery (ACM) in textbooks and academic sources until object-oriented languages came around, for more than thirty years.
In the sense that the syntax of most modern languages is "Algol-like", it was arguably the most influential of the four high-level programming languages among which it was roughly contemporary: FORTRAN, Lisp, and COBOL.
ALGOL introduced code blocks and the begin...end pairs for delimiting them.
Moreover, it was the first programming language which gave detailed attention to formal language definition and through the Algol 60 Report introduced Backus–Naur form, a principal formal grammar notation for language design.
There were three major specifications, named after the years they were first published:
- ALGOL 58 – originally proposed to be called IAL, for International Algebraic Language.
- ALGOL 60 – first implemented as X1 ALGOL 60 in mid-1960. Revised 1963.
- ALGOL 68 – introduced new elements including flexible arrays, slices, parallelism, operator identification. Revised 1973.
ALGOL 68 is substantially different from ALGOL 60 and was not well received, so that in general "Algol" means ALGOL 60 and dialects thereof.
The International Algebraic Language (IAL), renamed ALGOL 58, was highly influential and generally considered the ancestor of most of the modern programming languages (the so-called Algol-like languages).
Further, ALGOL object code was a simple, compact, and stack-based instruction set architecture commonly used in teaching compiler construction and other high order languages; 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 the Swiss Federal Institute of Technology in Zurich (ETH Zurich; cf.
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 other than Burroughs Corporation.
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.
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 1 to 16 January):
- 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 and programming language research
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 methodologic argument regarding the suitability of local effects in the context of call-by-name languages, in contrast 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 Programming Computable Functions (PCF) and ML.
IAL implementations timeline
To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.
The latter is still used for Unisys MCP system software.
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 (input/output) facilities.
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 is reevaluated.
Say i := 1 and A[i] := 2, so every time swap is referenced it will 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.
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.
Examples and portability issues
Code sample comparisons
(The way the bold text has to be written depends on the implementation, e.g. 'INTEGER'—quotation marks included—for integer.
This is known as stropping.)
Here is an example of how to produce a table using Elliott 803 ALGOL.
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 implementations used ALGOL 60's approaches to stropping.
In ALGOL 68's case tokens with the bold typeface are reserved words, types (modes) or operators.
Note: lower (⌊) and upper (⌈) bounds of an array, and array slicing, are directly available to the programmer.
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)
Main article: ALGOL 58
ALGOL 58 had no I/O facilities.
ALGOL 60 family
Main article: ALGOL 60
Since ALGOL 60 had no I/O facilities, there is no portable hello world program in ALGOL.
The next three examples are in Burroughs Extended Algol.
The first two direct output at the interactive terminal they are run on.
The first uses a character array, similar to C. The language allows the array identifier to be used as a pointer to the array, and hence in a REPLACE statement.
A simpler program using an inline format:
An even simpler program using the Display statement.
Note that its output would end up at the system console ('SPO'):
An alternative example, using Elliott Algol I/O is as follows.
Elliott Algol used different characters for "open-string-quote" and "close-string-quote":
Here is 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.
produced a new line on the teleprinter).
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.
The open and close quote characters were represented using '(' and ')' and spaces by %.
Main article: ALGOL 68
ALGOL 68 code was published with reserved words typically in lowercase, but bolded or underlined.
In the language of the "Algol 68 Report" the input/output facilities were collectively called the "Transput".
Timeline of ALGOL special characters
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 unsupported on most computing systems at the time.
For instance: ×, ÷, ≤, ≥, ≠, ¬, ∨, ∧, ⊂, ≡, ␣ and ⏨.
1962: ALCOR – This character set included the unusual "᛭" runic cross character for multiplication and the "⏨" Decimal Exponent Symbol for floating point notation.
1968: The "Algol 68 Report" – used extant ALGOL characters, and further adopted →, ↓, ↑, □, ⌊, ⌈, ⎩, ⎧, ○, ⊥, and ¢ characters which can be found on the IBM 2741 keyboard with typeball (or golf ball) print heads inserted (such as the APL golf ball).
These became available in the mid-1960s while ALGOL 68 was being drafted.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/ALGOL.