“ALGOL 60”的意思、由来-开放百科全书

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. 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.

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

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.

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]

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] 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.^[5]

ALGOL 60 implementations timeline

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

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 allowed for{{clarify|Was the choice of evaluation strategy made by the implementation, or left to programmers?|date=November 2018}} two evaluation strategies for parameter passing: the common call-by-value, and 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.^[10] 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:

There are 71 such restricted identifiers in the standard Burroughs large systems sub-language:

Standard Operators

Examples and portability issues

Code sample comparisons

ALGOL 60

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.

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 [https://web.archive.org/web/20100209040840/http://www.engin.umd.umich.edu/CIS/course.des/cis400/index.html The Language Guide] at the University of Michigan-Dearborn Computer and Information

Science Department [https://web.archive.org/web/20100204112923/http://www.engin.umd.umich.edu/CIS/course.des/cis400/algol/hworld.html Hello world! ALGOL Example Program page].

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 ’.

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).

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 %.^[12]

See also

References

1. ^{{cite journal |author=Knuth, Donald E. |title=Backus normal Form vs Backus Naur Form |journal=Comm. ACM |volume=7 |issue=12 |pages=735–6 |date=December 1964 |doi=10.1145/355588.365140 }}
2. ^ACM Award Citation / Peter Naur, 2005
3. ^{{cite web|url=https://vanemden.wordpress.com/2014/06/18/how-recursion-got-into-programming-a-comedy-of-errors-3/|title=How recursion got into programming: a tale of intrigue, betrayal, and advanced programming-language semantics|first=Maarten|last=van Emden|date=2014|series=A Programmer's Place}}
4. ^{{cite web |first=C.A.R. |last=Hoare |title=Hints on Programming Language Design |date=December 1973 |url=http://www.eecs.umich.edu/~bchandra/courses/papers/Hoare_Hints.pdf |page=27}} (This statement is sometimes erroneously attributed to Edsger W. Dijkstra, also involved in implementing the first ALGOL 60 compiler.)
5. ^{{cite web |editor-first1=Jonathan |editor-last1=Rees |editor-first2=William |editor-last2=Clinger |author-first1=Hal |author-last1=Abelson |author-first2=R. K. |author-last2=Dybvig |title=Revised(3) Report on the Algorithmic Language Scheme, (Dedicated to the Memory of ALGOL 60)| url=http://groups.csail.mit.edu/mac/ftpdir/scheme-reports/r3rs-html/r3rs_toc.html|access-date=2009-10-20|display-authors=etal}}
6. ^The Encyclopedia of Computer Languages {{webarchive |url=https://web.archive.org/web/20110927014141/http://hopl.murdoch.edu.au/showlanguage.prx?exp=1807 |date=September 27, 2011 }}
7. ^{{cite journal|url=http://www.dijkstrascry.com/node/4|title=Dijkstra's Rallying Cry for Generalization: the Advent of the Recursive Procedure, late 1950s — early 1960s|last=Daylight |first=E. G.|journal=The Computer Journal|volume=54|issue=11|year=2011|pages=1756–1772|doi=10.1093/comjnl/bxr002}}
8. ^{{Cite book | last1 = Kruseman Aretz | first1 = F.E.J. | chapter = The Dijkstra-Zonneveld ALGOL 60 compiler for the Electrologica X1 | title=Software Engineering | series = History of Computer Science | publisher = Centrum Wiskunde & Informatica | place = Kruislaan 413, 1098 SJ Amsterdam | date =30 June 2003 |chapter-url = http://www.cwi.nl/ftp/CWIreports/SEN/SEN-N0301.pdf }}
9. ^Irons, Edgar T., A syntax directed compiler for ALGOL 60, Communications of the ACM, Vol. 4, p. 51. (Jan. 1961)
10. ^{{cite book | last=Aho | first=Alfred V. | authorlink=Alfred V. Aho | author2=Ravi Sethi |author3=Jeffrey D. Ullman | title=Compilers: Principles, Techniques, and Tools | year=1986 | edition=1st | publisher=Addison-Wesley | isbn=978-0-201-10194-2| title-link=Dragon Book (computer science) }}, Section 7.5, and references therein
11. ^"803 ALGOL", the manual for Elliott 803 ALGOL
12. ^{{cite web|url=http://www.icl1900.co.uk/techpub/tp3340.djvu|title=ICL 1900 series: Algol Language|publisher=ICL Technical Publication 3340|year=1965}}