ALGOL 68

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ALGOL 68 (short for ALGOrithmic Language 1968) is an imperative computer programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and a more rigorously defined syntax and semantics. Contributions of ALGOL 68 to the field of computer science are deep and wide ranging, although some of them were not publicly identified until they were passed, in one form or another, to one of many subsequently developed programming languages.

ALGOL 68 was defined using a two-level grammar formalism invented by Adriaan van Wijngaarden. 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 main aims and principles of design of ALGOL 68 are:

1. Completeness and clarity of design, [1]
2. Orthogonal design, [2]
3. Security, [3]
4. Efficiency: [4]
   • Static mode checking,
   • Mode-independent parsing,
   • Independent compilation,
   • Loop optimization,
   • Representations - in minimal & larger character sets.

Critics of ALGOL 68, prominently C. A. R. Hoare, point out that it abandoned the simplicity of ALGOL 60 and became a vehicle for various complex ideas of its designers. The language also did little to make the compiler writer's task easy, in contrast to deliberately simple contemporaries (and competitors) C, S-algol and Pascal.

Though European defence agencies (in Britain Royal Signals and Radar Establishment - RSRE) promoted the use of ALGOL 68 for its expected security advantages, the American side of the NATO alliance decided to develop a different project, the Ada programming language. The use of Ada was made obligatory for defence contracts. Apparently there was no room for two languages of similar application range in the NATO. Perhaps the acceptance of ALGOL 68 (Алгол 68) on the Russian side, then Soviet Union, was not helpful on this, either.

The ALGOL 68 heritage is acknowledged by [[C++]].

For a full length treatment of the language, see Programming Algol 68 Made Easy by Dr. Sian Leitch.

Time-line of ALGOL 68

• "A Shorter History of Algol 68"
• Summary of "True Algol68s"

Report on the Algorithmic Language Algol 68

Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster

"Van Wijngaarden once characterized the four authors, somewhat tongue-in-cheek, as: Koster: transputter, Peck: syntaxer, Mailloux: implementer, Van Wijngaarden: party ideologist." -- Koster.

Revised Report on the Algorithmic Language Algol 68

Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck, C.H.A. Koster, M. Sintzoff, C.H. Lindsey, L.G.L.T. Meertens and R.G. Fisker.

Notable Language Elements

Bold Symbols and Reserved Words

There are 61 such reserved words ( some with "brief symbol" equivalents ) in the standard sub-language:

mode, op, prio, proc,
flex, heap, loc, long, ref, short,
bits, bool, bytes, char, compl, int, real, sema, string, void,
channel, file, format, struct, union, 
of, at "@", is ":==:", isnt ":/=:", true, false, empty, nil "∘", skip "~",
co "¢", comment "¢", pr, pragmat,
case in ouse in out esac "( ~ | ~ |: ~ | ~ | ~ )", 
for from to by while do od,
if then elif then else fi "( ~ | ~ |: ~ | ~ | ~ )", 
par begin end "( ~ )", go to, goto, exit.

Units: Expressions

The basic language construct is the unit. A unit may be a formula, an enclosed clause, a routine text or one of several technically needed constructs (assignation, jump, skip, nihil). The technical term enclosed clause unifies some of the inherently bracketting constructs known as block, do statement, switch statement in other contemporary languages. When keywords are used, generally the reversed character sequence of the introducing keyword is used for terminating the enclosure, eg. ( if ~ then ~ else ~ fi, case ~ in ~ out ~ esac, for ~ while ~ do ~ od ). This feature was reused by Stephen Bourne in the common Unix Bourne shell. An expression may also yield a multiple value, which is constructed from other values by a collateral clause. This construct just looks like the parameter pack of a procedure call.

mode: Declarations

The basic datatypes (called modes in ALGOL 68 parlance) are real, int, compl (complex number), bool and char. For example:

int n = 2, m:=3; co n is a fixed constant of 2, but m is variable which is initially 3 co
real avogadro = 6.0221415₁₀23; co Avogadro's number co
long long real pi = 3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37510;
compl square root of minus one = 0 ⊥ 1;

However, the declaration real x; is just syntactic sugar for ref real x = loc real;. That is, x is really the constant name for a reference to a newly generated local real variable.

Furthermore, instead of defining both float and double, or int and long and short, etc., ALGOL 68 provided modifiers, so that the presently common double would be written as long real or long long real instead, for example. Type queries of the kind of max real and min long int are provided to adapt programs to different implementations.

All variables need to be declared, the declaration does not have to appear prior to the first use.

primitive-declarer: int, real, compl, complex^G, bool, char, string, bits, bytes, format, file, pipe^G, channel, sema

• bits - a "packed vector" of bool.
• bytes - a "packed vector" of char.
• string - a flexible array of char.
• sema - a semaphore which can be initialised with the operator level.

Other declaration symbols include: flex, heap, loc, ref, long, short, event^S

• flex - declare the array to be flexible, i.e. it can grow in length on demand.
• heap - allocate variable some free space from the global heap.
• loc - allocate variable some free space of the local stack.
• ref - declare the variable to be a pointer, similar to using "&" in a C++ declaration.
• long - declare an int, real or compl to be of a longer size.
• short - declare an int, real or compl to be of a shorter size.

A new mode (type) may be declared using a mode declaration:

 int max=99;
 mode newtype = [0:9][0:max]struct ( long real a, b, c; short int i, j, k; ref real r );

This has the similar effect as the following C++ code:

 const int max=99;
 typedef class { public: double a, b, c; short i, j, k; float &r; } newtype[10][max+1];

Note that for ALGOL 68 only the newtype name appears to the left of the equality, and most notably the construction is made - and can be read - from left to right without regard to priorities.

Coercions: casting

The coercions produce a coercend from a coercee according to three criteria: the a priori mode of the coercend before the application of any coercion, the a posteriori mode of the coercee required after those coercions, and the syntactic position or "sort" of the coercee. Coercions may be cascaded.

There are six possible coercions, termed "deproceduring", "dereferencing", "uniting", "widening", "rowing" and "voiding". Each Coercion, except "uniting", prescribes a corresponding dynamic effect on the associated values. Hence, a number of primitive actions can be programmed implicitly by coercions.

Context strength - Allowed coercions:

• soft - deproceduring
• weak - dereferencing or deproceduring, yielding a name
• meek - dereferencing or deproceduring
• firm - meek, followed by uniting
• strong - firm, followed by widening, rowing or voiding

prag & co: Code Pragments and Comments

Pragmats are directives in the program, typically hints to the compiler. eg.

pragmat heap=32 pragmat
pr heap=32 pr

Comments can be inserted in variety of ways:

¢ The original way of adding your 2 cents worth to a program ¢
comment "bold" comment comment
co Style i comment co 
# Style ii comment #
£ This is a hash/pound comment for a UK keyboard £ 

Normally, comments cannot be nested in ALGOL 68. This restriction can be circumvented by using different comment delimiters (e.g. use hash only for temporary code deletions).

Expressions and compound statements

ALGOL 68 being an expression-oriented programming language, the value returned by an assignment statement is a reference to the destination. Thus, the following is valid ALGOL 68 code:

 real spi, twice pi; twice pi := 2 * ( spi := 3.1415926 );

This notion is present in C and Perl, among others. Note that twice pi is a single identifier, i.e., blanks are permitted within ALGOL 68 names (effectively avoiding the underscores versus camel case versus all lowercase issues at once, but at the price of introducing a cornucopia of more serious problems in software engineering).

As another example, to express the mathematical idea of a sum of f(i) from i=1 to n, the following ALGOL 68 integer expression suffices:

 (int sum := 0; for i to n do sum +:= f(i) od; sum)

Note that, being an integer expression, the former block of code can be used in any context where an integer value can be used. A block of code returns the value of the last expression it evaluated; this idea is present in Perl, among other languages.

Compound statements are all terminated by distinctive (if somewhat irreverent) closing brackets:

• if choice clauses:

 if condition then statements [ else statements ] fi
 "brief" form of if statement:  ( condition | statements | statements )
 
 if condition1 then statements elif condition2 then [ else statements ] fi
 "brief" form:  ( condition1 | statements :| condition2 | statements | statements )

• case choice clauses:

 case switch in statements, statements,... [ out statements ] esac
 "brief" form:  ( switch | statements,statements,... | statements )
 
 case switch1 in statements, statements,... ouse switch2 in statements, statements,... [ out statements ] esac
 "brief" form of case statement:  ( switch1 | statements,statements,... |: switch2 | statements,statements,... | statements )

• do loop clause:

 [ for index ] [ from first ] [ by increment ] [ to last ] [ while condition ] do statements od
 The minimum form of a "loop clause" is thus: do statements od

This scheme not only avoids the dangling else problem but also avoids having to use begin and end in embedded statement sequences. Some actual examples can be found below.

struct, union & [:]: Structures, unions and arrays</u>

ALGOL 68 supports arrays with any number of dimensions, and it allows for the slicing of whole or partial rows or columns, among many other choices.

 mode vector =    [3]real; # vector mode declaration (typedef) #
 mode matrix = [3][3]real; # matrix mode declaration (typedef) #
 vector r1 :=   (1,2,3);   # array variable initially (1,2,3) #
 [ ]real r2 = (4,5,6);     # constant array, length is implied #
 op + = (vector a,b)vector:  # binary operator definition #
   (vector out; int i; for i from ⌊a to ⌈a do out[i]:=a[i]+b[i] od; out);
 matrix m:=(r1,r2,r1+r2);  
 print ((m[ ][2:]));  # print the 2nd and 3rd element of each row #

Matrices can be sliced either way, eg:

 ref vec col:=m[ ][2]  # extract a ref (pointer) to the 2nd column!! #

ALGOL 68 supports multiple field structures (struct). Union types (modes) are supported. Reference variables may point to both array slices and structure fields. For an example of all this, here is the traditional linked list declaration:

 mode node = union (real, int, compl, string);
 
 mode list = struct (node val, ref list next);

Usage example for union case of node:

 node n:="1234";

 case n in
   (real):print(("real:",n)),
   (int):print(("int:",n)),
   (compl):print(("compl:",n)),
   (string):print(("string:",n))
   out print ((n,"?"))
 esac

proc: Procedures

Procedure (proc) declarations require type specifications for both the parameters and the result (void if none):

 proc max of real = (real a, b) real:
 
    if a > b then a else b fi;

or, using the "brief" form of the conditional statement:

 proc max of real = (real a, b) real: (a>b | a | b);

The return value of a proc is the value of the last expression evaluated in the procedure. References to procedures (ref proc) are also permitted. Call-by-reference parameters are provided by specifying references (such as ref real) in the formal argument list. The following example defines a procedure that applies a function (specified as a parameter) to each element of an array:

 proc apply = (ref [] real a, proc (real) real f):
 
    for i from lwb a to upb a do a[i] := f(a[i]) od;

This simplicity of code was unachievable in ALGOL 68's predecessor ALGOL 60.

op: Operators

The programmer may define new operators and both those and the pre-defined ones may be overloaded. The following example defines operator max with both dyadic and monadic versions (scanning across the elements of an array).

 prio max = 9;
 
 op max = (int a,b) int: ( a>b | a | b );
 op max = (real a,b) real: ( a>b | a | b );
 op max = (compl a,b) compl: ( abs a > abs b | a | b );
 
 op max = ([]real a) real:
    (real out := - max real;
     for i from lwb a to upb a do ( a[i]>out | out:=a[i] ) od;
     out);

• Monadic Operators:

• Standard dyadic operators with associated priorities:

• Assignation and Identity Relations: These are technically not operators, rather they are considered "units associated with names"

Special characters for operators

Template:SpecialChars The ∨, ∧, ¬, ≠, ≤, ≥, ×, ÷, ⌷, ↑, ↓, ⌊, ⌈ and ⊥ characters can be found on the IBM 2741 terminal with the APL "golf-ball" print head, these became available in the mid 1960s while ALGOL 68 was being drafted.

Image:APL keyboard.gif

transput: Input and output

Transput is the term used to refer to ALGOL 68's input and output facilities. There are pre-defined procedures for unformatted, formatted and binary transput. Files and other transput devices are handled in a consistent and machine-independent manner. The following example prints out some unformatted output to the standard output device:

print ((newpage, "Title", newline, "Value of i is ",
  i, "and x[i] is ", x[i], newline));

Note the predefined procedures newpage and newline passed as arguments.

Books, channels and files

The transput is considered to be of books, channels and files:

• Books are made up of pages, and lines, and may be locked and selected via chains.
  • A specific book can be located by name with a call to match.
• channels correspond to physical devices. eg. card punches and printers.
  • There are three standard channels: stand in channel, stand out channel, stand back channel.
• A file is a means of communicating between a particular program and a book that has been opened via some channel.
  • The mood of a file may be read, write, char, bin, and opened.
  • transput procedures include: establish, create, open, associate, lock, close, scratch.
  • position enquires: char number, line number, page number.
  • layout routines include:
    • space, backspace, newline, newpage.
    • get good line, get good page, get good book, and proc set=(ref file f, int page,line,char)void:
  • A file has event routines. eg. on logical file end, on physical file end, on page end, on line end, on format end, on value error, on char error.

formatted transput

"Formatted transput" in ALGOL 68's transput has its own syntax and patterns (functions), with formats embedded between two $ characters. Examples:

printf (($2l"The sum is:"x, g(0)$, m + n)); ¢ prints the same as ¢
print ((new line, new line, "The sum is:", space, whole (m + n, 0));

• and many more...

par: Parallel processing

ALGOL 68 supports programming of parallel processing. Using the keyword par, a collateral clause is converted to a parallel clause, where the synchronisation of actions is controlled using semaphores. In A68G the parallel actions are mapped to threads when available on the hosting operating system. In A68S a different paradigm of parallel processing was implemented (see below).

mode foot = [5]bool;

foot left, right;

sema left toe := level ⌈left,
     right toe := level  ⌈right;

proc shoot left toe = void: (
       shoot(left toe);
       print("Left: Ouch!!");
       newline
     ),
     shoot right toe = void:(
       shoot(right toe);
       print("Right: Ouch!!");
       newline
     );

¢ 10 round clip in a 1955 Colt Python .357 Magnum ¢
sema rounds = level 10;

¢ the Magnum needs more barrels to take full advantage of parallelism ¢
sema aqcuire target = level 1;

proc shoot = (ref sema target)void: (
  ↓ acquire target; 
  ↓ rounds; 
  print("BANG! ");
  ↓ target; 
  ↑ acquire target
);

¢ do shooting in parallel to cater for someone hoping to stand on just one foot ¢
par (
  for toe from ⌊left to ⌈left do
    shoot left toe 
  od, ¢ <= this comma is important ¢
  for toe from ⌊right to ⌈right do
    shoot right toe
  od
)

Code sample

This sample program implements the Sieve of Eratosthenes to find all the prime numbers that are less than 100. nil is the ALGOL 68 analogue of the null pointer in other languages. The notation x of y accesses a member x of a struct or union y.

begin # Algol-68 prime number sieve, functional style #

  proc error = (string s) void:
     (print(( newline, " error: ", s, newline)); goto stop);
  proc one to = (int n) list:
     (proc f = (int m,n) list: (m>n | nil | cons(m, f(m+1,n))); f(1,n));

  mode list = ref node;
  mode node = struct (int h, list t);
  proc cons = (int n, list l) list: heap node := (n,l);
  proc hd   = (list l) int: ( l is nil | error("hd nil"); skip | h of l );
  proc tl   = (list l) list: ( l is nil | error("tl nil"); skip | t of l );
  proc show = (list l) void: ( l isnt nil | print((" ",whole(hd(l),0))); show(tl(l)));

  proc filter = (proc (int) bool p, list l) list:
     if l is nil then nil 
     elif p(hd(l)) then cons(hd(l), filter(p,tl(l)))
     else filter(p, tl(l))
     fi;

  proc sieve = (list l) list:
     if l is nil then nil 
     else
        proc not multiple = (int n) bool: n mod hd(l) ≠ 0;
        cons(hd(l), sieve( filter( not multiple, tl(l) )))
     fi;

  proc primes = (int n) list: sieve( tl( one to(n) ));

  show( primes(100) )
end

Program representation

A feature of ALGOL 68, inherited from ALGOL tradition, is its different representations. There is a representation language used to describe algorithms in printed work, a strict language (rigorously defined in the Report) and an official reference language intended to be used in actual compiler input. In the examples above you will observe underlined words. This is the formal representation of the language. ALGOL 68's reserved words are effectively in a different namespace from identifiers, and spaces are allowed in identifiers, so the fragment:

 int a real int = 3 ;

is legal. The programmer who actually writes the code does not have the option of underlining the code. Depending on hardware and cultural issues, different methods to denote these identifiers, have been devised, called stropping regimes. So all or some of the following may be possible programming representations:

 'INT' A REAL INT = 3; 
 .INT A REAL INT = 3;
 INT a real int = 3;
 int a_real_int = 3;

The following characters were recommended for portability, and termed "worthy characters" in the Report on the Standard Hardware Representation of Algol 68:

• Template:NoteWorthy Characters: ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 "#$%'()*+,-./:;<=>@[ ]_|

This reflected a problem in the 1960s where some hardware didn't support lowercase, nor some other non ASCII characters, indeed in the 1973 report it was written: "Four worthy characters -- "|", "_", "[", and "]" -- are often coded differently, even at installations which nominally use the same character set."

• Base characters: "Worthy characters" are a subset of "base characters".

Some Vanitas

For its technical intricacies, ALGOL 68 needs a cornucopia of methods to deny the existence of something:

skip, "~" or "?"C - an undefined value always syntactically valid,
void - syntactically like a mode, but not one,
nil or "∘" - a name not denoting anything, of no mode,
empty - the only value admissible to void, needed for selecting void in a union,
[1:0]int - an empty array of integral values, with mode []int,
undefined - a procedure raising an exception in the runtime system.

The term nil is var always evaluates to true for any variable, whereas it is not known to which value a comparison x < skip evaluates for any integer x.

ALGOL 68 leaves intentionally undefined, what happens in case of integer overflow, the integer bit representation, and the degree of numerical accuracy for floating point. In contrast, the language Java has been criticized for over-specifying the latter.

Comparison to C++

Regarding the computing features, the nearest living sibling to ALGOL 68 may be [[C++]], making this a good comparison candidate:

C++ has not:

• nested functions,
• definable operator symbols and priorities,
• garbage collection,
• use before define,
• formatted transput using complex formatting declarations,
• assignment operation symbol (to avoid confusion with equal sign),
• arrays (and slice operations on them, but in layered libraries),
• automatic UNIONs,
• CASE expressions,
• nonlocal GOTO (not a good idea in most cases, anyway).
• intuitive declaration syntax due to its origin from C.

ALGOL 68 has not:

• public/private access protection,
• overloaded procedures (in contrast to operators),
• explicit memory allocation and deallocation,
• forward declarations,
• textual preprocessing (header files),
• confusion between &- and pointer-style,
• comment lines (only bracketed comments),
• hierarchical classes.

Variants

Except where noted (with a ^superscript), the language described above is that of the "Revised Report^(2)".

The language of the unrevised Report

The original language^{(1) differs in syntax of the mode cast, and it had the feature of proceduring, i.e. coercing the value of a term into a procedure which evaluates the term. Proceduring effectively can make evaluations lazy. The most useful application could have been the short-circuited evaluation of boolean operators. In}

op andf = (boola,proc boolb)bool:(a | b | false);

b is only evaluated, if a is true. As defined in ALGOL 68, it did not work as expected. Most implementations emulate the correct behaviour for this special case by extension of the language.

Before revision, the programmer could decide to have the arguments of a procedure evaluated serially instead of collaterally by using semicolons instead of commas (gommas).

Extension proposals from IFIP WG 2.1

After the revision of the report, some extensions to the language have been proposed to widen the applicability:

• partial parametrisation: creation of functions (with less parameters) by specification of some, but not all parameters for a call, e.g. a function logarithm of two parameters, base and argument, could be specialised to natural, binary or decadic log,
• module extension for support of external linkage,
• mode parameters for implementation of limited parametrical polymorphism (most operation on data structures like lists, trees or other data containers can be specified without touching the pay load).

So far, only partial parametrisation has been implemented, in ALGOL68G.

True ALGOL 68s Specification and Implementation Timeline

The S3 programming language that was used to write the VME operating system and much other system software on the ICL 2900 Series was a direct derivative of Algol 68. However, it omitted many of the more complex features, and replaced the basic modes with a set of data types that mapped directly to the 2900 Series hardware architecture.

Implementation specific extensions

ALGOL 68R^{(R) from RRE was the first ALGOL 68 subset implementation, running on the ICL 1900. Based on the original language, the main subset restrictions were definition before use and no parallel processing. This compiler was popular in UK universities in the 1970s, where many computer science students learnt ALGOL 68 as their first programming language; the compiler was renowned for good error messages.}

ALGOL 68RS^{(RS) from RSRE was a portable compiler system written in ALGOL 68RS (bootstrapped from ALGOL 68R), and implemented on a variety of systems including the ICL 2900 Series, Multics and DEC VAX/VMS. The language was based on the Revised Report, but with similar subset restrictions to ALGOL 68R. This compiler survives in the form of an Algol68-to-C-Compiler.}

In ALGOL 68S^{(S) from Carnegie_Mellon_University the power of parallel processing was improved by adding an orthogonal extension, eventing. Any variable declaration containing keyword event made assignments to this variable eligible for parallel evaluation, i.e. the right hand side was made into a procedure which was moved to one of the processors of the C.mmp multiprocessor system. Accesses to such variables were delayed after termination of the assignment.}

Cambridge ALGOL 68C^{(C) was a portable compiler that implemented a subset of ALGOL 68 by enforcing definition before use, restricting operator definitions and omitting garbage collection.}

ALGOL 68G^{(G) by M. van der Veer implements a usable ALGOL 68 interpreter for today's computers and operating systems. A minor restriction is that formatted transput is still not conforming to the Revised Report.}

"Despite good intentions, a programmer may violate portability by inadvertently employing a local extension. To guard against this, each implementation should provide a PORTCHECK pragmat option. While this option is in force, the compiler prints a message for each construct that it recognizes as violating some portability constraint."[5]

Quotes

• [...] it was said that A68's popularity was inversely proportional to [...] the distance from Amsterdam [6]
• ... C does not descend from Algol 68 is true, yet there was influence, much of it so subtle that it is hard to recover even when I think hard. In particular, the union type (a late addition to C) does owe to A68, not in any details, but in the idea of having such a type at all. More deeply, the type structure in general and even, in some strange way, the declaration syntax (the type-constructor part) was inspired by A68. And yes, of course, "long". Dennis Ritchie, 18 June 1988 [7]
• "Congratulations, your Master has done it" - Niklaus Wirth, Aug 1968.
• 'The best we could do was to send with it a minority report, stating our considered view that, "... as a tool for the creation of sophisticated programs, the language was a failure." ' - C. A. R. Hoare, Oct 1980, re: "Dec 1968"

References

• Brailsford, D.F. and Walker, A.N., Introductory ALGOL 68 Programming, Ellis Horwood/Wiley, 1979
• McGettrick, A.D., ALGOL 68, A First and Second Course, Cambridge Univ. Press, 1978
• Peck, J.E.L., An ALGOL 68 Companion, Univ. of British Columbia, October 1971
• Tanenbaum, A.S., A Tutorial on ALGOL 68, Computing Surveys 8, 155-190, June 1976 and 9, 255-256, September 1977, http://vestein.arb-phys.uni-dortmund.de/~wb/RR/tanenbaum.pdf

External links

• Revised Report on the Algorithmic Language ALGOL 68 The official reference for users and implementors of the language
• Revised Report on the Algorithmic Language ALGOL 68 HTML version of the above
• Charles Lindsey's paper on the development of ALGOL 68 for the second History of Programming Languages conference proceedings
• Algol 68 Genie - a GNU GPL Algol 68 interpreter
• Algol68 Standard Hardware representation (.pdf)
• Из истории создания компилятора с Алгол 68
• Algol 68 – 25 Years in the USSR
• Система программ динамической поддержки для транслятора с Алгола 68de:Algol 68

ru:Алгол 68