ALGOL 68

ALGOL 68

File:Algol68RevisedReportCover.jpg Revised Report on the Algorithmic Language - ALGOL 68 Edited by: A. van Wijngaarden & al, Sep 1973
Paradigm	multi-paradigm: concurrent, imperative
Appeared in	1968, last revised 1973
Designed by	A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster, et al.
Typing discipline	static, strong, safe, structural
Major implementations	ALGOL 68C, ALGOL 68G, ALGOL 68R, ALGOL 68RS, ALGOL 68S, FLACC, Алгол 68 Ленинград/Leningrad Unit
Dialects	ALGOL 68/FR (Final Report: 1968), Algol 68/RR (Revised Report: 1973)
Influenced by	ALGOL 60, ALGOL Y
Influenced	ALGOL 68C, C, C++, Bourne shell, Bash, Steelman, Ada, Python

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 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 designed by IFIP Working Group 2.1. On December 20th 1968 the language was formally adopted by Working Group 2.1 and subsequently approved for publication by the General Assembly of IFIP.

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 labeled "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: Completeness and clarity of design, Orthogonal design, Security, Efficiency: Static mode checking Mode-independent parsing Independent compilation Loop optimization Representations - in minimal & larger character sets

“ ALGOL 68 was the first (and possibly one of the last) major language for which a full formal definition was made before it was implemented. ” —C.H.A. Koster,

ALGOL 68 has been criticized, most prominently by some members of its design committee such as C. A. R. Hoare and Edsger Dijkstra, for abandoning the simplicity of ALGOL 60 becoming a vehicle for complex or overly general ideas, and doing little to make the compiler writer's task easy, in contrast to deliberately simple contemporaries (and competitors) such as C, S-algol and Pascal.

In the 1973 revision, certain features - such as proceduring, gommas and formal bounds - were omitted. c.f. The language of the unrevised report.

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, making its use obligatory for U.S. defense contracts.

Steve Bourne, who was on the Algol 68 revision committee, took some of its ideas to his Bourne shell (and thereby, to descendant shells such as Bash) and to C (and thereby to descendants such as C++).

The complete history of the project can be found in C.H. Lindsey's A History of ALGOL 68..

For a full length treatment of the language, see Programming Algol 68 Made Easy by Dr. Sian Leitch, or the Algol 68 Genie Documentation by Dr. Marcel van der Veer which includes the Revised Report.

Time-line of ALGOL 68

Year	Event	Contributor
Mar 1959	ALGOL Bulletin Issue 1 (First)	Peter Naur / ACM
Feb 1968	Draft Report(DR) Published	IFIP Working Group 2.1
Mar 1968	Algol 68 Final Report(FR) Presented at Munich Meeting	IFIP Working Group 2.1
Jun 1968	Meeting in Tirrenia, Italy	IFIP Working Group 2.1
Aug 1968	Meeting in North Berwick, Scotland	IFIP Working Group 2.1
Dec 1968	Algol 68 Final Report(FR) Presented at Munich Meeting	IFIP Working Group 2.1
Apr 1970	ALGOL 68R(R) under GEORGE 3 on an ICL 1907F.	Royal Signals and Radar Est.
Sep 1973	Revised Report (RR)Published	IFIP Working Group 2.1
1975	ALGOL 68C(C) - transportable compiler (zcode VM)	S. Bourne, Andrew Birrell, and Mike Guy
Jun 1977	Strathclyde ALGOL 68 conference, Scotland	ACM
May 1978	Proposals for ALGOL H - A Superlanguage of ALGOL 68	A. P. Black, V. J. Rayward-Smith
1980	ALGOL 68+(+) Super Language	Lambert Meertens & van Vliet
1984	Full ALGOL 68S(S) compiler for Sun, Sparc, and PCs	C.H.Lindsey ea, Manchester
Aug 1988	ALGOL Bulletin Issue 52 (last)	Ed. C. H. Lindsey / ACM
May 1997	Algol68 S(S) published on the internet	Charles H. Lindsey
Nov 2001	ALGOL 68G(G) released (GNU GPL open source licensing)	Marcel van der Veer

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

The Algorithmic Language ALGOL 68 Reports

• Mar. 1968: Draft 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.	”

• Oct. 1968: Penultimate Draft Report on the Algorithmic Language ALGOL 68 - Chapters 1-9 Chapters 10-12 - Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster.
• Dec. 1968: Report on the Algorithmic Language ALGOL 68 - Offprint from Numerische Mathematik, 14, 79-218 (1969); Springer-Verlag. - Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster.
  • WG 2.1 members active in the original design of ALGOL 68:. Fritz Bauer • Hans Bekic • Edsger Dijkstra† • Fraser Duncan† • Jan Garwick† • Gerhard Goos • Tony Hoare† • P.Z. Ingerman • Kees Koster • Peter Landin • Charles Lindsey • Barry Mailloux • John McCarthy • J.N. Merrier • Peter Naur‡ • Manfred Paul • John Peck • Willem van der Poel • Brian Randell† • Doug Ross • Klaus Samelson • Gerhard Seegmüller† • Michel Sintzoff • W.M. Turski† • Aad van Wijngaarden • Niklaus Wirth‡ • Mike Woodger† • Nobuo Yoneda; †Signatories to the Minority Report. ‡Resigned after [MR 93].
• Sep 1973: Revised Report on the Algorithmic Language Algol 68 - Springer-Verlag 1976 - 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.

Standardization

On December 20, 1968 the "Final Report" (MR 101) was adopted by the Working Group, then subsequently approved by the General Assembly of IFIP for publication. Translations were made for Russian, German, French and Bulgarian. Subsequently ALGOL 68 became one of the GOST standards in Russia.

• GOST 27974-88 Programming language ALGOL 68 - Язык программирования АЛГОЛ 68
• GOST 27975-88 Programming language ALGOL 68 extended - Язык программирования АЛГОЛ 68 расширенный

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

This article contains Unicode 5.2 "Miscellaneous Technical" characters. Without proper rendering support, you may see question marks, boxes, or other symbols instead of something like "10" (Decimal Exponent Symbol U+23E8).

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

int n = 2;
co n is a fixed constant of 2.co
int m := 3;
co m is a newly created local variable whose value is initially set to 3. 
    This is short for ref int m = loc int := 3; co
real avogadro = 6.0221415⏨23; 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 identifier 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 struct { 
     double a, b, c; short i, j, k; float *r;
 } newtype[9+1][max+1];

Note that for ALGOL 68 only the newtype mode-indication appears to the left of the equals symbol, 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

pr & co: Pragmats 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 ignored even within ALGOL 68 identifiers (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 (and somewhat reverent) 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 )

This scheme not only avoids the dangling else problem but also avoids having to use begin and end in embedded statement sequences.

• 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 )

Example:

proc days in month = (int year, month)int: 
  (month|31,(year%×4=0 ∧ year%×400≠0|29|28),31,30,31,30,31,31,30,31,30,31)

Algol68 allowed the switch to be of either type int or (uniquely) union. The latter allows the enforcing strong typing onto union variables. c.f. union below for example.

• 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 was considered the "universal" loop, the full syntax is:

for i from 1 by 1 to 3 while i≠4 do ~ od

There are several unusual aspects of the construct:

• • only the do ~ od portion was compulsory, in which case the loop will iterate indefinitely.
  • thus the clause to 100 do ~ od, will iterate only 100 times.
  • the while "syntactic element" allowed a programmer to break from a for loop early. eg

int sum sq:=0;
for i
while 
  print(("So far:",i,newline));
  sum sq≤1000 
do 
  sum sq+:=i↑2
od

Subsequent "extensions" to the standard Algol68 allowed the to syntactic element to be replaced with upto and downto to achieve a small optimisation. The same compilers also incorporated:

• until^(C) - for late loop termination.
• foreach^(S) - for working on arrays in parallel.

Further examples can be found in the code examples below.

struct, union & [:]: Structures, unions and arrays

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

 mode vector = [1:3]    real;   # vector mode declaration (typedef)  #
 mode matrix = [1:3,1:3]real;   # matrix mode declaration (typedef)  #
 vector v1  := (1,2,3);         # array variable initially (1,2,3)   #
 []real v2   = (4,5,6);         # constant array, type equivalent to vector, bounds are implied  #
 op + = (vector a,b) vector:    # binary operator definition         #
   (vector out; for i from ⌊a to ⌈a do out[i] := a[i]+b[i] od; out);
 matrix m := (v1, v2, v1+v2);  
 print ((m[,2:]));              # a slice of the 2nd and 3rd columns #

Matrices can be sliced either way, eg:

 ref vector row = m[2,];  # define a ref (pointer) to the 2nd row #
 ref vector col = m[,2];  # define a ref (pointer) to the 2nd column #

ALGOL 68 supports multiple field structures (struct) and united modes. Reference variables may point to any mode including 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),
      list = struct (node val, ref list next);

Usage example for union case of node:

Algol68 as in the 1968 Final Report node n := "1234"; real r; int i; compl c; string s case r,i,c,s::=n in print(("real:", r)), print(("int:", i)), print(("compl:", c)), print(("string:", s)) out print(("?:", n)) esac

Algol68 as in the 1973 Revised Report node n := "1234";   case n in (real r): print(("real:", r)), (int i): print(("int:", i)), (compl c): print(("compl:", c)), (string s): print(("string:", s)) 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

priority	Algol68 "Worthy characters"	+Algol68RR	+Algol68C,G	+Algol68FR
10	not, up, down, lwb, upb, -, abs, arg, bin, entier, leng, level, odd, repr, round, shorten	¬, ↑, ↓, ⌊, ⌈	~	⎩, ⎧

Standard dyadic operators with associated priorities

priority	Algol68 "Worthy characters"	+Algol68RR	+Algol68C,G	+Algol68FR
9	+*, i	+×, ⊥		!
8	shl, shr, **, up, down, lwb, upb	↑, ↓, ⌊, ⌈		⎩, ⎧
7	*, /,  %, over,  %*, mod, elem	×, ÷, ÷×, ÷*, %×, □		÷:
6	-, +
5	<, lt, <=, le, >=, ge, >, gt	≤, ≥
4	=, eq, /=, ne	≠	~=
3	&, and	∧
2	or	∨
1	minusab, plusab, timesab, divab, overab, modab, plusto, -:=, +:=, *:=, /:=, %:=, %*:=, +=:	×:=, ÷:=, ÷×:=, ÷*:=,  %×:=		÷::=

Assignation and identity relations etc

These are technically not operators, rather they are considered "units associated with names"

priority	Algol68 "Worthy characters"	+Algol68RR	+Algol68C	+Algol68FR
effectively 0	:=, =:, = , :=:, :/=:, is, isnt, of, at	:≠:, :	:~=:	ct, ::, ctab, ::=, .., is not, →, @

“:=:” (alternatively “is”) tests if two pointers are equal; “:/=:” (alternatively “isnt”) tests if they are unequal.

Why :=: and :/=: are needed: Consider trying to compare two pointer values, such as the following variables, declared as pointers-to-integer:

ref int ip, jp

Now consider how to decide whether these two are pointing to the same location, or whether one of them is pointing to nil. The following expression

ip = jp

will dereference both pointers down to values of type int, and compare those, since the “=” operator is defined for int, but not ref int. It is not legal to define “=” for operands of type ref int and int at the same time, because then calls become ambiguous, due to the implicit coercions that can be applied: should the operands be left as ref int and that version of the operator called? Or should they be dereferenced further to int and that version used instead? Therefore the following expression can never be made legal:

ip = nil

Hence the need for separate constructs not subject to the normal coercion rules for operands to operators. But there is a gotcha. The following calls:

ip :=: jp

ip :=: nil

while legal, will probably not do what you expect. They will always return false, because they are comparing the actual addresses of the variables ip and jp, rather than what they point to. To achieve the right effect, you have to write

ip :=: ref int(jp)

ip :=: ref int(nil)

Patent application: On 14 May 2003, software patent application No. 20040230959 was filed for the ISNOT operator by employees of Microsoft. This patent was granted on 18 November 2004.

Special characters

This article contains Unicode 5.2 "Miscellaneous Technical" characters. Without proper rendering support, you may see question marks, boxes, or other symbols instead of something like "10" (Decimal Exponent Symbol U+23E8).

Most of Algol's special characters (×, ÷, ≤, ≥, ≠, ¬, ∨, ∧, ⏨, →, ↓, ↑, □, ⌊, ⌈, ⎩, ⎧, ○ and ⊥) can be found on the IBM 2741 keyboard with the APL "golf-ball" print head inserted, these became available in the mid 1960s while ALGOL 68 was being drafted:

File:APL-keybd2.svg

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))

• Format syntax in ALGOL 68G

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).

int initial foot width = 5;
mode foot = struct(
   string name,
   sema width,
   bits toe ¢ packed vector of BOOL ¢ 
);
 
foot left foot:= foot("Left", level initial foot width, 2r11111), 
     right foot:= foot("Right", level initial foot width, 2r11111);
 
¢ 10 round clip in a 1968 Colt Python .357 Magnum ¢ 
sema rounds = level 10;
 
¢ the Magnum needs more barrels to take full advantage of parallelism ¢ 
sema acquire target = level 1;
 
prio ∧:= = 1;
op ∧:= = (ref bits lhs, bits rhs)ref bits: lhs := lhs ∧ rhs;
 
proc shoot = (ref foot foot)void: (
  ↓acquire target; 
  ↓rounds; 
  print("BANG! ");
  ↓width → foot; 
  toe → foot ∧:= ¬(bin 1 shl level width → foot);
  printf(($g": Ouch!! - "5(g)l$, name → foot, []bool(toe → foot)[bits width - initial foot width + 1:]));
  ↑acquire target
);
 
¢ do shooting in parallel to cater for someone hoping to stand on just one foot ¢ 
par (
  for toe to initial foot width do
    shoot(left foot)
  od, ¢ <= initial this comma is important ¢ 
  for toe to initial foot width do
    shoot(right foot)
  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 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; # the POINT stropping style #
 INT a real int = 3;# the UPPER stropping style #
 int a_real_int = 3;# the RES stropping style, there are 61 accepted reserved words #

All implementations must recognise at least POINT, UPPER and RES inside PRAGMAT sections.

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

• ^ Worthy 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".

Example of different program representations

Algol68 as typically published ¢ bold/underline typeface ¢ mode xint = int; xint sum sq:=0; for i while sum sq≠70×70 do sum sq+:=i↑2 od

quote stropping (similar to wiki) 'pr' quote 'pr' 'mode' 'xint' = 'int'; 'xint' sum sq:=0; 'for' i 'while' sum sq≠70×70 'do' sum sq+:=i↑2 'od'

Code for a 7-bit/ascii compiler .PR UPPER .PR MODE XINT = INT; XINT sum sq:=0; FOR i WHILE sum sq/=70*70 DO sum sq+:=i**2 OD

Code for a 6-bits/byte compiler .PR POINT .PR .MODE .XINT = .INT; .XINT SUM SQ:=0; .FOR I .WHILE SUM SQ .NE 70*70 .DO SUM SQ .PLUSAB I .UP 2 .OD

Algol68 using RES stropping .PR RES .PR mode .xint = int; .xint sum sq:=0; for i while sum sq≠70×70 do sum sq+:=i↑2 od

ALGOL 68 allows for every natural language to define its own set of keywords Algol-68. As a result, programmers are able to write programs using keywords from their native language. Below is an example of a simple procedure that calculates "the day following", the code is in two languages: English and German.

 # Next day date - English variant #
 mode date = struct(int day, string month, int year);
 proc the day following = (date x) date:
      if day of  x < length of month (month of x, year of x)
      then (day of x + 1, month of x, year of x)
      elif month of x = "December"
      then (1, "January", year of x + 1)
      else (1, successor of month (month of x), year of x)
      fi;

 # Nachfolgetag - Deutsche Variante #
 menge datum = tupel(ganz tag, wort monat, ganz Jahr);
 funktion naechster tag nach = (datum x) datum:
          wenn tag von x < monatslaenge(monat von x, jahr von x)
          dann (tag von x + 1, monat von x, jahr von x)
          wennaber monat von x = "Dezember"
          dann (1, "Januar", jahr von x + 1)
          ansonsten (1, nachfolgemonat(monat von x), jahr von x)
          endewenn;      

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 an unspecified reference 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 standards reports procedure raising an exception in the runtime system.
ℵ - Used in the standards report to inhibit introspection of certain types. eg sema

c.f. below for other examples of ℵ.

The term nil is var always evaluates to true for any variable (but see above for correct use of is :/=:), 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.

Both official reports included some advanced features that were not part of the standard language. This were indicated with an ℵ and considered effectively private. Examples include "≮" and "≯" for templates, the outtype/intype for crude duck typing, and the straightout and straightin operators for "straightening" nested arrays and structures.

Extract from the 1973 report:

§10.3.2.2. Transput modes
a) mode ℵ simplout = union (≮ℒ int≯, ≮ℒ real≯, ≮ℒ compl≯, bool, ≮ℒ bits≯, 
           char, [ ] char);
b) mode ℵ outtype = ¢ an actual - declarer specifying a mode united 
   from a sufficient set of modes none of which is 'void' or contains 'flexible', 
   'reference to', 'procedure' or 'union of' ¢;
c) mode ℵ simplin = union (≮ref ℒ int≯, ≮ref ℒ real≯, ≮ref ℒ compl≯, ref bool, 
           ≮ref ℒ bits≯, ref char, ref [ ] char, ref string);
d) mode ℵ intype = ¢ ... ¢;

§10.3.2.3. Straightening
a) op ℵ straightout = (outtype x) [ ] simplout: ¢ the result of "straightening" 'x' ¢;
b) op ℵ straightin = (intype x) [ ] simplin: ¢ the result of straightening 'x' ¢;

Comparisons with other languages

• 1973 - Comparative Notes on Algol 68 and PL/I - S. H. Valentine - February 1973
• 1976 - Evaluation of ALGOL 68, Jovial J3B, PASCAL, SIMULA 67, and TACPOL Versus TINMAN - Requirements for a Common High Order Programming Language.
• 1977 - A comparison of PASCAL and ALGOL 68[2] - Andrew S. Tanenbaum - June 1977.
• 1993 - Five Little Languages and How They Grew - BLISS, Pascal, Algol 68, BCPL & C - Dennis M. Ritchie - April 1993.
• 2009 - On Go - oh, go on - How well will Google's Go stand up against Brand X programming language? - David Given - November 2009
• Comparison of ALGOL 68 and C++.

Variants

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

The language of the unrevised report

The original language^(FR) 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 = (bool a,proc bool b)bool:(a | b | false);
op orf = (bool a,proc bool b)bool:(a | true | b);

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 (aka Currying): creation of functions (with fewer 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 operations 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

Name	Year	Purpose	State	Description	Target CPU	Licencing
Generalized ALGOL	1962	Scientific	NL	ALGOL for generalised grammars
ALGOL YY	1966	Scientific	Intl	First version of Algol 68	Specification	ACM
ALGOL 68DR	1968	Scientific	Intl	IFIP WG 2.1 Draft Report	Specification - March	ACM
ALGOL 68FR	1968	Scientific	Intl	IFIP WG 2.1 Final Report	Specification - August	ACM
ALGOL 68RR	1970	Scientific	UK		ICL 1900
EPOS ALGOLE	1971	Scientific
ALGOL 68RSRS	1972	Scientific	UK	Portable compiler system	ICL 2900/Series 39, Multics, VMS & C generator	Open Source
Algol 68 with areasA	1972	Experimental & other	UK	Addition of areas to Algol 68
OREGANO	1973	Research	US	"The importance of implementation models."		UCLA
ALGOL 68CC	1975	Scientific	UK	Cambridge Algol 68	ICL, IBM 360, PDP 10 & Unix, Telefunken, Tesla	Cambridge
ALGOL 68 RevisedRR	1975	Scientific	Intl	IFIP WG 2.1 Revised Report	Specification	ACM
Algol HH	1975	Experimental & other	UK	Proposed extensions to the mode system of Algol 68	Specification
Algol 68 ORDAo	1976	practical uses	USSR		ODRA 1204/IL
FLACCF	1977	Multi-purpose	CA	Mailloux's Algol 68
ALGOL 68-RTRT	1979	Scientific	UK	Parallel ALGOL 68-R
RS Algolrs	1979	Scientific	UK
ALGOL 68++	1980	Scientific	NL	Superlanguage of ALGOL 68
ALGOL 68 LGUL	1980	Telecommuni- cations	USSR	Full Language + Modules	IBM, DEC, CAMCOH, PS 1001 & PC
Interactive ALGOL 68I	1983		UK	Incremental compilation	PC	Noncommercial shareware
ALGOL 68SS	1985	Scientific	Intl	Sun version of ALGOL 68	Sun-3, Sun Sparc (under SunOS 4.1 & Solaris 2), Atari ST (under GEMDOS), Acorn Archimedes (under RISCOS), VAX-11 under Ultrix-32
MK2 Interactive ALGOL 68	1992		UK	Incremental compilation	PC	Noncommercial shareware[3]
ALGOL 68GG	2001	Full Language	NL	Includes standard collateral clause	Interpreter in C	GPL
ALGOL 68G Version 1.18.0	2009	Full Language	NL		Interpreter in C	GPL

The S3 programming language that was used to write the ICL 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 39, 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, restricting operator definitions and omitting garbage collection, flexible rows and formatted transput.

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 does not fully conform 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."[4]

Quotes

• ... The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of. The central notion I captured from Algol was a type structure based on atomic types (including structures), composed into arrays, pointers (references), and functions (procedures). Algol 68's concept of unions and casts also had an influence that appeared later. Dennis Ritchie Apr 1993.
• ... 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
• "Congratulations, your Master has done it" - Niklaus Wirth
• The more I see of it, the more unhappy I become - E.W. Dijkstra, 1968
• [...] it was said that A68's popularity was inversely proportional to [...] the distance from Amsterdam - Guido van Rossum
• 1980 quote: '[...] 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"
• Original 1968 version: "[...] More than ever it will be required from an adequate programming tool that it assists, by structure, the programmer in the most difficult aspects of his job, viz. in the reliable creation of sophisticated programs. In this respect we fail to see how the language proposed here [Algol68] is a significant step forward: on the contrary, we feel that its implicit view of the programmer's task is very much the same as, say, ten years ago. This forces upon us the conclusion that, regarded as a programming tool, the language must be regarded as obsolete. [...]" Signed by: DIJKSTRA, DUNCAN, GARWICK, HOARE, RANDELL, SEEGMUELLER, TURSKI, WOODGER. And then on Dec. 23, 1968, Jan V. Garwick

See also

• ALGOL 60
• ALGOL Y
• ALGOL N
• ALGOL 68C
• ALGOL 68G

• C (programming language)
• C++
• Bourne shell
• Bash

• Steelman language requirements
• Ada (programming language)
• Python (programming language)

References

• Brailsford, D.F. and Walker, A.N., Introductory ALGOL 68 Programming, Ellis Horwood/Wiley, 1979
• Lindsey, C.H. and van der Meulen, S.G., Informal Introduction to ALGOL 68, North-Holland, 1971
• 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
• Woodward, P.M. and Bond, S.G., ALGOL 68-R Users^sic Guide, London, Her Majesty's Stationery Office, 1972

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
• Revised Report on the Algorithmic Language ALGOL 68 Enhanced HTML version, based on the version above
• A Tutorial on Algol 68, by Andrew S. Tanenbaum, in Computing Surveys, Vol. 8, No. 2, June 1976, with Corrigenda (Vol. 9, No. 3, September 1977)
• 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 Алгол 68
• C history with Algol68 heritage