ALGOL 68

[[File:Algol68.png|64px|left|alt=Language|link=https://sourceforge.net/projects/algol68]] 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.

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.

Execute an ALGOL 68 program online

Grammar

The grammar for ALGOL 68 is officially in the two level, Van Wijngaarden grammar but a subset has been done in the one level Backus–Naur Form:

Resources

FYI: There are two online manual pages:

Or - if you prefer a hardcopy - you can try and pick up a hard cover manual like "''Informal Introduction to Algol 68''" - by C. H. Lindsey & S. V. Vander Meulen. Be sure to get the 1977 edition:

Editor modes:

Status

  • 20th December 1968 - '''ALGOL 68's''' Final Report was ratified by UNESCO's IFIP working group 2.1 in Munich.
  • 20th December 2008 - Zig Zag - the 100th '''ALGOL 68''' code contribution on rosettacode.org! ** Happy 40th Birthday '''ALGOL 68''', ** AND 50th Birthday '''ALGOL 58'''.
  • 23rd August 2009 - algol68g-1.18.0-9h released
  • 20th December 2009 - Happy 51st/41st Birthdays with Hamming numbers - the 200th '''ALGOL 68''' code contribution on rosettacode.org! ** This time code was by Marcel van der Veer, author of Algol 68 Genie
  • 25th October 2011 - Jejones3141 added Soundex - the 300th '''ALGOL 68''' code specimen.

Revisions

*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. *Oct. 1968: Penultimate Draft Report on the Algorithmic Language ALGOL 68 - Chapters 1-9 - 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. *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.

Code samples

Most of the code samples provided here have a leading main:( and a matching ) at the end. These are not actually required in the language, but are included so as to highlight that the code sample is complete, and works with (at least) ALGOL 68G unmodified.

On some compilers, it may be necessary to include appropriate "job cards" or precludes in order for the programs to compile successfully. Hopefully not too much else is required. Examples: {|border="1" style="border-collapse: collapse; border: 5px double grey;" align="center" || Brief Algol68 || Algol68 as in rosettacode || Actual ELLA Algol 68RS code |- || print(("Hello, world!",new line)) || main:( print(("Hello, world!",new line)) ) || PROGRAM helloworld CONTEXT VOID USE standard BEGIN print(("Hello, world!", new line)) END FINISH |}

Example of different program representations

At the time when ALGOL 68 was defined some predominant computers had 24 or 36 bit words, with 6 bit character sets. Hence it was desirable that ALGOL 68 should be able to run on machines with only uppercase. The official spec provided for different representations of the same program. Quote stropping (enclosing the bold words in single quotes) and Point stropping (preceeding the bold words with a dot) were used. A variant of Point stropping called RES stropping was also defined. In RES stropping some language-defined bold words are not preceded by a dot. A pragmatic comment may have been required to indicate which stropping convention was to be used, as in some of the examples below. Upper stropping (representing the bold words by upper case and non-bold words in lower case) was introduced by Algol 68R. Upper stropping is used by Algol 68RS and is one of the options for Algol 68G. Rutgers ALGOL 68 uses quote stropping. Most of the samples on Rosetta Code use Upper stropping. Example: {|border="1" style="border-collapse: collapse; border: 5px double grey;" align="center" || 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' || 7-bit/ascii compiler .PR UPPER .PR MODE XINT = INT; XINT sum sq:=0; FOR i WHILE sum sq/=7070 DO sum sq+:=i**2 OD || 6-bits/byte compiler .PR POINT .PR .MODE .XINT = .INT; .XINT SUM SQ:=0; .FOR I .WHILE SUM SQ .NE 7070 .DO SUM SQ .PLUSAB I .UP 2 .OD || 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 || Upper stropping

upper case = bold

MODE XINT = INT; XINT sum sq:=0; FOR i WHILE sum sq /= 70*70 DO sum sq PLUSAB i UP 2 OD |}

Coercion (casting)

ALGOL 68 has a hierarchy of contexts which determine which kind of coercions are available at a particular point in the program. These contexts are: {|class="wikitable" !rowspan=2| N

a

m

e !rowspan=2| Context location !colspan=5| Coercions available in this context !rowspan=2| Coercion examples |- |bgcolor=aaaaff|Soft |bgcolor=aaeeaa|Weak |bgcolor=ffee99|Meek |bgcolor=ffcc99|Firm |bgcolor=ffcccc|Strong |- !S

t

r

o

n

g ||Right hand side of:

  • Identity-declarations, as "~" in:
REAL x = ~
  • Initialisations, as "~" in:
REAL x := ~

Also:

  • Actual-parameters of calls, as "~" in:
PROC: sin(~)
  • Enclosed clauses of casts, as "~" in:
REAL(~)
  • Units of routine-texts
  • Statements yielding VOID
  • All parts (but one) of a balanced clause
  • One side of an identity relation, as "~" in:
 ~ IS ~

|bgcolor=aaaaff rowspan=4 width="50px"| deproc- eduring |bgcolor=aaeeaa rowspan=3 width="50px"| all '''soft''' then weak deref- erencing |bgcolor=ffee99 rowspan=2 width="50px"| all '''weak''' then deref- erencing |bgcolor=ffcc99 rowspan=1 width="50px"| all '''meek''' then uniting |bgcolor=ffcccc width="50px"| all '''firm''' then widening, rowing and voiding |colspan=1 bgcolor=ffcccc| Widening occurs if there is no loss of precision. For example: An INT will be coerced to a REAL, and a REAL will be coerced to a LONG REAL. But not vice-versa. Examples:

INT to LONG INT
INT to REAL
REAL to COMPL
BITS to []BOOL
BYTES to STRING

A variable can also be coerced (rowed) to an array of length 1.

For example:

INT to [1]INT
REAL to [1]REAL

etc |- !F

i

r

m || *Operands of formulas as "~" in:

OP: ~ * ~

*Parameters of transput calls |colspan=3 bgcolor=ffcc99| Example:

UNION(INT,REAL) var := 1

|- !M

e

e

k ||

  • Trimscripts (yielding INT)
  • Enquiries: e.g. as "~" in the following
IF ~ THEN ... FI

and

FROM ~ BY ~ TO ~ WHILE ~ DO ... OD etc
  • Primaries of calls (e.g. sin in sin(x)) |colspan=4 bgcolor=ffee99|Examples:
REF REF BOOL to BOOL
REF REF REF INT to INT

|- !W

e

a

k ||

  • Primaries of slices, as in "~" in:
~[1:99]
  • Secondaries of selections, as "~" in:
value OF ~

|colspan=5 bgcolor=aaeeaa|Examples:

REF BOOL to REF BOOL
REF REF INT to REF INT
REF REF REF REAL to REF REAL
REF REF REF REF STRUCT to REF STRUCT

|- !S

o

f

t || The LHS of assignments, as "~" in:

~ := ...

|colspan=6 bgcolor=aaaaff| Example:

  • deproceduring of:
PROC REAL random: e.g. random

|} For more details about Primaries and Secondaries refer to [[Operator_precedence#ALGOL_68|Operator precedence]].

See also

*[[Web 68]] *[[ALGOL 60]] *[[ALGOL W]]

Code Specimen

Merged content

An implementation of BF in [[ALGOL 68]] for Rosetta Code.

MODE BYTE = SHORT SHORT SHORT INT;

 MODE CADDR = BYTE; # code address #
 MODE OPCODE = BYTE;
 OPCODE nop = 0;

 MODE DADDR = BYTE; # data address #
 MODE DATA = BYTE;
 DATA zero = 0;

 PROC run = ([] OPCODE code list)VOID:(
   [-255:255]DATA data list;  # finite data space #
   FOR i FROM LWB data list TO UPB data list DO data list[i] := zero OD;

   DADDR data addr := ( UPB data list + LWB data list ) OVER 2;
   CADDR code addr := LWB code list;

   [0:127]OPCODE assembler; # 7 bit ascii only #

   STRING op code list="><+-.,[]";
   []PROC VOID op list= []PROC VOID(
     # ? # VOID: SKIP, # NOP #
     # > # VOID: data addr +:= 1,
     # < # VOID: data addr -:= 1,
     # + # VOID: data list[data addr] +:= 1,
     # - # VOID: data list[data addr] -:= 1,
     # . # VOID: print(REPR data list[data addr]),
     # , # VOID: data list[data addr]:=ABS read char,
     # [ # VOID:
             IF data list[data addr] = zero THEN
               # skip to the end of the loop, allowing for nested loops #
               INT br level := 0;
               WHILE
                 IF   code list[code addr] = ABS "["
                 THEN
                   br level +:= 1;
                 ELIF code list[code addr] = ABS "]"
                 THEN
                   br level -:= 1
                 FI;
                 IF br level > 0
                 THEN
                   code addr +:= 1;
                   TRUE
                 ELSE
                   FALSE
                 FI
               DO SKIP OD
             FI,
     # ] # VOID:
             IF data list[data addr] /= zero THEN
               # skip to the start of the loop, allowing for nested loops #
               INT br level := 0;
               WHILE
                 IF   code list[code addr] = ABS "["
                 THEN
                   br level +:= 1
                 ELIF code list[code addr] = ABS "]"
                 THEN
                   br level -:= 1
                 FI;
                 code addr -:= 1;
                 br level < 0
               DO SKIP OD
             FI
   )[:@0];

   FOR op TO UPB assembler DO assembler[op] := nop OD; # insert NOP #
   FOR op TO UPB op code list DO assembler[ABS op code list[op]] := op OD;

   WHILE code addr <= UPB code list DO
     op list[ABS assembler[ABS code list[code addr]]];
     code addr +:= 1
   OD
 );

 STRING code list := "++++++[>+++++++++++<-]>-."; # print the ascii letter A #

 [UPB code list]BYTE byte code list; # convert to bytes #
 FOR i TO UPB code list DO byte code list[i] := ABS code list[i] OD;

 run(byte code list)

Tasks