ALGOL 68C#The ENVIRON and USING clauses

A very early predecessor of this compiler was used by Guy and Bourne to write the first Game of Life programs on the PDP-7 with a DEC 340 display.{{cite web |url=http://www.aceware.iinet.net.au/acms/ItemDetail.asp?lngItemId=175 |title=Item Detail: Digital Equipment Corporation Model 340 |website=Australian Computer Museum Society (ACMS) |publisher=AceWare Web Hosting |access-date=2020-04-17 }}{{Dead link|date=March 2023 |bot=InternetArchiveBot |fix-attempted=yes }}

Various Liverpool Software Gazette issues detail the Z80 implementation. The compiler required about 120 KB of memory to run; hence the Z80's 64 KB memory is actually too small to run the compiler. So ALGOL 68C programs for the Z80 had to be cross-compiled from the larger CAP computer, or an IBM System/370 mainframe computer.

Stephen Bourne subsequently reused ALGOL 68's if ~ then ~ else ~ fi, case ~ in ~ out ~ esac and for ~ while ~ do ~ od clauses in the common Unix Bourne shell, but with in's syntax changed, out removed, and od replaced with done (to avoid conflict with the od utility).

After Cambridge, Bourne spent nine years at Bell Labs with the Version 7 Unix (Seventh Edition Unix) team. As well as developing the Bourne shell, he ported ALGOL 68C to Unix on the DEC PDP-11-45 and included a special option in his Unix debugger Advanced Debugger (adb) to obtain a stack backtrace for programs written in ALGOL 68C. Here is an extract from the Unix 7th edition manual pages:{{cite web |url=http://modman.unixdev.net/?sektion=1&page=adb&manpath=v7man |title=The Modular Manual Browser: Adb |website=UnixDev.net |url-status=dead |archive-url=https://web.archive.org/web/20160303175742/http://modman.unixdev.net/?sektion=1&page=adb&manpath=v7man |archive-date=2016-03-03 |access-date=2020-04-17}}

NAME

adb - debugger

SYNOPSIS

adb [-w] [ objfil [ corfil ] ]

[...]

COMMANDS

[...]

$modifier

Miscellaneous commands. The available modifiers

are:

[...]

a ALGOL 68 stack backtrace. If address is

given then it is taken to be the address of

the current frame (instead of r4). If count

is given then only the first count frames

are printed.

Below is a sampling of some notable extensions:

• Automatic op:= for any operator, e.g. *:= and +:=
• UPTO, DOWNTO and UNTIL in loop-clauses;
• displacement operator (:=:=)
• ANDF, ORF and THEF syntactic elements.
• separate compilation - ENVIRON clause and USING clause
• scopes not checked
• bounds in formal-declarers
• CODE ... EDOC clause - for embedding ZCODE

Separate compilation in ALGOL 68C is done using the ENVIRON and USING clauses. The ENVIRON saves the complete environment at the point it appears. A separate module written starting with a USING clause is effectively inserted into the first module at the point the ENVIRON clause appears.

ENVIRON and USING are useful for a top-down style of programming, in contrast to the bottom-up style implied by traditional library mechanisms.

These clauses are kind of the inverse of the #include found in the C programming language, or import found in Python. The purpose of the ENVIRON mechanism is to allow a program source to be broken into manageable sized pieces. It is only necessary to parse the shared source file once, unlike a #include found in the C programming language where the include file needs to be parsed for each source file that includes it.

A file called mylib.a68:

BEGIN
INT dim = 3; # a constant #
INT a number := 120; # a variable #
ENVIRON EXAMPLE1;
MODE MATRIX = [dim, dim]REAL; # a type definition #
MATRIX m1;
a number := ENVIRON EXAMPLE2;
print((a number))
END

A file called usemylib.a68:

USING EXAMPLE2 FROM "mylib"
BEGIN
MATRIX m2; # example only #
print((a number)); # declared in mylib.a68 #
print((2 UPB m1)); # also declared in mylib.a68 #
ENVIRON EXAMPLE3;  # ENVIRONs can be nested #
666
END

• No ALGOL 68 FLEX and variable length arrays
• MODE STRING implemented without FLEX
• The PAR parallel clause was not implemented
• Nonstandard transput
• others...

A translator–compiler for ALGOL 68C was available for the PDP-10, IBM System/360 and several other computers.

{{cite book |last=Bourne |first=Stephen R. |author-link=Stephen R. Bourne |last2=Birrell |first2=Andrew D. |last3=Walker |first3=Ian |title=ALGOL 68C reference manual |publisher=Cambridge University Computer Laboratory |date=1975}}

{{cite magazine |magazine=Scientific American |number=223 |date=October 1970 |pages=120–123 |title=The fantastic combinations of John Conway's new solitaire game "life" |department=Mathematical Games |first=Martin |last=Gardner |url=https://web.stanford.edu/class/sts145/Library/life.pdf |quote=For long-lived populations such as this one Conway sometimes uses a PDP-7 computer with a screen on which he can observe the changes [...] The program was written by M. J. T. Guy and S. R. Bourne. Without its help some discoveries about the game would have been difficult to make.}}

• {{Official website|bitbucket.org/algol68c/dl}}
• [https://web.archive.org/web/20050724183114/http://hopl.murdoch.edu.au/showlanguage.prx?exp=667 Cambridge ALGOL 68: on the historical roster of computer languages] – includes 10+ publication references.
• [http://portal.acm.org/ft_gateway.cfm?id=807148&type=pdf A Transportation of ALGOL 68C – PJ Gardner, University of Essex] – March 1977 (From 370 to DECsystem-10)
• [https://jmvdveer.home.xs4all.nl/en.post.algol-68-mvs.html Running ALGOL 68C on MVS] - how to install ALGOL 68C on an emulated MVS system

{{ALGOL programming}}

{{DEFAULTSORT:Algol 68c}}

Cambridge CHAOS CAP computer

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