This manual describes how to use ga68, the GNU compiler for
Algol 68. This manual is specifically about how to invoke
ga68, as well as its features. For more information about
the Algol 68 language in general, the reader is referred to the
bibliography.
The ga68 command is the GNU compiler for the Algol 68 language and
supports many of the same options as gcc. See Option Summary in Using the GNU Compiler Collection (GCC).
This manual only documents the options specific to ga68.
The following options control how the compiler handles certain dialect variations of the language.
-std=std ¶Specify the standard to which the program is expected to conform, which may be one of ‘algol68’ or ‘gnu68’. The default value for std is ‘gnu68’, which specifies a strict super language of Algol 68 allowing GNU extensions. The ‘algol68’ value specifies that the program strictly conform to the Revised Report.
-fstropping=stropping_regimeSpecify the stropping regime to expect in the input programs. The default value for stropping_regime is ‘supper’, which specifies the modern SUPPER stropping which is a GNU extension. The ‘upper’ value specifies the classic UPPER stropping of Algol 68 programs. See Stropping regimes.
-fbracketsThis option controls whether [ .. ] and { .. } are
considered equivalent to ( .. ). This syntactic variation is
blessed by the Revised Report and is still strict Algol 68.
This option is disabled by default.
These options specify directories to search for files, libraries, and other parts of the compiler:
-Idir ¶Add the directory dir to the list of directories to be searched
for files when processing the pragmat include. Multiple
-I options can be used, and the directories specified are
scanned in left-to-right order, as with gcc.
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there is likely to be a bug in the program. Unless -Werror is specified, they do not prevent compilation of the program.
-Wvoiding ¶Warn on non-void units being voided due to a strong context.
-WscopeWarn when a potential name scope violation is found.
-Whidden-declarationsWarn when a declaration hides another declaration in a larger reach.
-WextensionsWarn when a non-portable Algol 68 construct is used, like GNU extensions to Algol 68.
These options affect the runtime behavior of programs compiled with
ga68.
-fno-a68-assert ¶Turn off code generation for ASSERT contracts.
-fa68-nil-checking ¶Turn on code generation of run-time checks for NIL while dereferencing.
-fno-a68-bounds-checking ¶Turn off code generation of run-time array bounds checks while slicing.
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
-shared-libga68 ¶On systems that provide libga68 as a shared and a static library, this option forces the use of the shared version. If no shared version was built when the compiler was configured, this option has no effect.
-static-libga68 ¶On systems that provide libga68 as a shared and a static library, this option forces the use of the static version. If no static version was built when the compiler was configured, this option has no effect. This is the default.
This chapter documents how to compose full Algol 68 programs using the modules and separated compilation support provided by this compiler.
Each Algol 68 source file contains a packet. Packets therefore play the role of compilation units, and each packet can be compiled separately to an object file. A set of compiled object files can then be linked in the usual fashion into an executable, archive or shared object by the system linker, without the need of any language-specific link editor or build system.
This compiler supports three different kind of packets:
main function of other languages like C.
See Particular programs.
access construct. Prelude packets
are so-called because their contents get stuffed in the
user-prelude in the case of user-defined modules, or the
library-prelude in the case of module packets provided by the
compiler. They are usually used to compose libraries that can be used
in a bottom-up fashion.
See Modules.
egg construct, that can be stuffed in a matching
formal hole in another package via a nest construct.
Formal holes are used in order to achieve separated compilation in a
top-bottom fashion, and also to invoke procedures written in other
languages, such as C functions or Fortran subroutines.
See Holes.
A collection of packets, all of which must be compatible with each other, constitutes either a program or a library. Exactly one of the packets constituting a program shall be a particular program. In libraries at least one packet must be a prelude packet.
Definition modules, often referred as just modules in the sequel, fulfill two different but related purposes. On one side, they provide some degree of protection by preventing accessing indicators defined within the module but not explicitly publicized. On the other, they allow to define interfaces, allow separated compilation based on these interfaces, and conform libraries.
Modules are usually associated with bottom-up development strategies, where several already written components are grouped and combined to conform bigger components.
A definition module is a construct that provides access to a set of publicized definitions. They can appear anywhere, but are typically found in the outer reach and compiled separately, in which case they conform a prelude packet (see Packets). They are composed of a prelude and a postlude. The publicized definitions appear in the module’s prelude.
Consider for example the following definition module, which implements a very simple logging facility:
module logger =
def int fd = stderr;
pub string originator;
pub proc log = (string msg) void:
fputs (fd, (originator /= "" | ": ") + msg + "\n");
log ("beginning of log\n");
postlude
log ("end of log\n");
fed
The module text delimited by def and fed
gets ascribed to the module indicator logger. Module
indicators are bold tags. Once defined, the module logger
is accessible anywhere within its reach.
The prelude of the module spans from def to either
postlude, or to fed in case of modules not
featuring a postlude. It consists on a restricted serial clause in a
void strong context, which can contain units and declarations, but no
labels or completers. The declarations in the prelude may be either
publicized or no publicized. As we shall see, publicized indicators
are accessible within the reach of the defining module publicizing
them. Publicized declarations are marked by preceding them with
pub.
In our example the module prelude consists on three declarations and
one unit. The indicator fd is not publicized and is to be used
internally by the module. The indicators originator and
log, on the other hand, are publicized and conform the
interface of the module. Note how the range of the prelude also
covers the postlude: the log procedure is reachable there, as
it would be fd as well.
The postlude of the module is optional and spans from
postlude to fed. It consists on a serial clause
in a void strong context, where definitions, labels and
module accesses are not allowed, just units.
Once a module is defined (see Writing modules) it can be accessed, provided it is within reach, using an access clause. The access clause identifies the modules to access and then makes the publicized definitions of these modules visible within a control clause.
For example, this is how we could use the logger definition module defined in a previous section to log the progress of some program that reads an input file and writes some output file:
access logger
begin # Identify ourselves with the program name #
originator := argv (1);
# Read input file. #
if not parse input (argv (2))
then log ("error parsing input file"); stop fi;
# Write output file. #
if not write output (argv (3))
then log ("error writing output file"); stop fi;
log ("success")
end
In this case the controlled clause is the closed clause conforming the
particular program, and the definitions publicized by the logger
module, in this case originator and log, can be used
within it.
An access clause is not restricted to just provide access to a single module: any number of module indicators can be specified in an access clause. Suppose that our example processing program has to read and write the data in JSON format, and that a suitable JSON library is available in the form of a reachable module. We could then make both logger and json modules accessible like this:
access logger, json
begin # Identify ourselves with the program name #
originator := argv (1);
jsonval data;
# Read input file. #
if data := json from file (argv (2));
data = json no val
then log ("error parsing input file"); stop fi;
# Write output file. #
if not json to file (argv (3), data)
then log ("error writing output file"); stop fi;
log ("success")
end
In this version of the program the access clause includes the module
indicator json, and that makes the mode indicator
jsonval and the tags json no val, json
from file and json to file visible within the program’s
closed clause.
Note that the following two access clauses are not equivalent:
access logger, json c ... c; access logger access json c ... c;
In the first case, a compilation time error is emitted if there is a
conflict among the publicized definitions of both modules; for
example, if both modules were to publicize a procedure called
log. In the second case, the declaration of log
publicized by logger would hide the declaration of
log publicized by json. This also has implications
related to activation, that we will be discussing in a later section.
The controlled clause in an access clause doesn’t have to be a serial clause, like in the examples above. It can be any enclosed clause, like for example a loop clause:
proc frobnicate frobs = ([]frob frobs) void:
access logger to upb frobs
do log ("frobnicating " + name of frob);
frobnicate (frob)
od
The elaboration of an access clause yields a value, which is the value yielded by the elaboration of the controlled clause. Since the later is an enclosed clause, coercions get passed into them whenever required, the usual fashion.
We can see an example of this in the following procedure, whose body
is a controlled closed clause that yields a real value:
proc incr factor = (ref[]real factors, int idx) real:
access logger (log ("factor increased"); factors[idx] +:= 1.0)
Note how the access clause above is in a strong context requiring a
value of mode real. The value yielded by the access clause
is the value yielded by the controlled enclosed clause, which in this
case is a closed clause. The strong context and required mode gets
passed to the last unit of the closed clause (the assignation) which
in this case yields a value of mode ref real. The unit
is coerced to real by dereferencing, and the resulting
value becomes the value yielded by the access clause.
A definition module may itself access other modules. This is done by placing the module text as a controlled clause of an access clause. Suppose we rewrite our logger module so it uses JSON internally to log JSON objects rather than raw strings. We could do it this way:
module logger =
access json
def int fd = stderr;
pub string originator;
pub proc log = (string msg) void:
fputs (fd, json array (json string (originator),
json string (msg)));
log (json string ("beginning of log\n"));
postlude
log (json string ("end of log\n"));
fed
Note how this version of logger uses a few definitions
publicized by the json module.
A program accessing logger will not see the definitions
publicized by the json module. If that is what we
intended, for example to allow the users of the logger to tweak their
own JSON, we would need to specify it this way:
module logger = access pub json def c ...as before... c fed
In this version the definitions publicized by json become
visible to programs accessing logger.
In all the examples seen so far the modules were accessed just once. In these cases, accessing the module via an access-clause caused the activation of the module.
Activating a module involves elaborating all the declarations and units that conform its prelude. Depending on the particular module definition that gets activated, this may involve all sort of side effects, such as allocating space for values and initializing them, opening files, etc. Once the modules specified in the access clause are successfully activated, the controlled clause gets elaborated itself, within the reach of all the publicized definitions by the activated modules as we saw in the last section. Finally, once the controlled clause has been elaborated, the module gets revoked by elaborating the serial clause in its postlude.
However, nothing prevents some given definition module to be accessed
more than once in the same program. The following program, that makes
use of the logger module, exemplifies this:
access logger
begin originator := argv (1);
log ("executing program");
c ... c
access logger (originator := argv (1) + ":subtask";
log ("doing subtask")
c ... c)
end
In this program the module logger is accessed twice. The
code is obviously written assuming that the inner access clause
triggers a new activation of the logger module, allocating
new storage and executing its prelude. This would result in the
following log contents:
a.out: beginning of log a.out: executing program a.out:subtask: beginning of log a.out:subtask: doing subtask a.out:subtask: end of log a.out: end of log
However, this is not what happens. The module gets only activated once, as the result of the outer access clause. The inner access clause just makes the publicized indicators visible in its controlled clause. The actual resulting log output is:
a.out: beginning of log a.out: executing program a.out:subtask: doing subtask a.out:subtask: end of log
Which is not what we intended. Modules are not classes. If we wanted the logger to support several originators that can be nested, we would need to add support for it in the definition module. Something like:
module logger =
def int fd = stderr, max originators = 10;
int orig := 0;
[max originators]string originators;
pub proc push originator = (string str) void:
(assert (orig < max originators);
orig +:= 1;
originators[orig] := str);
pub proc pop originator = void:
(assert (max originators > 0);
orig -:= 1);
pub proc log = (string msg) void:
fputs (fd, (originator[orig] /= "" | ": ") + msg + "\n");
log ("beginning of log\n");
postlude
log ("end of log\n");
fed
Note how in this version of logger originators acts
as a stack of originator strings, and it is not publicized. The
management of the stack is done via a pair of publicized procedures
push originator and pop originator. Our program will
now look like:
access logger
begin push originator (argv (1));
log ("executing program");
c ... c
access logger (push originator ("subtask");
log ("doing subtask")
c ... c;
pop originator)
end
And the log output is:
a.out: beginning of log a.out: executing program a.out:subtask: doing subtask a.out: end of log
————————————————————–
module-indications are used to find interface-definitions of modules:
ACCESS FOO SKIP
Looks for (in order):
foo.m68 foo.o libfoo.so
Should we use instead:
ACCESS "foo" SKIP
That would use for module indicators the same syntax than hole indicators.
Modules get accessed, invoked and revoked.
Access clauses:
: ACCESS A, B <enclosed clause>
Where A and B are “revelations”.
In
: MODULE A = ACCESS B DEF ... FED
Doesn’t reveals any contents of B. Whereas in:
: MODULE A = ACCESS PUB B DEF .. FED
The module A is also revealing B’s public declarations. So accessing A provides access to B.
User-defined preludes go to the user-prelude.
Invocation and revocation:: How modules are executed.
It is possible for a definition module to not publicize any definition. Such modules would be used only for the side effects produced from executing the prelude and postlude when the module gets invoked and revoked. XXX: provide an example?
XXX
An Algol 68 particular program consists on an enclosed clause in
a strong context with target mode void, possibly preceded
by a set of zero of more labels. For example:
hello:
begin puts ("Hello, world!\n")
end
Note that the enclosed clause conforming the particular program doesn’t have to be a closed clause. Consider for example the following program, that prints out its command line arguments:
for i to argc do puts (argv (i) + "\n") od
Some operating systems have the notion of exit status of a
process. In such systems, by default the execution of the particular
program results in an exit status of success. It is possible for the
program to specify an explicit exit status by using the standard
procedure set exit status, like:
begin # ... program code ... #
if error found;
then set exit status (1) fi
end
In POSIX systems the status is an integer, and the system interprets a
value other than zero as a run-time error. In other systems the
status may be of some other type. To support this, the set
error status procedure accepts as an argument an united value that
accommodates all the supported systems.
The following example shows a very simple program that prints “Hello world” on the standard output and then returns to the operating system with a success status:
begin puts ("Hello world\n")
end
The environment in which particular programs run is expressed here in the form of pseudo code:
(c standard-prelude c;
c library-prelude c;
c system-prelude c;
par begin c system-task-1 c,
c system-task-2 c,
c system-task-n c,
c user-task-1 c,
c user-task-2 c,
c user-task-m c
end)
Where each user task consists on:
(c particular-prelude c; c user-prelude c; c particular-program c; c particular-postlude c)
The only standard system task described in the report is expressed in pseudo-code as:
do down gremlins; undefined; up bfileprotect od
Which denotes that, once a book (file) is closed, anything may happen. Other system tasks may exist, depending on the operating system. In general these tasks in the parallel clause denote the fact that the operating system is running in parallel (intercalated) with the user’s particular programs.
Subsequent sections in this manual include a detailed description of the contents of these preludes.
Pragmats (also known as pragmas in other programming
languages) are directives and annotations for the compiler, and they
impact the compilation process in several ways. A pragmat starts with
either pragmat or pr and finished with either
pragmat or pr respectively. Pragmats cannot be
nested. For example:
pr include "foo.a68" pr
The interpretation of pragmats is compiler-specific. This chapter documents the pragmats supported by GCC.
An include pragmat has the form:
pr include "PATH" pr
Where PATH is the path of the file whose contents are to be
included at the location of the pragmat. If the provided path is
relative then it is interpreted as relative to the directory
containing the source file that contains the pragmat.
The reference language specified by the Revised Report describes Algol 68 particular programs as composed by symbols. However, the Report leaves the matter of the concrete representation of these symbols, the representation language, open to the several implementations. This was motivated by the very heterogeneous computer systems in existence at the time the Report was written. Flexibility in terms of representation was crucial.
This flexibility was indeed exploited by the early implementations, and there was a price to pay for it. A few years after the publication of the Revised Report the different implementations had already given rise to a plethora of many related languages that, albeit being strict Algol 68, differed considerably in appearance. This, and the fact that people were already engrossed in writing programs other than compilers that needed to process Algol 68 programs, such as code formatters and macro processors, prompted the WG 2.1 to develop and publish a Report on the Standard Hardware Representation for ALGOL 68, which came out in 1975.
This compiler follows the Standard Hardware Representation, but deviates from it in a few aspects. This chapter provides an overview of the hardware representation and documents any deviation.
A program in the strict Algol 68 language is composed by a series of
symbols. These symbols have names such as letter-a-symbol and
assigns-to-symbol which are, well, purely symbolic. In fact,
these are notions in the two-level grammar that defines the strict
language.
A representation language provides a mapping between symbols in
the strict language and the representation of these symbols. Each
representation is a sequence of syntactic marks. For example, the
completion symbol may be represented by exit, where
the marks are the bold letters. The tilde symbol may be
represented by ~, which is a single mark. The representation of
assigns to symbol is :=, which is composed by the two
marks : and =. The representation of letter-a
is, not surprising, the single mark a.
The section 9.4 of the Report describes the recommended representation for all the symbols of the language, which constitutes the so-called reference language. Algol 68 implementations are strongly encouraged to use representation languages which are similar enough to the reference language, but it is not mandatory.
A representation language may specify more than one representation for
a given symbol. For example, in the reference language the is
not symbol is represented by isnt, :/=: and a variant
of the later where the slash sign is superimposed on the equal sign.
In this case, an implementation can choose to implement any number of
the representations.
Spaces, tabs and newlines are typographical display features
that, when they appear between symbols, are of no significance an do
not alter the meaning of the program. However, when a space or a tab
appear in string or character denotations, they represent the
space symbol and the tab symbol
respectively1.
The syntactic marks of a representation language, both symbols and typographical display features, are realized as a set of worthy characters and the newline. Effectively, an Algol 68 program is a sequence of worthy characters and newlines. The worthy characters are:
a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 space tab " # $ % & ' ( ) * + , - . / : ; < = > [ \ ] ^ _ | ! ? ~
Some of the characters above were considered unworthy by the original Standard Hardware Representation:
!It was considered unworthy because many installations didn’t have a
vertical bar base character, and ! was used as a base character
for |. Today every computer system features a vertical bar
character, so ! can qualify as a worthy character.
&The Revised Report specifies that & is a monad, used as a
symbol for the dyadic and operator. The Standard Hardware
representation decided to turn it into an unworthy character,
motivated by the fact that no nomads existed for the other logical
operators not and or, and also with the goal of
maintaining the set of worthy characters as small as possible to
improve portability. Recognizing that the first motivation still
holds, but not the second, this compiler re-instates & as a
monad but doesn’t use it as an alternative representation of the
and operator.
~The Standard Hardware Representation vaguely cites some “severe
difficulties” with the hardware representation of the tilde
character. Whatever these difficulties were at the time, they surely
don’t exist anymore. This compiler recognizes ~ as a worthy
character, and is used as a monad.
?The question mark character was omitted as a worthy character to limit
the size of the worthy set. This compiler recognizes ? as a
worthy character, and is used as a monad.
\Back-slash wasn’t included as a worthy character because back in 1975
it wasn’t supported in EBCDIC (it is now). This compiler recognizes
\ as a worthy character, and it is used to introduce string
breaks.
tabThis compiler recognizes the tabulator character as a worthy character, and it is used as a typographical display feature.
The worthy characters described in the previous section are to be
interpreted symbolically rather than visually. The worthy character
|, for example, is the vertical line character and generally
looks the same in every system. The worthy character space is
obviously referred by a symbolic name.
The actual visually distinguishable characters available in an
installation are known as base characters. The Standard
Hardware Representation allows implementations the possibility of
using two or more base characters to represent a single worthy
character. This was the case of the | character, which was
represented in many implementations by either | or !.
This compiler uses the set of base characters corresponding to the subset of the Unicode character set that maps one to one to the set of worthy characters described in the previous section:
A-Z 65-90 a-z 97-122 space 32 tab 9 ! 33 " 34 $ 36 % 37 & 38 ' 39 ( 40 ) 41 * 42 + 43 , 44 - 45 . 46 / 47 : 58 ; 59 < 60 = 61 > 62 ? 63 @ 64 [ 91 \ 92 ] 93 ^ 94 _ 95 | 124 ~ 126
The Algol 68 reference language establishes that certain source constructs, namely mode indications and operator indications, consist in a sequence of bold letters and bold digits, known as a bold word. In contrast, other constructs like identifiers, field selectors and labels are composed of regular or non-bold letters and digits, known as a tag.
What is precisely a bold letter or digit, and how it differs from a non-bold letter or digit, is not specified by the Report. This is no negligence, but a conscious effort at abstracting the definition of the so-called strict language from its representation. This allows different representations of the same language.
Some representations of Algol 68 are intended to be published in books, be it paper or electronic devices, and be consumed by persons. These are called publication languages. In publication languages bold letters and digits are typically represented by actual bold alphanumeric typographic marks, or sometimes underlined alphanumeric marks.
Other representations of Algol 68 are intended to be automatically processed by a computer. These representations are called hardware languages. Sometimes the hardware languages are also intended to be written and read by programmers; these are called programming languages.
Unfortunately, computer systems today usually do not yet provide readily usable and ergonomic bold or underline alphanumeric marks, despite the existence of Unicode and modern and sophisticated editing environments. The lack of appropriate input methods surely plays a role on this. Thus, the programming representation languages of Algol 68 should resort to a technique known as stropping in order to differentiate bold letters and digits from non-bold letters and digits. The set of rules specifying the representation of these characters is called a stropping regime.
There are three classical stropping regimes for Algol 68, which are standardized and specified in the Standard Hardware Representation normative document. These are POINT stropping, RES stropping and UPPER stropping. The following sections do a cursory tour over them; for more details the reader is referred to the Standard Hardware Representation.
This compiler implements UPPER stropping and SUPPER stropping.
POINT stropping is in a way the most fundamental of the three standard regimes. It was designed to work in installations with limited character sets that provide only one alphabet, one set of digits, and a very restricted set of other symbols.
In POINT stropping a bold word is represented by its constituent
letters and digits preceded by a point character. For example, the
symbol bold begin symbol in the strict language, which is
represented as begin in bold face in the reference language,
would be represented as .BEGIN in POINT stropping.
More examples are summarized in the following table.
| Strict language | Reference language | POINT stropping |
|---|---|---|
true symbol | true | .TRUE |
false symbol | false | .FALSE |
integral symbol | int | .INT |
completion symbol | exit | .EXIT |
bold-letter-c-... | crc32 | .CRC32 |
In POINT stropping a tag is represented by writing its constituent non-bold letters and digits in order. But they are organized in several taggles.
Each taggle is a sequence of one or more letters and digits,
optionally followed by an underscore character. For example, the tag
PRINT is composed of a single taggle, but the tag
PRINT_TABLE is composed of a first taggle PRINT_
followed by a second taggle TABLE.
To improve readability it is possible to insert zero or more white
space characters between the taggles in a tag. Therefore, the tag
PRINT_TABLE could have been written PRINT TABLE, or even
PRINT_ TABLE. This is the reason why Algol 68 identifiers,
labels and field selectors can and do usually feature white spaces in
them.
It is important to note that both the trailing underscore characters
in taggles and the white spaces in a tag do not contribute anything to
the denoted tag: these are just stropping artifacts aimed to improve
readability. Therefore FOOBAR FOO BAR, FOO_BAR
and FOO_BAR_ are all representations of the same tag, that
represents the
letter-f-letter-o-letter-o-letter-b-letter-a-letter-r language
construct.
Below is the text of an example Algol 68 procedure encoded in POINT stropping.
.PROC RECSEL OUTPUT RECORDS = .VOID:
.BEGIN .BITS FLAGS
:= (INCLUDE DESCRIPTORS | REC F DESCRIPTOR | REC F NONE);
.RECRSET RES = REC DB QUERY (DB, RECUTL TYPE,
RECUTL QUICK, FLAGS);
.RECWRITER WRITER := REC WRITER FILE NEW (STDOUT);
SKIP COMMENTS .OF WRITER := .TRUE;
.IF RECUTL PRINT SEXPS
.THEN MODE .OF WRITER := REC WRITER SEXP .FI;
REC WRITE (WRITER, RES)
.END
The early installations where Algol 68 ran not only featured a very restricted character set, but also suffered from limited storage and complex to use and time consuming input methods such as card punchers and readers. It was important for the representation of programs to be as compact as possible.
It is likely that is what motivated the introduction of the RES stropping regime. As its name implies, it removes the need of stropping many bold words by introducing reserved words.
A reserved word is one of the bold words specified in the section 9.4.1 of the Report as a representation of some symbol. Examples are at, begin, if, int and real.
RES stropping encodes bold words and tags like POINT stropping, but if a bold word is a reserved word then it can then be written without a preceding point, achieving this way a more compact, and easier to read, representation for programs.
Introducing reserved words has the obvious disadvantage that some tags
cannot be written the obvious way due to the possibility of conflicts.
For example, to represent a tag if it is not possible to just
write IF, because it conflicts with a reserved word, but this
can be overcome easily (if not elegantly) by writing IF_
instead.
Below is the recsel output records procedure again, this time
encoded in RES stropping.
PROC RECSEL OUTPUT RECORDS = VOID:
BEGIN BITS FLAGS
:= (INCLUDE DESCRIPTORS | REC F DESCRIPTOR | REC F NONE);
.RECRSET RES = REC DB QUERY (DB, RECUTL TYPE,
RECUTL QUICK, FLAGS);
.RECWRITER WRITER := REC WRITER FILE NEW (STDOUT);
SKIP COMMENTS OF WRITER := TRUE;
IF RECUTL PRINT SEXPS
THEN MODE .OF WRITER := REC WRITER SEXP FI;
REC WRITE (WRITER, RES)
END
Note how user-defined mode an operator indications still require explicit stropping.
In time computers added support for more than one alphabet by introducing character sets with both upper and lower case letters, along with convenient ways to both input and display these.
In UPPER stropping the bold letters in bold word are represented by upper-case letters, whereas the letters in tags are represented by lower-case letters.
The notions of upper- and lower-case are not applicable to digits, but since the language syntax assures that it is not possible to have a bold word that starts with a digit, digits are considered to be bold if they follow a bold letter or another bold digit.
Below is the recsel output records procedure again, this time
encoded in UPPER stropping.
PROC recsel output records = VOID:
BEGIN BITS flags
:= (include descriptors | rec f descriptor | rec f none);
RECRSET res = rec db query (db, recutl type,
recutl quick, flags);
RECWRITER writer := rec writer file new (stdout);
skip comments of writer := TRUE;
IF recutl print sexps
THEN mode OF writer := rec writer sexp FI;
rec write (writer, res)
END
Note how in this regime it is almost never necessary to introduce bold tags with points. All in all, it looks much more natural to contemporary readers. UPPER stropping is in fact the stropping regime of choice today. It is difficult to think of any reason why anyone would resort to use POINT or RES stropping.
In the SUPPER stropping regime bold words are written by writing a sequence of one or more taggles. Each taggle is written by writing a letter followed by zero or more other letters and digits and is optionally followed by a trailing underscore character. The first letter in a bold word shall be an upper-case letter. The rest of the letters in the bold word may be either upper- or lower-case.
For example, RecRset, Rec_Rset and RECRset are
all different ways to represent the same mode indication. This allows
to recreate popular naming conventions such as CamelCase.
As in the other stropping regimes, the casing of the letters and the underscore characters are not really part of the mode or operator indication.
Operator indications are also bold words and are written in exactly
the same way than mode indications, but it is usually better to always
use upper-case letters in operator indications. On one side, it looks
better, especially in the case of dyadic operators where the asymmetry
of, for example Equal would look odd, consider m1 Equal
m2 as opposed to m1 EQUAL m2. On the other side, tools like
editors can make use of this convention in order to highlight operator
indications differently than mode indications.
In the SUPPER stropping regime tags are written by writing a sequence of one or more taggles. Each taggle is written by writing a letter followed by zero or more other letters and digits and is optionally followed by a trailing underscore character. All letters in a tag shall be lower-case letters.
For example, the identifier list is represented by a single
taggle, and it is composed by the letters l, i, s
and t, in order. In the jargon of the strict language we would
spell the tag as letter-l-letter-i-letter-s-letter-t.
The label found_zero is represented by two taggles,
found_ and zero, and it is composed by the letters
f, o, u, n, d, z, e,
r and o, in order. In the jargon of the strict language
we would spell the tag as letter-f-letter-o-letter-u-letter-n
-letter-d-letter-z-letter-e-letter-r-letter-o.
The identifier crc_32 is likewise represented by two taggles,
crc_ and 32. Note how the second taggle contains only
digits. In the jargon of the strict language we would spell the tag
as letter-c-letter-r-letter-c-digit-three-digit-two.
The underscore characters are not really part of the tag, but part of
the stropping. For example, both goto found_zero and
goto foundzero jump to the same label.
The recsel output records procedure, encoded in SUPPER
stropping, looks like below.
proc recsel_output_records = void:
begin bits flags
:= (include_descriptors | rec_f_descriptor | rec_f_none);
RecRset res = rec_db_query (db, recutl_type,
recutl_uick, flags);
RecWriter writer := rec_writer_file_new (stdout);
skip_comments of writer := true;
if recutl_print_sexps
then mode_ of writer := rec_writer_sexp fi;
rec_write (writer, res)
end
Algol68 operators, be them predefined or defined by the programmer,
can be referred via either bold tags or sequences of certain
non-alphabetic symbols. For example, the dyadic operator + is
defined for many modes to perform addition, the monadic operator
entier gets a real value and rounds it to an integral
value, and the operator :=: is the identity relation. Many
operators provide both bold tag names and symbols names, like in the
case of :/=: that can also be written as isnt.
Bold tags are lexically well delimited, and if the same tag is used to
refer to a monadic operator and to a dyadic operator, no ambiguity can
arise. For example, in the following program it is clear that the
second instance of plus refers to the monadic operator, and
the first instance refers to the dyadic operator2.
op plus = (int a, b) int: a + b, plus = (int a): a; int val = 2 plus plus 3;
On the other hand, symbols are not lexically delimited as words, and
one symbol can appear immediately following another symbol. This can
lead to ambiguities. For example, if we were to define a C-like
monadic operator ++ like:
op ++ = (ref int a) int: (int t = a; a +:=1; t);
Then the expression ++a would be ambiguous: is it ++a or
+(+a)?. In a similar way, if we would use ++ as the
name of a dyadic operator, an expression like a++b could be
also interpreted as both a++b and a+(+b).
To avoid these problems Algol 68 divides the symbols which are suitable to appear in the name of an operator into two classes: monads and nomads. Monads are symbols that can be used as monadic operators. Nomads are symbols which can be used as both monadic or dyadic operators. Given these two sets, the rules to conform a valid operator are:
:= or =:, but not by both.
In the GNU Algol 68 compiler:
%^&+-~!?.
></=*.
The intrinsic value of each worthy character that appears inside a
string denotation is itself. The string "/abc", for example,
contains a slash character followed by the three letters a,
b and c.
Sometimes, however, it becomes necessary to represent some non-worthy character in a string denotation. In these cases, an escape convention has to be used to represent these extra string-items. It is up to the implementation to decide this convention, and the only requirement imposed by the Standard Hardware Representation on this regard is that the character used to introduce escapes, the escape character, shall be the apostrophe. This section documents the escape conventions implemented by the GNU compiler.
Two characters have special meaning inside string denotations: double
quote (") and apostrophe ('). The first finishes the
string denotation, and the second starts a string break, which
is the Algol 68 term for what is known as an “escape sequence” in
other programming languages. Two consecutive double-quote characters
specify a single double-quote character.
The following string breaks are recognized by this compiler:
''Apostrophe character '.
'nNewline character.
'fForm feed character.
'rCarriage return (no line feed).
'tTab.
'(list of character codes separated by commas)The indicated characters, where each code has the form uhhhh or
Uhhhhhhhh, where hhhh and hhhhhhhh are integers
expressing the character code in hexadecimal. The list must contain
at least one entry.
A string break can appear as the single string-item in a character denotation, subject to the following restrictions:
'(...) that contain more than
one character code are not allowed in character denotations. If the
specified code point is not a valid Unicode character then the value
of the denotation is invalid char.
The Algol 68 Revised Report defines an extensive set of standard modes, operators, procedures and values, collectively known as the standard prelude.
The standard prelude is available to Algol 68 programs without needing to import any module.
For brevity, in this section the pseudo-mode l represents a
shortsety, i.e. a sequence of either zero or more
LONG or zero or more SHORT.
An environment enquiry is a constant or a procedure, whose elaboration yields a value that may be useful to the programmer, that reflects some characteristic of the particular implementation. The values of these enquiries are also determined by the architecture and operating system targeted by the compiler.
int int lengths ¶1 plus the number of extra lenghts of integers which are meaningful.
int int shorths ¶1 plus the number of extra shorths of integers which are meaningful.
l int L max int ¶The largest integral value.
int real lengths ¶1 plus the number of extra lenghts of real numbers which are meaningful.
int real shorths ¶1 plus the number of extra shorths of real numbers which are meaningful.
l real L max real ¶The largest real value.
l real L small real ¶The smallest real value such that both 1 + small real > 1 and
1 - small real < 1.
int bits lengths ¶1 plus the number of extra widths of bits which are meaningful.
int bits shorths ¶1 plus the number of extra shorths of bits which are meaningful.
int bits width ¶int long bits width ¶int long long bits width ¶The number of bits in a bits value.
int bytes lengths ¶1 plus the number of extra widths of bytes which are meaningful.
int bytes shorths ¶1 plus the number of extra shorths of bytes which are meaningful.
int bytes width ¶int long bytes width ¶int long long bytes width ¶The number of chars in a bytes value.
int max abs char ¶The largest value which abs of a char can yield.
char null character ¶Some character.
char flip ¶char flop ¶Characters used to represent true and false
boolean values in textual transput.
char error char ¶Character used to represent the digit of a value resulting from a conversion error in textual transput.
char blank ¶The space character.
l real L pi ¶The number pi.
The only value of this mode is empty.
Mode for the boolean truth values true and false.
Modes for signed integral values. Each long or
short may increase or decrease the range of the domain,
depending on the characteristics of the current target. Further
longs and shorts may be specified with no
effect.
Modes for signed real values. Each long may increase the
upper range of the domain, depending on the characteristics of the
current target. Further longs may be specified but with no
effect.
Mode for character values. The character values are mapped one-to-one to code points in the 21-bit space of Unicode.
Mode for sequences of characters. This is implemented as a flexible
row of char values.
Modes for complex values. Each long may increase the
precision of both the real and imaginary parts of the numbers,
depending on the characteristics of the current target. Further
longs may be specified with no effect.
Compact and efficient representation of a row of boolean values. Each
long may increase the number of booleans that can be packed
in a bits, depending on the characteristics of the current target.
Compact and efficient representation of a row of character values.
Each long may increase the number of characters that can be
packed in a bytes, depending on the characteristics of the current
target.
1plusab, +:=
minusab, -:=
timesab, *:=
divab, /:=
overab, %:=
modab, %*:=
plusto, +=:
2or
3and
xor
4eq, =
ne, /=
5lt, <,
le, <=
gt, >
ge, >=
6+
-
7*
/
over, %
mod, %*
elem
8**
shl, up
shr, down
up, down
^
lwb
upb
9i
+*
The following operators work on any row mode, denoted below using the
pseudo-mode rows.
= (rows a) int ¶Monadic operator that yields the lower bound of the first bound pair
of the descriptor of the value of a.
= (rows a) int ¶Monadic operator that yields the upper bound of the first bound pair
of the descriptor of the value of a.
= (int n, rows a) int ¶Dyadic operator that yields the lower bound in the n-th bound pair of
the descriptor of the value of a, if that bound pair exists.
Attempting to access a non-existing bound pair results in a run-time
error.
= (int n, rows a) int ¶Dyadic operator that yields the upper bound in the n-th bound pair of
the descriptor of the value of a, if that bound pair exists.
Attempting to access a non-existing bound pair results in a run-time
error.
= (bool a) bool ¶= (bool a) bool ¶Monadic operator that yields the logical negation of its operand.
= (bool a, b) bool ¶Dyadic operator that yields the logical “or” of its operands.
= (bool a, b) bool ¶= (bool a, b) bool ¶Dyadic operator that yields the logical “and” of its operands.
= (bool a, b) bool ¶= (bool a, b) bool ¶Dyadic operator that yields true if its operands are the
same truth value, false otherwise.
= (bool a, b) bool ¶= (bool a, b) bool ¶Dyadic operator that yields false if its operands are the
same truth value, true otherwise.
= (bool a) int ¶Monadic operator that yields 1 if its operand is true, and
0 if its operand is false.
= (l int a) l int ¶Monadic operator that yields the affirmation of its operand.
= (l int a) l int ¶Monadic operator that yields the negative of its operand.
= (l int a) l int ¶Monadic operator that yields the absolute value of its operand.
= (l int a) int ¶Monadic operator that yields -1 if a if negative, 0 if a
is zero and 1 if a is positive.
= (l int a) bool ¶Monadic operator that yields true if its operand is odd,
false otherwise.
= (l int a, b) l int ¶Dyadic operator that yields the addition of its operands.
= (l int a, b) l int ¶Dyadic operator that yields b subtracted from a.
= (l int a, b) l int ¶Dyadic operator that yields the multiplication of its operands.
= (l int a, b) l int ¶= (l int a, b) l int ¶Dyadic operator that yields the integer division of a by
b, rounding the quotient toward zero.
= (l int a, b) l int ¶= (l int a, b) l int ¶Dyadic operator that yields the remainder of the division of a
by b.
= (l int a, b) l real ¶Dyadic operator that yields the integer division with real result of
a by b.
= (ref l int a, l int b) ref l int ¶= (ref l int a, l int b) ref l int ¶Plus and become. Dyadic operator that calculates a + b,
assigns the result of the operation to the name a and then
yields a.
= (ref l int a, l int b) ref l int ¶= (ref l int a, l int b) ref l int ¶Dyadic operator that calculates a - b, assigns the result of
the operation to the name a and then yields a.
= (ref l int a, l int b) ref l int ¶= (ref l int a, l int b) ref l int ¶Dyadic operator that calculates a * b, assigns the result of
the operation to the name a and then yields a.
= (l int a, b) bool ¶= (l int a, b) bool ¶Dyadic operator that yields whether its operands are equal.
= (l int a, b) bool ¶= (l int a, b) bool ¶Dyadic operator that yields whether its operands are not equal.
= (l int a, b) bool ¶= (l int a, b) bool ¶Dyadic operator that yields whether a is less than b.
= (l int a, b) bool ¶= (l int a, b) bool ¶Dyadic operator that yields whether a is less than, or equal to
b.
= (short int a) short short int ¶= (int a) short int ¶= (long int a) int ¶= (long long int a) long int ¶Monadic operator that yields, if it exists, the integral value that
can be lengthened to the value of a. If the value doesn’t
exist then the operator yields either the most positive integral value
in the destination mode, if a is bigger than that value, or the
most negative integral value in the destination mode, if a is
smaller than that value.
= (short short int a) short int ¶= (short int a) int ¶= (int a) long int ¶= (long int a) long long int ¶Monadic operator that yields the integral value lengthened from the
value of a.
= (l real a) l real ¶Monadic operator that yields the affirmation of its operand.
= (l real a) l real ¶Monadic operator that yields the negative of its operand.
= (l real a) l real ¶Monadic operator that yields the absolute value of its operand.
= (l real a) int ¶Monadic operator that yields -1 if a if negative, 0 if a
is zero and 1 if a is positive.
= (l real a, b) l real ¶Dyadic operator that yields the addition of its operands.
= (l real a, b) l real ¶Dyadic operator that yields b subtracted from a.
= (l real a, b) l real ¶Dyadic operator that yields the multiplication of its operands.
= (l real a, b) l real ¶Dyadic operator that yields the realeger division with real result of
a by b.
= (ref l real a, l real b) ref l real ¶= (ref l real a, l real b) ref l real ¶Plus and become. Dyadic operator that calculates a + b,
assigns the result of the operation to the name a and then
yields a.
= (ref l real a, l real b) ref l real ¶= (ref l real a, l real b) ref l real ¶Dyadic operator that calculates a - b, assigns the result of
the operation to the name a and then yields a.
= (l real a, b) bool ¶= (l real a, b) bool ¶Dyadic operator that yields whether its operands are equal.
= (l real a, b) bool ¶= (l real a, b) bool ¶Dyadic operator that yields whether its operands are not equal.
= (l real a, b) bool ¶= (l real a, b) bool ¶Dyadic operator that yields whether a is less than b.
= (l real a, b) bool ¶= (l real a, b) bool ¶Dyadic operator that yields whether a is less than, or equal to
b.
= (l real a) int ¶Monadic operator that yields the nearest integer to its operand.
= (l real a) int ¶Monadic operator that yields the integer equal to a, or the
next integer below (more negative than) a.
= (long real a) real ¶= (long long real a) long real ¶Monadic operator that yields, if it exists, the real value that
can be lengthened to the value of a. If the value doesn’t
exist then the operator yields either the most positive real value
in the destination mode, if a is bigger than that value, or the
most negative real value in the destination mode, if a is
smaller than that value.
= (real a) long real ¶= (long real a) long long real ¶Monadic operator that yields the real value lengthened from the
value of a.
= (char a, b) bool ¶= (char a, b) bool ¶Dyadic operator that yields whether its operands are equal.
= (char a, b) bool ¶= (char a, b) bool ¶Dyadic operator that yields whether its operands are not equal.
= (char a, b) bool ¶= (char a, b) bool ¶Dyadic operator that yields whether a is less than b.
= (char a, b) bool ¶= (char a, b) bool ¶Dyadic operator that yields whether a is less than, or equal to
b.
= (string a, b) bool ¶= (string a, b) bool ¶Dyadic operator that yields whether its operands are equal. Two strings are equal if they contain the same sequence of characters.
= (string a, b) bool ¶= (string a, b) bool ¶Dyadic operator that yields whether its operands are not equal.
= (string a, b) bool ¶= (string a, b) bool ¶Dyadic operator that yields whether the string a is less than
the string b.
= (string a, b) bool ¶= (string a, b) bool ¶Dyadic operator that yields whether the string a is less than,
or equal to string b.
= (string a, b) string ¶Dyadic operator that yields the concatenation of the two given strings as a new string.
= (string a, char b) string ¶Dyadic operator that yields the concatenation of the given string
a and a string whose contents are the character b.
= (ref string a, string b) ref string ¶= (ref string a, string b) ref string ¶Plus and become. Dyadic operator that calculates a + b,
assigns the result of the operation to the name a and then
yields a.
= (string b, ref string a) ref string ¶= (string b, ref string b) ref string ¶Dyadic operator that calculates a + b, assigns the result of
the operation to the name a and then yields a.
= (ref string a, string b) ref string ¶= (ref string a, string b) ref stringl ¶Plus and become. Dyadic operator that calculates a * b,
assigns the result of the operation to the name a and then
yields a.
= (l bits a, b) l bits ¶= (l bits a, b) l bits ¶Monadic operator that yields the element-wise not logical operation in the elements of the given bits operand.
= (l bits a, b) l bits ¶= (l bits a, b) l bits ¶Dyadic operator that yields the element-wise and logical operation in the elements of the given bits operands.
= (l bits a, b) l bits ¶Dyadic operator that yields the element-wise “or” logical operation in the elements of the given bits operands.
= (l bits a, b) bool ¶= (l bits a, b) bool ¶Dyadic operator that yields whether its operands are equal. Two bits are equal if they contain the same sequence of booleans.
= (l bits a, b) bool ¶= (l bits a, b) bool ¶Dyadic operator that yields whether its operands are not equal.
= (l bits a, b) bool ¶= (l bits a, b) bool ¶Dyadic operator that yields whether the bits a is less than
the bits b.
= (l bits a, b) bool ¶= (l bits a, b) bool ¶Dyadic operator that yields whether the bits a is less than,
or equal to bits b.
= (l bits a) l int ¶Monadic operator that yields the integral value whose constituent bits
correspond to the booleans stored in a. See bin and abs of negative integral values.
= (l int a) l bits ¶Monadic operator that yields the bits value whose boolean elements map
the bits in the given integral operand. See bin and abs of negative integral values.
= (long bits a) bits ¶= (long long bits a) long bits ¶Monadic operator that yields the bits value that can be lengthened to
the value of a.
= (bits a) long bits ¶= (long bits a) long long bits ¶Monadic operator that yields the bits value lengthened from the value
of a. The lengthened value features false in the
extra left positions added to match the lengthened size.
= (l real a) l real ¶Procedure that yields the square root of the given real argument.
= (l real a) l real ¶Procedure that yields the base e logarithm of the given real
argument.
= (l real a) l real ¶Procedure that yields the exponential function of the given real
argument. This is the inverse of ln.
= (l real a) l real ¶Procedure that yields the sin trigonometric function of the given real argument.
= (l real a) l real ¶Procedure that yields the arc-sin trigonometric function of the given real argument.
= (l real a) l real ¶Procedure that yields the cos trigonometric function of the given real argument.
= (l real a) l real ¶Procedure that yields the arc-cos trigonometric function of the given real argument.
= (l real a) l real ¶Procedure that yields the tan trigonometric function of the given real argument.
= (l real a) l real ¶Procedure that yields the arc-tan trigonometric function of the given real argument.
This chapter documents the GNU extensions to the standard prelude. The facilities documented below are available to Algol 68 programs only if the gnu68 language dialect is selected, which is the default.
The extended prelude is available to Algol 68 programs without needing
to import any module, provided they are compiled as gnu68 code,
which is the default.
An environment enquiry is a constant, whose value may be useful to the programmer, that reflects some characteristic of the particular implementation. The values of these enquiries are also determined by the architecture and operating system targeted by the compiler.
l int L min int ¶The most negative integral value.
l real L min real ¶The most negative real value.
l real L infinity ¶Positive infinity expressed in a real value.
l real L minus infinity ¶Negative infinity expressed in a real value.
char invalid char ¶A character that is unknown or unrepresentable in Unicode.
The following operators work on any row mode, denoted below using the
pseudo-mode rows.
= (rows a) int ¶Monadic operator that yields the number of elements implied by the
first bound pair of the descriptor of the value of a.
= (int n, rows a) int ¶Dyadic operator that yields the number of elements implied by the n-th
bound pair of the descriptor of the value of a.
= (bool a, b) bool ¶Dyadic operator that yields the exclusive-or operation of the given boolean arguments.
= (l bits a, b) l bits ¶Dyadic operator that yields the bit exclusive-or operation of the given bits arguments.
The POSIX prelude provides facilities to perform simple transput (I/O) based on POSIX file descriptors, accessing the file system, command-line arguments, environment variables, etc.
This prelude is available to Algol 68 programs without needing to
import any module, provided they are compiled as gnu68 code,
which is the default.
The Algol 68 program can report an exit status to the operating system once they stop running. The exit status reported by default is zero, which corresponds to success.
= (int status) ¶Procedure that sets the exit status to report to the operating system once the program stop executing. The default exit status is 0 which, by convention, is interpreted by POSIX systems as success. A value different to zero is interpreted as an error status. This procedure can be invoked more than one, the previous exit status being overwritten.
Algol 68 programs can access the command-line arguments passed to them by using the following procedures.
= int ¶Procedure that yields the number of arguments passed in the command line, including the name of the program.
= (int n) string ¶Procedure that yields the nth argument passed in the command
line. The first argument is always the name used to invoke the
program. If n is out of range then this procedure returns the
empty string.
= (string varname) string ¶Procedure that yields the value of the environment variable
varname as a string.
When a call to a procedure in this prelude results in an error, the
called procedure signals the error in some particular way and also
sets a global errno to a code describing the error. For
example, trying to opening a file that doesn’t exist will result in
fopen returning -1, which signals an error. The caller can
then inspect the global errno to see what particular error
prevented the operation to be completed: in this case, errno
will contain the error code corresponding to “file doesn’t exist”.
= int ¶This procedure yields the current value of the global errno.
The yielded value reflects the error status of the last executed POSIX
prelude operation.
= (int ecode) string ¶This procedure gets an error code and yields a string containing an
explanatory short description of the error. It is typical to pass the
output of errno to this procedure.
= (string msg) void ¶This procedure prints the given string msg in the standard
error output, followed by a colon character, a space character and
finally the string error of the current value of errno.
File descriptors are int values that identify open files
that can be accessed by the program. The fopen procedure
allocates file descriptors as it opens files, and the descriptor is
used in subsequent transput calls to perform operations on the files.
There are three descriptors, however, which are automatically opened when the program starts executing and automatically closed when the program finishes. These are:
int stdin ¶File descriptor associated with the standard input. Its value is 0.
int stdout ¶File descriptor associated with the standard output. Its value is 1.
int stderr ¶File descriptor associated with the standard error. Its value is 2.
= (string pathname, bits flags) int ¶Open the file specified by pathname. The argument flags
is a combination of file o flags as defined below. If the
specified file is successfully opened while satisfying the constraints
implied by flags then this procedure yields a file descriptor
that is used in subsequent I/O calls to refer to the open
file. Otherwise, this procedure yields -1. The particular error can
be inspected by calling the errno procedure.
= (int fd) int ¶Close the given file descriptor, which no longer refers to any file.
This procedure yields zero on success, and -1 on error. In the later
case, the program can look at the particular error by calling the
errno procedure.
= (string pathname, bits mode) int ¶Create a file with name pathname. The argument mode is
a bits value containing a bit pattern that determines the
permissions on the created file. The bit pattern has the form
8rUGO, where U reflects the permissions of the user who
owns the file, U reflects the permissions of the users
pertaining to the file’s group, and O reflects the permissions
of all other users. The permission bits are 1 for execute, 2 for
write and 4 for read. If the file is successfully created then this
procedure yields a file descriptor that is used in subsequent I/O
calls to refer to the newly created file. Otherwise, this procedure
yields -1. The particular error can be inspected by calling the
errno procedure.
fopen ¶The following flags can be combined using bit-wise operators. Note that in POSIX systems the effective mode of the created file is the mode specified by the programmer masked with the process’s umask.
bits file o default ¶Flag for fopen indicating that the file shall be opened with
whatever capabilities allowed by its permissions.
bits file o rdwr ¶Flag for fopen indicating that the file shall be opened for
both reading and writing.
bits file o rdonly ¶Flag for fopen indicating that the file shall be opened for
reading only. This flag is not compatible with file o rdwr nor
with file o wronly.
bits file o wronly ¶Flag for fopen indicating that the file shall be opened for
write only. This flag is not compatible with file o rdwr nor
with file o rdonly.
bits file o trunc ¶Flag for fopen indicating that the opened file shall be
truncated upon opening it. The file must allow writing for this flag
to take effect. The effect of combining file o trunc and
file o rdonly is undefined and varies among implementations.
A program can communicate with other computers, or with other processes running in the same computer, via sockets. The sockets are identified by file descriptors.
= (string host, int port) int ¶This procedure creates a stream socket and connects it to the given
host using port port. The established communication is
full-duplex, and allows sending and receiving data using transput
until it gets closed. On success this procedure yields a file
descriptor. On error this procedure yields -1 and errno is set
appropriately.
The following procedures read or write characters and strings from and
to open files. The external encoding of the files is assumed to be
UTF-8. Since Algol 68 chars are USC-4, this means that
reading or writing a character may involve reading or writing more
than one byte, depending on the particular Unicode code points
involved.
= (char c) char ¶Write the given character to the standard output. This procedure
yields c in case the character got successfully written, or
invalid char otherwise.
= (string str) void ¶Write the given string to the standard output.
= (int fd, char c) int ¶Write given character c to the file with descriptor fd.
This procedure yields c on success, or invalid char
on error.
= (int fd, string str) int ¶Write the given string str to the file with descriptor
fd. This procedure yields the number of bytes written on
success, or 0 on error.
= char ¶Read a character from the standard input. This procedure yields the
read character in case the character got successfully read, or
invalid char otherwise.
= (int n) string ¶Read a string composed of n characters from the standard input.
If n is bigger than zero then characters get read until either
n characters have been read or the end of line is reached. If
n is zero or negative then characters get read until either a
new line character is read or the end of line is reached.
= (int fd) int ¶Read a character from the file with descriptor fd. This
procedure yields the read character in case the character got
successfully read, or invalid char otherwise.
= (int fd, int n) string ¶Read a string from the file with descriptor fd. If n is
bigger than zero then characters get read until either n
characters have been read or the end of line is reached. If n
is zero or negative then characters get read until either a new line
character is read or the end of line is reached.
This chapter documents the GNU extensions implemented by this compiler on top of the Algol 68 programming language. These extensions collectively conform a strict superlanguage of Algol 68, and are enabled by default. To disable them the user can select the strict Algol 68 standard by passing the option -std=algol68 when invoking the compiler.
bin and abs of negative integral values ¶The bin operator gets an integral value and yields a
bits value that reflects the internal bits of the integral
value. The semantics of this operator, as defined in the Algol 68
standard prelude, are:
op bin = (L int a) L bits:
if a >= L 0
then L int b := a; L bits;
for i from L bits width by -1 to 1
do (L F of c)[i] := odd b; b := b % L 2 od;
c
fi;
The abs operator performs the inverse operation of
bits. Given a L bits value, it yields the
L int value whose bits representation is the bits value.
The semantics of this operator, as defined in the Algol 68 prelude,
are:
op abs = (L bits a) L int:
begin L int c := L 0;
for i to L bits width
do c := L 2 * c + K abs (L F of a)[i] od;
c
end
Note how the bin of a negative integral value is not
defined: the implicit else-part of the conditional yields
skip, which is defined as any bits value in that context.
Note also how abs doesn’t make any provision to check
whether the resulting value is positive: it assumes it is so.
The GNU Algol 68 compiler, when working in strict Algol 68 mode
(-std=algol68), makes bin to always yield L
bits (skip) when given a negative value, as mandated by the
report. But the skip value is always the bits representation of zero,
i.e. 2r0. Strict Algol 68 programs, however, must not rely on
this.
When GNU extensions are enabled (-std=gnu68) the
bin of a negative value yields the two’s complement bit
pattern of the value rather than zero. Therefore, bin -
short short 2 yields 2r11111110. And abs
short short 2r11111110 yields -2.
This compiler supports the stropping regimes known as UPPER and SUPPER. In both regimes bold words are written by writing their constituent bold letters and digits, in order. In UPPER regime all the letters of a bold word are to be written using upper-case. In SUPPER regime, only the first bold letter is required to be written using upper-case, and this only when the bold word is not a reserved word.
When a bold word comprises several natural words, it may be a little difficult to distinguish them at first sight. Consider for example the following code, written fist in UPPER stropping:
MODE TREENODE = STRUCT (TREENODEPAYLOAD data, REF TREENODE next),
TREENODEPAYLOAD = STRUCT (INT code, REAL average, mean);
Then written in SUPPER stropping:
mode TreeNode = struct (TreeNodePayload data, REF TreeNode next),
TreeNodePayload = struct (int code, real average, mean);
Particularly in UPPER stropping, it may be difficult to distinguish the constituent natural words at first sight.
In order to improve this, this compiler implements a GNU extension
called bold taggles that allows to use underscore characters
(_) within mode and operator indications as a visual aid to
improve readability. When this extension is enabled, mode indications
and operator indications consist in a sequence of the so-called
bold taggles, which are themselves sequences of one or more bold
letters or digits optionally terminated by an underscore character.
With bold taggles enabled the program above could have been written using UPPER stropping as:
MODE TREE_NODE = STRUCT (TREE_NODE_PAYLOAD data, REF TREE_NODE next),
TREE_NODE_PAYLOAD = STRUCT (INT code, REAL average, mean);
And using SUPPER stropping as:
mode Tree_Node = struct (Tree_Node_Payload data, ref Tree_Node next),
Tree_Node_Payload = struct (int code, real average, mean);
Which is perhaps more readable for most people. Note that the
underscore characters are not really part of the mode or operator
indication. Both TREE_NODE and TREENODE denote the same
mode indication. Note also that, following the definition, constructs
like Foo__bar and _Baz are not valid indications.
Bold taggles are available when the gnu68 dialect of the language is selected. See Dialect options.
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The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
ga68’s command line options are indexed here without any initial
‘-’ or ‘--’. Where an option has both positive and negative forms
(such as -foption and -fno-option), relevant
entries in the manual are indexed under the most appropriate form; it may
sometimes be useful to look up both forms.
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