2 Lexical Elements
1/3 {AI05-0299-1} [The text of a program consists of the texts of one or more
compilations. The text of a compilation is a sequence of lexical elements,
each composed of characters; the rules of composition are given in this
clause. Pragmas, which provide certain information for the compiler, are also
described in this clause.]
2.1 Character Set
1/3 {AI95-00285-01} {AI95-00395-01} {AI05-0266-1} The character repertoire for
the text of an Ada program consists of the entire coding space described by
the ISO/IEC 10646:2011 Universal Multiple-Octet Coded Character Set. This
coding space is organized in planes, each plane comprising 65536 characters.
1.a/2 This paragraph was deleted.{AI95-00285-01}
1.b/2 This paragraph was deleted.{AI95-00285-01}
1.c/3 Discussion: {AI95-00285-01} {AI05-0266-1} It is our intent to
follow the terminology of ISO/IEC 10646:2011 where appropriate,
and to remain compatible with the character classifications
defined in A.3, "Character Handling".
Syntax
Paragraphs 2 and 3 were deleted.
3.1/3 {AI95-00285-01} {AI95-00395-01} {AI05-0266-1} A character is defined
by this International Standard for each cell in the coding space
described by ISO/IEC 10646:2011, regardless of whether or not ISO/IEC
10646:2011 allocates a character to that cell.
Static Semantics
4/3 {AI95-00285-01} {AI95-00395-01} {AI05-0079-1} {AI05-0262-1} {AI05-0266-1}
The coded representation for characters is implementation defined [(it need
not be a representation defined within ISO/IEC 10646:2011)]. A character whose
relative code point in its plane is 16#FFFE# or 16#FFFF# is not allowed
anywhere in the text of a program. The only characters allowed outside of
comments are those in categories other_format, format_effector, and
graphic_character.
4.a Implementation defined: The coded representation for the text of
an Ada program.
4.b/2 Ramification: {AI95-00285-01} Note that this rule doesn't really
have much force, since the implementation can represent characters
in the source in any way it sees fit. For example, an
implementation could simply define that what seems to be an
other_private_use character is actually a representation of the
space character.
4.1/3 {AI95-00285-01} {AI05-0266-1} {AI05-0299-1} The semantics of an Ada
program whose text is not in Normalization Form KC (as defined by Clause 21 of
ISO/IEC 10646:2011) is implementation defined.
4.c/2 Implementation defined: The semantics of an Ada program whose text
is not in Normalization Form KC.
5/3 {AI95-00285-01} {AI05-0266-1} {AI05-0299-1} The description of the
language definition in this International Standard uses the character
properties General Category, Simple Uppercase Mapping, Uppercase Mapping, and
Special Case Condition of the documents referenced by the note in Clause 1 of
ISO/IEC 10646:2011. The actual set of graphic symbols used by an
implementation for the visual representation of the text of an Ada program is
not specified.
6/3 {AI95-00285-01} {AI05-0266-1} Characters are categorized as follows:
6.a/3 Discussion: {AI05-0005-1} {AI05-0262-1} {AI05-0266-1} Our
character classification considers that the cells not allocated in
ISO/IEC 10646:2011 are graphic characters, except for those whose
relative code point in their plane is 16#FFFE# or 16#FFFF#. This
seems to provide the best compatibility with future versions of
ISO/IEC 10646, as future characters can already be used in Ada
character and string literals.
7/2 This paragraph was deleted.{AI95-00285-01}
8/2 {AI95-00285-01} letter_uppercase
Any character whose General Category is defined to be "
Letter, Uppercase".
9/2 {AI95-00285-01} letter_lowercase
Any character whose General Category is defined to be "
Letter, Lowercase".
9.a/1 This paragraph was deleted.{8652/0001} {AI95-00124-01}
9.1/2 {AI95-00285-01} letter_titlecase
Any character whose General Category is defined to be "
Letter, Titlecase".
9.2/2 {AI95-00285-01} letter_modifier
Any character whose General Category is defined to be "
Letter, Modifier".
9.3/2 {AI95-00285-01} letter_other
Any character whose General Category is defined to be "
Letter, Other".
9.4/2 {AI95-00285-01} mark_non_spacing
Any character whose General Category is defined to be "Mark,
Non-Spacing".
9.5/2 {AI95-00285-01} mark_spacing_combining
Any character whose General Category is defined to be "Mark,
Spacing Combining".
10/2 {AI95-00285-01} number_decimal
Any character whose General Category is defined to be "
Number, Decimal".
10.1/2 {AI95-00285-01} number_letter
Any character whose General Category is defined to be "
Number, Letter".
10.2/2 {AI95-00285-01} punctuation_connector
Any character whose General Category is defined to be "
Punctuation, Connector".
10.3/2 {AI95-00285-01} other_format
Any character whose General Category is defined to be "Other,
Format".
11/2 {AI95-00285-01} separator_space
Any character whose General Category is defined to be "
Separator, Space".
12/2 {AI95-00285-01} separator_line
Any character whose General Category is defined to be "
Separator, Line".
12.1/2 {AI95-00285-01} separator_paragraph
Any character whose General Category is defined to be "
Separator, Paragraph".
13/3 {AI95-00285-01} {AI05-0262-1} format_effector
The characters whose code points are 16#09# (CHARACTER
TABULATION), 16#0A# (LINE FEED), 16#0B# (LINE TABULATION),
16#0C# (FORM FEED), 16#0D# (CARRIAGE RETURN), 16#85# (NEXT
LINE), and the characters in categories separator_line and
separator_paragraph.
13.a/2 Discussion: ISO/IEC 10646:2003 does not define the names of
control characters, but rather refers to the names defined by
ISO/IEC 6429:1992. These are the names that we use here.
13.1/2 {AI95-00285-01} other_control
Any character whose General Category is defined to be "Other,
Control", and which is not defined to be a format_effector.
13.2/2 {AI95-00285-01} other_private_use
Any character whose General Category is defined to be "Other,
Private Use".
13.3/2 {AI95-00285-01} other_surrogate
Any character whose General Category is defined to be "Other,
Surrogate".
14/3 {AI95-00285-01} {AI95-00395-01} {AI05-0262-1} graphic_character
Any character that is not in the categories other_control,
other_private_use, other_surrogate, format_effector, and whose
relative code point in its plane is neither 16#FFFE# nor
16#FFFF#.
14.a/2 This paragraph was deleted.
14.b/2 Discussion: {AI95-00285-01} We considered basing the definition of
lexical elements on Annex A of ISO/IEC TR 10176 (4th edition),
which lists the characters which should be supported in
identifiers for all programming languages, but we finally decided
against this option. Note that it is not our intent to diverge
from ISO/IEC TR 10176, except to the extent that ISO/IEC TR 10176
itself diverges from ISO/IEC 10646:2003 (which is the case at the
time of this writing [January 2005]).
14.c/2 More precisely, we intend to align strictly with ISO/IEC
10646:2003. It must be noted that ISO/IEC TR 10176 is a Technical
Report while ISO/IEC 10646:2003 is a Standard. If one has to make
a choice, one should conform with the Standard rather than with
the Technical Report. And, it turns out that one must make a
choice because there are important differences between the two:
14.d/2 * ISO/IEC TR 10176 is still based on ISO/IEC 10646:2000 while
ISO/IEC 10646:2003 has already been published for a year. We
cannot afford to delay the adoption of our amendment until
ISO/IEC TR 10176 has been revised.
14.e/2 * There are considerable differences between the two editions of
ISO/IEC 10646, notably in supporting characters beyond the BMP
(this might be significant for some languages, e.g. Korean).
14.f/2 * ISO/IEC TR 10176 does not define case conversion tables, which
are essential for a case-insensitive language like Ada. To get
case conversion tables, we would have to reference either
ISO/IEC 10646:2003 or Unicode, or we would have to invent our
own.
14.g/2 For the purpose of defining the lexical elements of the language,
we need character properties like categorization, as well as case
conversion tables. These are mentioned in ISO/IEC 10646:2003 as
useful for implementations, with a reference to Unicode.
Machine-readable tables are available on the web at URLs:
14.h/2 http://www.unicode.org/Public/4.0-Update/UnicodeData-4.0.0.txt
http://www.unicode.org/Public/4.0-Update/CaseFolding-4.0.0.txt
14.i/2 with an explanatory document found at URL:
14.j/2 http://www.unicode.org/Public/4.0-Update/UCD-4.0.0.html
14.k/2 The actual text of the standard only makes specific references to
the corresponding clauses of ISO/IEC 10646:2003, not to Unicode.
15/3 {AI95-00285-01} {AI05-0266-1} The following names are used when referring
to certain characters (the first name is that given in ISO/IEC 10646:2011):
15.a/3 Discussion: {AI95-00285-01} {AI05-0266-1} This table serves to
show the correspondence between ISO/IEC 10646:2011 names and the
graphic symbols (glyphs) used in this International Standard.
These are the characters that play a special role in the syntax of
Ada.
graphic symbol
"
#
&
'
(
)
*
+
,
-
.
name
quotation mark
number sign
ampersand
apostrophe, tick
left parenthesis
right parenthesis
asterisk, multiply
plus sign
comma
hyphen-minus, minus
full stop, dot, point
graphic symbol
:
;
<
=
>
_
|
/
!
%
name
colon
semicolon
less-than sign
equals sign
greater-than sign
low line, underline
vertical line
solidus, divide
exclamation point
percent sign
Implementation Requirements
16/3 {AI05-0286-1} An Ada implementation shall accept Ada source code in UTF-8
encoding, with or without a BOM (see A.4.11), where every character is
represented by its code point. The character pair CARRIAGE RETURN/LINE FEED
(code points 16#0D# 16#0A#) signifies a single end of line (see 2.2); every
other occurrence of a format_effector other than the character whose code
point position is 16#09# (CHARACTER TABULATION) also signifies a single end of
line.
16.a/3 Reason: {AI05-0079-1} {AI05-0286-1} This is simply requiring that
an Ada implementation be able to directly process the ACATS, which
is provided in the described format. Note that files that only
contain characters with code points in the first 128 (which is the
majority of the ACATS) are represented in the same way in both
UTF-8 and in "plain" string format. The ACATS includes a BOM in
files that have any characters with code points greater than 127.
Note that the BOM contains characters not legal in Ada source
code, so an implementation can use that to automatically
distinguish between files formatted as plain Latin-1 strings and
UTF-8 with BOM.
16.b/3 We allow line endings to be both represented as the pair CR LF (as
in Windows and the ACATS), and as single format_effector
characters (usually LF, as in Linux), in order that files created
by standard tools on most operating systems will meet the standard
format. We specify how many line endings each represent so that
compilers use the same line numbering for standard source files.
16.c/3 This requirement increases portability by having a format that is
accepted by all Ada compilers. Note that implementations can
support other source representations, including structured
representations like a parse tree.
Implementation Permissions
17/3 {AI95-00285-01} {AI05-0266-1} The categories defined above, as well as
case mapping and folding, may be based on an implementation-defined version of
ISO/IEC 10646 (2003 edition or later).
17.b/3 Ramification: The exact categories, case mapping, and case folding
chosen affects identifiers, the result of '[[Wide_]Wide_]Image,
and packages Wide_Characters.Handling and
Wide_Wide_Characters.Handling.
17.c/3 Discussion: This permission allows implementations to upgrade to
using a newer character set standard whenever that makes sense,
rather than having to wait for the next Ada Standard. But the
character set standard used cannot be older than ISO/IEC
10646:2003 (which is essentially similar to Unicode 4.0).
NOTES
18/2 1 {AI95-00285-01} The characters in categories other_control,
other_private_use, and other_surrogate are only allowed in comments.
19.a/3 This paragraph was deleted.{AI05-0286-1}
Extensions to Ada 83
19.b Ada 95 allows 8-bit and 16-bit characters, as well as
implementation-specified character sets.
Wording Changes from Ada 83
19.c/3 {AI95-00285-01} {AI05-0299-1} The syntax rules in this subclause
are modified to remove the emphasis on basic characters vs.
others. (In this day and age, there is no need to point out that
you can write programs without using (for example) lower case
letters.) In particular, character (representing all characters
usable outside comments) is added, and basic_graphic_character,
other_special_character, and basic_character are removed.
Special_character is expanded to include Ada 83's
other_special_character, as well as new 8-bit characters not
present in Ada 83. Ada 2005 removes special_character altogether;
we want to stick to ISO/IEC 10646:2003 character classifications.
Note that the term "basic letter" is used in A.3, "
Character Handling" to refer to letters without diacritical marks.
19.d/2 {AI95-00285-01} Character names now come from ISO/IEC 10646:2003.
19.e/2 This paragraph was deleted.{AI95-00285-01}
Extensions to Ada 95
19.f/2 {AI95-00285-01} {AI95-00395-01} Program text can use most
characters defined by ISO-10646:2003. This subclause has been
rewritten to use the categories defined in that Standard. This
should ease programming in languages other than English.
Inconsistencies With Ada 2005
19.g/3 {AI05-0299-1} {AI05-0266-1} An implementation is allowed (but not
required) to use a newer character set standard to determine the
categories, case mapping, and case folding. Doing so will change
the results of attributes '[[Wide_]Wide_]Image and the packages
[Wide_]Wide_Characters.Handling in the case of a few rarely used
characters. (This also could make some identifiers illegal, for
characters that are no longer classified as letters.) This is
unlikely to be a problem in practice. Moreover, truly portable Ada
2012 programs should avoid using in these contexts any characters
that would have different classifications in any character set
standards issued since 10646:2003 (since the compiler can use any
such standard as the basis for its classifications).
Wording Changes from Ada 2005
19.h/3 {AI05-0079-1} Correction: Clarified that only characters in the
categories defined here are allowed in the source of an Ada
program. This was clear in Ada 95, but Amendment 1 dropped the
wording instead of correcting it.
19.i/3 {AI05-0286-1} A standard source representation is defined that all
compilers are expected to process. Since this is the same format
as the ACATS, it seems unlikely that there are any implementations
that don't meet this requirement. Moreover, other representations
are still permitted, and the "impossible or impractical" loophole
(see 1.1.3) can be invoked for any implementations that cannot
directly process the ACATS.
2.2 Lexical Elements, Separators, and Delimiters
Static Semantics
1 The text of a program consists of the texts of one or more compilations.
The text of each compilation is a sequence of separate lexical elements. Each
lexical element is formed from a sequence of characters, and is either a
delimiter, an identifier, a reserved word, a numeric_literal, a
character_literal, a string_literal, or a comment. The meaning of a program
depends only on the particular sequences of lexical elements that form its
compilations, excluding comments.
2/3 {AI95-00285-01} {AI05-0262-1} The text of a compilation is divided into
lines. In general, the representation for an end of line is implementation
defined. However, a sequence of one or more format_effectors other than the
character whose code point is 16#09# (CHARACTER TABULATION) signifies at least
one end of line.
2.a Implementation defined: The representation for an end of line.
3/2 {AI95-00285-01} [In some cases an explicit separator is required to
separate adjacent lexical elements.] A separator is any of a separator_space,
a format_effector, or the end of a line, as follows:
4/2 * {AI95-00285-01} A separator_space is a separator except within a
comment, a string_literal, or a character_literal.
5/3 * {AI95-00285-01} {AI05-0262-1} The character whose code point is 16#09#
(CHARACTER TABULATION) is a separator except within a comment.
6 * The end of a line is always a separator.
7 One or more separators are allowed between any two adjacent lexical
elements, before the first of each compilation, or after the last. At least
one separator is required between an identifier, a reserved word, or a
numeric_literal and an adjacent identifier, reserved word, or
numeric_literal.
7.1/3 {AI05-0079-1} One or more other_format characters are allowed anywhere
that a separator is[; any such characters have no effect on the meaning of an
Ada program].
8/2 {AI95-00285-01} A delimiter is either one of the following characters:
9 & ' ( ) * + , - . / : ; < = > |
10 or one of the following compound delimiters each composed of two adjacent
special characters
11 => .. ** := /= >= <= << >> <>
12 Each of the special characters listed for single character delimiters is a
single delimiter except if this character is used as a character of a compound
delimiter, or as a character of a comment, string_literal, character_literal,
or numeric_literal.
13 The following names are used when referring to compound delimiters:
delimiter name
=> arrow
.. double dot
** double star, exponentiate
:= assignment (pronounced: "becomes")
/= inequality (pronounced: "not equal")
>= greater than or equal
<= less than or equal
<< left label bracket
>> right label bracket
<> box
Implementation Requirements
14 An implementation shall support lines of at least 200 characters in
length, not counting any characters used to signify the end of a line. An
implementation shall support lexical elements of at least 200 characters in
length. The maximum supported line length and lexical element length are
implementation defined.
14.a Implementation defined: Maximum supported line length and lexical
element length.
14.b Discussion: From URG recommendation.
Wording Changes from Ada 95
14.c/3 {AI95-00285-01} {AI05-0299-1} The wording was updated to use the
new character categories defined in the preceding subclause.
Extensions to Ada 2005
14.d/3 {AI05-0079-1} Correction: Clarified that other_format characters
are allowed anywhere that separators are allowed. This was
intended in Ada 2005, but didn't actually make it into the
wording.
2.3 Identifiers
1 Identifiers are used as names.
Syntax
2/2 {AI95-00285-01} {AI95-00395-01} identifier ::=
identifier_start {identifier_start | identifier_extend}
3/2 {AI95-00285-01} {AI95-00395-01} identifier_start ::=
letter_uppercase
| letter_lowercase
| letter_titlecase
| letter_modifier
| letter_other
| number_letter
3.1/3 {AI95-00285-01} {AI95-00395-01} {AI05-0091-1} identifier_extend
::=
mark_non_spacing
| mark_spacing_combining
| number_decimal
| punctuation_connector
4/3 {AI95-00395-01} {AI05-0091-1} An identifier shall not contain two
consecutive characters in category punctuation_connector, or end with
a character in that category.
4.a/3 Reason: This rule was stated in the syntax in Ada 95, but that has
gotten too complex in Ada 2005.
Static Semantics
5/3 {AI95-00285-01} {AI05-0091-1} {AI05-0227-1} {AI05-0266-1} {AI05-0299-1}
Two identifiers are considered the same if they consist of the same sequence
of characters after applying locale-independent simple case folding, as
defined by documents referenced in the note in Clause 1 of ISO/IEC 10646:2011.
5.a/3 Discussion: {AI05-0227-1} Simple case folding is a mapping to
lower case, so this is matching the defining (lower case) version
of a reserved word. We could have mentioned case folding of the
reserved words, but as that is an identity function, it would have
no effect.
5.a.1/3 {AI05-0227-1} The "documents referenced" means Unicode. Note that
simple case folding is supposed to be compatible between Unicode
versions, so the Unicode version used doesn't matter.
5.3/3 {AI95-00395-01} {AI05-0091-1} {AI05-0227-1} After applying simple case
folding, an identifier shall not be identical to a reserved word.
5.b/3 Implementation Note: We match the reserved words after applying
case folding so that the rules for identifiers and reserved words
are the same. Since a compiler usually will lexically process
identifiers and reserved words the same way (often with the same
code), this will prevent a lot of headaches.
5.c/3 Ramification: {AI05-0227-1} The rules for reserved words differ in
one way: they define case conversion on letters rather than
sequences. This means that it is possible that there exist some
unusual sequences that are neither identifiers nor reserved words.
We are not aware of any such sequences so long as we use simple
case folding (as opposed to full case folding), but we have
defined the rules in case any are introduced in future character
set standards. This originally was a problem when converting to
upper case: "if" and "acceß" have upper case conversions of "
IF" and "ACCESS" respectively. We would not want these to be treated
as reserved words. But neither of these cases exist when using
simple case folding.
Implementation Permissions
6 In a nonstandard mode, an implementation may support other upper/lower
case equivalence rules for identifiers[, to accommodate local conventions].
6.a/3 Discussion: {AI95-00285-01} {AI05-0227-1} For instance, in most
languages, the simple case folded equivalent of LATIN CAPITAL
LETTER I (an upper case letter without a dot above) is LATIN SMALL
LETTER I (a lower case letter with a dot above). In Turkish,
though, LATIN CAPITAL LETTER I and LATIN CAPITAL LETTER I WITH DOT
ABOVE are two distinct letters, so the case folded equivalent of
LATIN CAPITAL LETTER I is LATIN SMALL LETTER DOTLESS I, and the
case folded equivalent of LATIN CAPITAL LETTER I WITH DOT ABOVE is
LATIN SMALL LETTER I. Take for instance the following identifier
(which is the name of a city on the Tigris river in Eastern
Anatolia):
6.b/3 DIYARBAKIR -- The first i is dotted, the second isn't.
6.c/3 A Turkish reader would expect that the above identifier is
equivalent to:
6.d/3 diyarbakir
6.d.1/3 However, locale-independent simple case folding (and thus Ada)
maps this to:
6.d.2/3 dIyarbakir
6.e/3 which is different from any of the following identifiers:
6.f/2 diyarbakir
diyarbakir
diyarbakir
diyarbakir
6.f.1/3 including the "correct" matching identifier for Turkish. Upper
case conversion (used in '[Wide_]Wide_Image) introduces additional
problems.
6.g/3 An implementation targeting the Turkish market is allowed (in
fact, expected) to provide a nonstandard mode where case folding
is appropriate for Turkish.
6.j/2 Lithuanian and Azeri are two other languages that present similar
idiosyncrasies.
NOTES
6.1/2 2 {AI95-00285-01} Identifiers differing only in the use of
corresponding upper and lower case letters are considered the same.
Examples
7 Examples of identifiers:
8/2 {AI95-00433-01} Count X Get_Symbol Ethelyn Marion
Snobol_4 X1 Page_Count Store_Next_Item
<Unicode-928><Unicode-955><Unicode-940><Unicode-964><Unicode-969>
<Unicode-957> -- Plato
<Unicode-1063><Unicode-1072><Unicode-1081><Unicode-1082>
<Unicode-1086><Unicode-1074><Unicode-1089><Unicode-1082>
<Unicode-1080><Unicode-1081> -- Tchaikovsky
<Unicode-952> <Unicode-966> -- Angles
Wording Changes from Ada 83
8.a We no longer include reserved words as identifiers. This is not a
language change. In Ada 83, identifier included reserved words.
However, this complicated several other rules (for example,
regarding implementation-defined attributes and pragmas, etc.). We
now explicitly allow certain reserved words for attribute
designators, to make up for the loss.
8.b Ramification: Because syntax rules are relevant to overload
resolution, it means that if it looks like a reserved word, it is
not an identifier. As a side effect, implementations cannot use
reserved words as implementation-defined attributes or pragma
names.
Extensions to Ada 95
8.c/2 {AI95-00285-01} An identifier can use any letter defined by
ISO-10646:2003, along with several other categories. This should
ease programming in languages other than English.
Incompatibilities With Ada 2005
8.d/3 {AI05-0091-1} Correction: other_format characters were removed
from identifiers as the Unicode recommendations have changed. This
change can only affect programs written for the original Ada 2005,
so there should be few such programs.
8.e/3 {AI05-0227-1} Correction: We now specify simple case folding
rather than full case folding. That potentially could change
identifier equivalence, although it is more likely that
identifiers that are considered the same in original Ada 2005 will
now be considered different. This change was made because the
original Ada 2005 definition was incompatible (and even
inconsistent in unusual cases) with the Ada 95 identifier
equivalence rules. As such, the Ada 2005 rules were rarely fully
implemented, and in any case, only Ada 2005 identifiers containing
wide characters could be affected.
2.4 Numeric Literals
1 There are two kinds of numeric_literals, real literals and integer
literals. A real literal is a numeric_literal that includes a point; an
integer literal is a numeric_literal without a point.
Syntax
2 numeric_literal ::= decimal_literal | based_literal
NOTES
3 3 The type of an integer literal is universal_integer. The type of a
real literal is universal_real.
2.4.1 Decimal Literals
1 A decimal_literal is a numeric_literal in the conventional decimal
notation (that is, the base is ten).
Syntax
2 decimal_literal ::= numeral [.numeral] [exponent]
3 numeral ::= digit {[underline] digit}
4 exponent ::= E [+] numeral | E - numeral
4.1/2 {AI95-00285-01} digit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
5 An exponent for an integer literal shall not have a minus sign.
5.a Ramification: Although this rule is in this subclause, it applies
also to the next subclause.
Static Semantics
6 An underline character in a numeric_literal does not affect its meaning.
The letter E of an exponent can be written either in lower case or in upper
case, with the same meaning.
6.a Ramification: Although these rules are in this subclause, they
apply also to the next subclause.
7 An exponent indicates the power of ten by which the value of the
decimal_literal without the exponent is to be multiplied to obtain the value
of the decimal_literal with the exponent.
Examples
8 Examples of decimal literals:
9 12 0 1E6 123_456 -- integer literals
12.0 0.0 0.456 3.14159_26 -- real literals
Wording Changes from Ada 83
9.a We have changed the syntactic category name integer to be
numeral. We got this idea from ACID. It avoids the confusion
between this and integers. (Other places don't offer similar
confusions. For example, a string_literal is different from a
string.)
2.4.2 Based Literals
1 [ A based_literal is a numeric_literal expressed in a form that specifies
the base explicitly.]
Syntax
2 based_literal ::=
base # based_numeral [.based_numeral] # [exponent]
3 base ::= numeral
4 based_numeral ::=
extended_digit {[underline] extended_digit}
5 extended_digit ::= digit | A | B | C | D | E | F
Legality Rules
6 The base (the numeric value of the decimal numeral preceding the first #)
shall be at least two and at most sixteen. The extended_digits A through F
represent the digits ten through fifteen, respectively. The value of each
extended_digit of a based_literal shall be less than the base.
Static Semantics
7 The conventional meaning of based notation is assumed. An exponent
indicates the power of the base by which the value of the based_literal
without the exponent is to be multiplied to obtain the value of the
based_literal with the exponent. The base and the exponent, if any, are in
decimal notation.
8 The extended_digits A through F can be written either in lower case or in
upper case, with the same meaning.
Examples
9 Examples of based literals:
10 2#1111_1111# 16#FF# 016#0ff# -- integer literals of value 255
16#E#E1 2#1110_0000# -- integer literals of value 224
16#F.FF#E+2 2#1.1111_1111_1110#E11 -- real literals of value 4095.0
Wording Changes from Ada 83
10.a The rule about which letters are allowed is now encoded in BNF, as
suggested by Mike Woodger. This is clearly more readable.
2.5 Character Literals
1 [A character_literal is formed by enclosing a graphic character between
two apostrophe characters.]
Syntax
2 character_literal ::= 'graphic_character'
NOTES
3 4 A character_literal is an enumeration literal of a character type.
See 3.5.2.
Examples
4 Examples of character literals:
5/2 {AI95-00433-01} 'A' '*' ''' ' '
'L' '<Unicode-1051>' '<Unicode-923>' -- Various els.
'<Unicode-8734>' '<Unicode-1488>
' -- Big numbers - infinity and aleph.
Wording Changes from Ada 83
5.a/3 {AI05-0299-1} The definitions of the values of literals are in
Clauses 3 and 4, rather than here, since it requires knowledge of
types.
2.6 String Literals
1 [A string_literal is formed by a sequence of graphic characters (possibly
none) enclosed between two quotation marks used as string brackets. They are
used to represent operator_symbols (see 6.1), values of a string type (see
4.2), and array subaggregates (see 4.3.3). ]
Syntax
2 string_literal ::= "{string_element}"
3 string_element ::= "" | non_quotation_mark_graphic_character
4 A string_element is either a pair of quotation marks (""), or a single
graphic_character other than a quotation mark.
Static Semantics
5 The sequence of characters of a string_literal is formed from the sequence
of string_elements between the bracketing quotation marks, in the given order,
with a string_element that is "" becoming a single quotation mark in the
sequence of characters, and any other string_element being reproduced in the
sequence.
6 A null string literal is a string_literal with no string_elements between
the quotation marks.
NOTES
7 5 An end of line cannot appear in a string_literal.
7.1/2 6 {AI95-00285-01} No transformation is performed on the sequence of
characters of a string_literal.
Examples
8 Examples of string literals:
9/2 {AI95-00433-01} "Message of the day:"
"" -- a null string literal
" " "A" """" -- three string literals of length 1
"Characters such as $, %, and } are allowed in string literals"
"Archimedes said ""<Unicode-917><Unicode-973><Unicode-961>
<Unicode-951><Unicode-954><Unicode-945>"""
"Volume of cylinder (PIr²h) = "
Wording Changes from Ada 83
9.a The wording has been changed to be strictly lexical. No mention is
made of string or character values, since string_literals are also
used to represent operator_symbols, which don't have a defined
value.
9.b The syntax is described differently.
Wording Changes from Ada 95
9.c/2 {AI95-00285-01} We explicitly say that the characters of a
string_literal should be used as is. In particular, no
normalization or folding should be performed on a string_literal.
2.7 Comments
1 A comment starts with two adjacent hyphens and extends up to the end of
the line.
Syntax
2 comment ::= --{non_end_of_line_character}
3 A comment may appear on any line of a program.
Static Semantics
4 The presence or absence of comments has no influence on whether a program
is legal or illegal. Furthermore, comments do not influence the meaning of a
program; their sole purpose is the enlightenment of the human reader.
Examples
5 Examples of comments:
6 -- the last sentence above echoes the Algol 68 report
end; -- processing of Line is complete
-- a long comment may be split onto
-- two or more consecutive lines
---------------- the first two hyphens start the comment
2.8 Pragmas
1 A pragma is a compiler directive. There are language-defined pragmas that
give instructions for optimization, listing control, etc. An implementation
may support additional (implementation-defined) pragmas.
Language Design Principles
1.a/3 {AI05-0100-1} {AI05-0163-1} In general, if all pragmas are treated
as unrecognized pragmas, the program should remain both
syntactically and semantically legal. There are a few exceptions
to this general principle (for example, pragma Import can
eliminate the need for a completion), but the principle remains,
and is strictly true at the syntactic level. Certainly any
implementation-defined pragmas should obey this principle both
syntactically and semantically, so that if the pragmas are not
recognized by some other implementation, the program will remain
legal.
Syntax
2 pragma ::=
pragma identifier [(pragma_argument_association
{, pragma_argument_association})];
3/3 {AI05-0290-1} pragma_argument_association ::=
[pragma_argument_identifier =>] name
| [pragma_argument_identifier =>] expression
| pragma_argument_aspect_mark => name
| pragma_argument_aspect_mark => expression
4/3 {AI05-0290-1} In a pragma, any pragma_argument_associations without a
pragma_argument_identifier or pragma_argument_aspect_mark shall
precede any associations with a pragma_argument_identifier or
pragma_argument_aspect_mark.
5 Pragmas are only allowed at the following places in a program:
6 * After a semicolon delimiter, but not within a formal_part or
discriminant_part.
7/3 * {AI05-0100-1} {AI05-0163-1} At any place where the syntax rules
allow a construct defined by a syntactic category whose name ends
with "declaration", "item", "statement", "clause", or "
alternative", or one of the syntactic categories variant or
exception_handler; but not in place of such a construct if the
construct is required, or is part of a list that is required to
have at least one such construct.
7.1/3 * {AI05-0163-1} In place of a statement in a
sequence_of_statements.
7.2/3 * {AI05-0100-1} At any place where a compilation_unit is allowed.
8 Additional syntax rules and placement restrictions exist for specific
pragmas.
8.a Discussion: The above rule is written in text, rather than in BNF;
the syntactic category pragma is not used in any BNF syntax rule.
8.b Ramification: A pragma is allowed where a
generic_formal_parameter_declaration is allowed.
9 The name of a pragma is the identifier following the reserved word pragma.
The name or expression of a pragma_argument_association is a pragma argument.
9.a/2 To be honest: {AI95-00284-02} For compatibility with Ada 83, the
name of a pragma may also be "interface", which is not an
identifier (because it is a reserved word). See J.12.
10/3 {AI05-0272-1} An identifier specific to a pragma is an identifier or
reserved word that is used in a pragma argument with special meaning for that
pragma.
10.a To be honest: Whenever the syntax rules for a given pragma allow
"identifier" as an argument of the pragma, that identifier is an
identifier specific to that pragma.
10.b/3 {AI05-0272-1} In a few cases, a reserved word is allowed as "an
identifier specific to a pragma". Even in these cases, the syntax
still is written as identifier (the reserved word(s) are not
shown). For example, the restriction No_Use_Of_Attribute (see
13.12.1) allows the reserved words which can be attribute
designators, but the syntax for a restriction does not include
these reserved words.
Static Semantics
11 If an implementation does not recognize the name of a pragma, then it has
no effect on the semantics of the program. Inside such a pragma, the only
rules that apply are the Syntax Rules.
11.a To be honest: This rule takes precedence over any other rules that
imply otherwise.
11.b Ramification: Note well: this rule applies only to pragmas whose
name is not recognized. If anything else is wrong with a pragma
(at compile time), the pragma is illegal. This is true whether the
pragma is language defined or implementation defined.
11.c For example, an expression in an unrecognized pragma does not
cause freezing, even though the rules in 13.14, "Freezing Rules
" say it does; the above rule overrules those other rules. On the
other hand, an expression in a recognized pragma causes freezing,
even if this makes something illegal.
11.d For another example, an expression that would be ambiguous is not
illegal if it is inside an unrecognized pragma.
11.e Note, however, that implementations have to recognize pragma
Inline(Foo) and freeze things accordingly, even if they choose to
never do inlining.
11.f Obviously, the contradiction needs to be resolved one way or the
other. The reasons for resolving it this way are: The
implementation is simple - the compiler can just ignore the
pragma altogether. The interpretation of constructs appearing
inside implementation-defined pragmas is implementation defined.
For example: "pragma Mumble(X);". If the current implementation
has never heard of Mumble, then it doesn't know whether X is a
name, an expression, or an identifier specific to the pragma
Mumble.
11.g To be honest: The syntax of individual pragmas overrides the
general syntax for pragma.
11.h Ramification: Thus, an identifier specific to a pragma is not a
name, syntactically; if it were, the visibility rules would be
invoked, which is not what we want.
11.i/3 {AI05-0229-1} This also implies that named associations do not
allow one to give the arguments in an arbitrary order - the order
given in the syntax rule for each individual pragma must be
obeyed. However, it is generally possible to leave out earlier
arguments when later ones are given; for example, this is allowed
by the syntax rule for pragma Import (see J.15.5, "
Interfacing Pragmas"). As for subprogram calls, positional
notation precedes named notation.
11.j Note that Ada 83 had no pragmas for which the order of named
associations mattered, since there was never more than one
argument that allowed named associations.
11.k To be honest: The interpretation of the arguments of
implementation-defined pragmas is implementation defined. However,
the syntax rules have to be obeyed.
Dynamic Semantics
12 Any pragma that appears at the place of an executable construct is
executed. Unless otherwise specified for a particular pragma, this execution
consists of the evaluation of each evaluable pragma argument in an arbitrary
order.
12.a Ramification: For a pragma that appears at the place of an
elaborable construct, execution is elaboration.
12.b An identifier specific to a pragma is neither a name nor an
expression - such identifiers are not evaluated (unless an
implementation defines them to be evaluated in the case of an
implementation-defined pragma).
12.c The "unless otherwise specified" part allows us (and
implementations) to make exceptions, so a pragma can contain an
expression that is not evaluated. Note that pragmas in
type_definitions may contain expressions that depend on
discriminants.
12.d When we wish to define a pragma with some run-time effect, we
usually make sure that it appears in an executable context;
otherwise, special rules are needed to define the run-time effect
and when it happens.
Implementation Requirements
13 The implementation shall give a warning message for an unrecognized pragma
name.
13.a Ramification: An implementation is also allowed to have modes in
which a warning message is suppressed, or in which the presence of
an unrecognized pragma is a compile-time error.
Implementation Permissions
14 An implementation may provide implementation-defined pragmas; the name of
an implementation-defined pragma shall differ from those of the
language-defined pragmas.
14.a Implementation defined: Implementation-defined pragmas.
14.b Ramification: The semantics of implementation-defined pragmas, and
any associated rules (such as restrictions on their placement or
arguments), are, of course, implementation defined.
Implementation-defined pragmas may have run-time effects.
15 An implementation may ignore an unrecognized pragma even if it violates
some of the Syntax Rules, if detecting the syntax error is too complex.
15.a Reason: Many compilers use extra post-parsing checks to enforce
the syntax rules, since the Ada syntax rules are not LR(k) (for
any k). (The grammar is ambiguous, in fact.) This paragraph allows
them to ignore an unrecognized pragma, without having to perform
such post-parsing checks.
Implementation Advice
16/3 {AI05-0163-1} Normally, implementation-defined pragmas should have no
semantic effect for error-free programs; that is, if the
implementation-defined pragmas in a working program are replaced with
unrecognized pragmas, the program should still be legal, and should still have
the same semantics.
16.a.1/2 Implementation Advice: Implementation-defined pragmas should have
no semantic effect for error-free programs.
16.a Ramification: Note that "semantics" is not the same as "
effect;" as explained in 1.1.3, the semantics defines a set of possible
effects.
16.b Note that adding a pragma to a program might cause an error
(either at compile time or at run time). On the other hand, if the
language-specified semantics for a feature are in part
implementation defined, it makes sense to support pragmas that
control the feature, and that have real semantics; thus, this
paragraph is merely a recommendation.
17 Normally, an implementation should not define pragmas that can make an
illegal program legal, except as follows:
18/3 * {AI05-0229-1} A pragma used to complete a declaration;
18.a/3 Discussion: {AI05-0229-1} There are no language-defined pragmas
which can be completions; pragma Import was defined this way in
Ada 95 and Ada 2005, but in Ada 2012 pragma Import just sets
aspect Import which disallows having any completion.
19 * A pragma used to configure the environment by adding, removing, or
replacing library_items.
19.a.1/2 Implementation Advice: Implementation-defined pragmas should not
make an illegal program legal, unless they complete a declaration
or configure the library_items in an environment.
19.a Ramification: For example, it is OK to support Interface,
System_Name, Storage_Unit, and Memory_Size pragmas for upward
compatibility reasons, even though all of these pragmas can make
an illegal program legal. (The latter three can affect legality in
a rather subtle way: They affect the value of named numbers in
System, and can therefore affect the legality in cases where
static expressions are required.)
19.b On the other hand, adding implementation-defined pragmas to a
legal program can make it illegal. For example, a common kind of
implementation-defined pragma is one that promises some property
that allows more efficient code to be generated. If the promise is
a lie, it is best if the user gets an error message.
Incompatibilities With Ada 83
19.c In Ada 83, "bad" pragmas are ignored. In Ada 95, they are illegal,
except in the case where the name of the pragma itself is not
recognized by the implementation.
Extensions to Ada 83
19.d Implementation-defined pragmas may affect the legality of a
program.
Wording Changes from Ada 83
19.e Implementation-defined pragmas may affect the run-time semantics
of the program. This was always true in Ada 83 (since it was not
explicitly forbidden by RM83), but it was not clear, because there
was no definition of "executing" or "elaborating" a pragma.
Extensions to Ada 2005
19.f/3 {AI05-0163-1} Correction: Allow pragmas in place of a statement,
even if there are no other statements in a
sequence_of_statements.
19.g/3 {AI05-0272-1} Identifiers specific to a pragma can be reserved
words.
19.h/3 {AI05-0290-1} Pragma arguments can be identified with
aspect_marks; this allows identifier'Class in this context. As
usual, this is only allowed if specifically allowed by a
particular pragma.
Wording Changes from Ada 2005
19.i/3 {AI05-0100-1} Correction: Clarified where pragmas are (and are
not) allowed.
Syntax
20 The forms of List, Page, and Optimize pragmas are as follows:
21 pragma List(identifier);
22 pragma Page;
23 pragma Optimize(identifier);
24 [Other pragmas are defined throughout this International Standard, and
are summarized in Annex L.]
24.a Ramification: The language-defined pragmas are supported by every
implementation, although "supporting" some of them (for example,
Inline) requires nothing more than checking the arguments, since
they act only as advice to the implementation.
Static Semantics
25 A pragma List takes one of the identifiers On or Off as the single
argument. This pragma is allowed anywhere a pragma is allowed. It specifies
that listing of the compilation is to be continued or suspended until a List
pragma with the opposite argument is given within the same compilation. The
pragma itself is always listed if the compiler is producing a listing.
26 A pragma Page is allowed anywhere a pragma is allowed. It specifies that
the program text which follows the pragma should start on a new page (if the
compiler is currently producing a listing).
27 A pragma Optimize takes one of the identifiers Time, Space, or Off as the
single argument. This pragma is allowed anywhere a pragma is allowed, and it
applies until the end of the immediately enclosing declarative region, or for
a pragma at the place of a compilation_unit, to the end of the compilation. It
gives advice to the implementation as to whether time or space is the primary
optimization criterion, or that optional optimizations should be turned off.
[It is implementation defined how this advice is followed.]
27.a Implementation defined: Effect of pragma Optimize.
27.b Discussion: For example, a compiler might use Time vs. Space to
control whether generic instantiations are implemented with a
macro-expansion model, versus a shared-generic-body model.
27.c We don't define what constitutes an "optimization" - in fact, it
cannot be formally defined in the context of Ada. One compiler
might call something an optional optimization, whereas another
compiler might consider that same thing to be a normal part of
code generation. Thus, the programmer cannot rely on this pragma
having any particular portable effect on the generated code. Some
compilers might even ignore the pragma altogether.
Examples
28 Examples of pragmas:
29/3 {AI95-00433-01} {AI05-0229-1}
pragma List(Off); -- turn off listing generation
pragma Optimize(Off); -- turn off optional optimizations
pragma Pure(Rational_Numbers); -- set categorization for package
pragma Assert(Exists(File_Name),
Message => "Nonexistent file"); -- assert file exists
Extensions to Ada 83
29.a The Optimize pragma now allows the identifier Off to request that
normal optimization be turned off.
29.b An Optimize pragma may appear anywhere pragmas are allowed.
Wording Changes from Ada 83
29.c We now describe the pragmas Page, List, and Optimize here, to act
as examples, and to remove the normative material from Annex L
, "Language-Defined Pragmas", so it can be entirely an informative
annex.
Wording Changes from Ada 95
29.d/2 {AI95-00433-01} Updated the example of named pragma parameters,
because the second parameter of pragma Suppress is obsolescent.
Wording Changes from Ada 2005
29.e/3 {AI05-0229-1} Updated the example of pragmas, because both
pragmas Inline and Import are obsolescent.
2.9 Reserved Words
Syntax
1/1 This paragraph was deleted.
2/3 {AI95-00284-02} {AI95-00395-01} {AI05-0091-1} The following are the
reserved words. Within a program, some or all of the letters of a
reserved word may be in upper case.
2.a Discussion: Reserved words have special meaning in the syntax. In
addition, certain reserved words are used as attribute names.
2.b The syntactic category identifier no longer allows reserved words.
We have added the few reserved words that are legal explicitly to
the syntax for attribute_reference. Allowing identifier to include
reserved words has been a source of confusion for some users, and
differs from the way they are treated in the C and Pascal language
definitions.
abort
abs
abstract
accept
access
aliased
all
and
array
at
begin
body
case
constant
declare
delay
delta
digits
do
else
elsif
end
entry
exception
exit
for
function
generic
goto
if
in
interface
is
limited
loop
mod
new
not
null
of
or
others
out
overriding
package
pragma
private
procedure
protected
raise
range
record
rem
renames
requeue
return
reverse
select
separate
some
subtype
synchronized
tagged
task
terminate
then
type
until
use
when
while
with
xor
NOTES
3 7 The reserved words appear in lower case boldface in this
International Standard, except when used in the designator of an
attribute (see 4.1.4). Lower case boldface is also used for a reserved
word in a string_literal used as an operator_symbol. This is merely a
convention - programs may be written in whatever typeface is desired
and available.
Incompatibilities With Ada 83
3.a The following words are not reserved in Ada 83, but are reserved
in Ada 95: abstract, aliased, protected, requeue, tagged, until.
Wording Changes from Ada 83
3.b/3 {AI05-0299-1} The subclause entitled "Allowed Replacements of
Characters" has been moved to Annex J, "Obsolescent Features".
Incompatibilities With Ada 95
3.c/2 {AI95-00284-02} The following words are not reserved in Ada 95,
but are reserved in Ada 2005: interface, overriding, synchronized.
A special allowance is made for pragma Interface (see J.12). Uses
of these words as identifiers will need to be changed, but we do
not expect them to be common.
Wording Changes from Ada 95
3.d/2 {AI95-00395-01} The definition of upper case equivalence has been
modified to allow identifiers using all of the characters of ISO
10646. This change has no effect on the character sequences that
are reserved words, but does make some unusual sequences of
characters illegal.
Incompatibilities With Ada 2005
3.e/3 {AI05-0091-1} Correction: Removed other_format characters from
reserved words in order to be compatible with the latest Unicode
recommendations. This change can only affect programs written for
original Ada 2005, and there is little reason to put other_format
characters into reserved words in the first place, so there should
be very few such programs.
3.f/3 {AI05-0176-1} The following word is not reserved in Ada 2005, but
is reserved in Ada 2012: some. Uses of this word as an identifier
will need to be changed, but we do not expect them to be common.
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