1.1.4 Method of Description and Syntax Notation
The form of an Ada program is described by means
of a context-free syntax together with context-dependent requirements
expressed by narrative rules.
The meaning of Ada programs is described by means
of narrative rules defining both the effects of each construct and the
composition rules for constructs.
The
context-free syntax of the language is described using a simple variant
of Backus-Naur Form. In particular:
Lower case words
in a sans-serif font, some containing embedded underlines, are used to
denote syntactic categories, for example:
Boldface words are
used to denote reserved words, for example:
array
Square brackets enclose
optional items. Thus the two following rules are equivalent.
Curly brackets enclose
a repeated item. The item may appear zero or more times; the repetitions
occur from left to right as with an equivalent left-recursive rule. Thus
the two following rules are equivalent.
A vertical line separates
alternative items unless it occurs immediately after an opening curly
bracket, in which case it stands for itself:
If the name of any syntactic
category starts with an italicized part, it is equivalent to the category
name without the italicized part. The italicized part is intended to
convey some semantic information. For example
subtype_name
and
task_name
are both equivalent to
name
alone.
Discussion: The
grammar given in this International Standard is not LR(1). In fact, it
is ambiguous; the ambiguities are resolved by the overload resolution
rules (see
8.6).
We often use “if” to mean “if
and only if” in definitions. For example, if we define “photogenic”
by saying, “A type is photogenic if it has the following properties...,”
we mean that a type is photogenic if and only if it has those
properties. It is usually clear from the context, and adding the “and
only if” seems too cumbersome.
When we refer to the name of a language-defined
entity (for example, Duration), we mean the language-defined entity even
in programs where the declaration of the language-defined entity is hidden
by another declaration. For example, when we say that the expected type
for the
expression
of a
delay_relative_statement
is Duration, we mean the language-defined type Duration that is declared
in Standard, not some type Duration the user might have declared.
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The delimiters, compound delimiters, reserved words, and
numeric_literals
are exclusively made of the characters whose code point is between 16#20#
and 16#7E#, inclusively. The special characters for which names are defined
in this International Standard (see
2.1) belong
to the same range. [For example, the character E in the definition of
exponent
is the character whose name is “LATIN CAPITAL LETTER E”,
not “GREEK CAPITAL LETTER EPSILON”.]
Discussion: This just means that programs
can be written in plain ASCII characters; no characters outside of the
7-bit range are required.
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When this International Standard mentions the conversion of some character
or sequence of characters to upper case, it means the character or sequence
of characters obtained by using simple upper case mapping, as defined
by documents referenced in the note in Clause 1 of ISO/IEC 10646:2011.
This paragraph
was deleted.
A
syntactic category
is a nonterminal in the grammar defined in BNF under “Syntax.”
Names of syntactic categories are set in a different font,
like_this.
A
construct is a piece
of text (explicit or implicit) that is an instance of a syntactic category
defined under “Syntax”.
Ramification: For example, an
expression
is a construct. A declaration is a construct, whereas the thing declared
by a declaration is an “entity.”
Discussion: “Explicit” and
“implicit” don't mean exactly what you might think they mean:
The text of an instance of a generic is considered explicit, even though
it does not appear explicitly (in the nontechnical sense) in the program
text, and even though its meaning is not defined entirely in terms of
that text.
A
constituent of a construct
is the construct itself, or any construct appearing within it.
Whenever the run-time semantics
defines certain actions to happen in an
arbitrary order, this
means that the implementation shall arrange for these actions to occur
in a way that is equivalent to some sequential order, following the rules
that result from that sequential order. When evaluations are defined
to happen in an arbitrary order, with conversion of the results to some
subtypes, or with some run-time checks, the evaluations, conversions,
and checks may be arbitrarily interspersed, so long as each expression
is evaluated before converting or checking its value.
[Note
that the effect of a program can depend on the order chosen by the implementation.
This can happen, for example, if two actual parameters of a given call
have side effects.]
Discussion: Programs will be more portable
if their external effect does not depend on the particular order chosen
by an implementation.
There is no requirement that the implementation
always choose the same order in a given kind of situation. In fact, the
implementation is allowed to choose a different order for two different
executions of the same construct. However, we expect most implementations
will behave in a relatively predictable manner in most situations.
Reason: The “sequential order”
wording is intended to allow the programmer to rely on “benign”
side effects. For example, if F is a function that returns a unique integer
by incrementing some global and returning the result, a call such as
P(F, F) is OK if the programmer cares only that the two results of F
are unique; the two calls of F cannot be executed in parallel, unless
the compiler can prove that parallel execution is equivalent to some
sequential order.
3 The syntax rules describing structured
constructs are presented in a form that corresponds to the recommended
paragraphing. For example, an
if_statement
is defined as:
4 The line breaks and indentation in the
syntax rules indicate the recommended line breaks and indentation in
the corresponding constructs. The preferred places for other line breaks
are after semicolons.
Wording Changes from Ada 95
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We now explicitly say that the lexical elements of the language (with
a few exceptions) are made up of characters in the lower half of the
Latin-1 character set. This is needed to avoid confusion given the new
capability to use most ISO 10646 characters in identifiers and strings.
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We now explicitly define what the Standard means by upper case, as there
are many possibilities for ISO 10646 characters.
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The example for square brackets has been changed as there is no longer
a
return_statement syntax rule.
Wording Changes from Ada 2005
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Correction: Upper case is defined by "simple upper case mapping",
because "full case folding" is a mapping (mostly) to lower
case.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
