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1.1.4 Method of Description and Syntax Notation
1
The form of an Ada program is described by means
of a context-free syntax together with context-dependent requirements
expressed by narrative rules.
2
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.
3
{syntax
(notation)} {grammar
(notation)} {context
free grammar (notation)} {BNF
(Backus-Naur Form) (notation)} {Backus-Naur
Form (BNF) (notation)} The context-free
syntax of the language is described using a simple variant of Backus-Naur
Form. In particular:
4
- Lower
case words in a sans-serif font, some containing embedded underlines,
are used to denote syntactic categories, for example:
5
case_statement
6
- Boldface
words are used to denote reserved words, for example:
7
array
8
- Square
brackets enclose optional items. Thus the two following rules are equivalent.
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return_statement ::= return [expression];
return_statement ::= return; | return expression;
10
- 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.
11
term ::= factor {multiplying_operator factor}
term ::= factor | term multiplying_operator factor
12
- A vertical
line separates alternative items unless it occurs immediately after an
opening curly bracket, in which case it stands for itself:
13
constraint ::= scalar_constraint | composite_constraint
discrete_choice_list ::= discrete_choice {| discrete_choice}
14
- {italics (syntax
rules)} 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.
14.a
Discussion: {LR(1)}
{ambiguous grammar} {grammar
(resolution of ambiguity)} {grammar (ambiguous)}
The grammar given in the RM95 is not LR(1). In fact,
it is ambiguous; the ambiguities are resolved by the overload resolution
rules (see 8.6).
14.b
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.
14.c
When we say, for example, ``a
declarative_item of a declarative_part'',
we are talking about a declarative_item
immediately within that declarative_part.
When we say ``a declarative_item
in, or within, a declarative_part'',
we are talking about a declarative_item
anywhere in the declarative_part,
possibly deeply nested within other declarative_parts.
(This notation doesn't work very well for names,
since the name ``of'' something also has another meaning.)
14.d
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.
15
{syntactic category}
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.
16
{Construct} [Glossary
Entry]
A
construct is a piece of text (explicit
or implicit) that is an instance of a syntactic category defined under
``Syntax.''
16.a
Ramification: For example,
an expression is a construct. A
declaration is a construct, whereas the thing declared by a declaration
is an ``entity.''
16.b
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 non-technical sense) in the program
text, and even though its meaning is not defined entirely in terms of
that text.
17
{constituent (of a construct)}
A
constituent of a construct is the construct
itself, or any construct appearing within it.
18
{arbitrary order}
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.
{type conversion (arbitrary order)
[partial]} {conversion
(arbitrary order) [partial]} [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.]
18.a
Discussion: Programs
will be more portable if their external effect does not depend on the
particular order chosen by an implementation.
18.b
18.c
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.
18.d
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.
19
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:
20
if_statement ::=
if condition then
sequence_of_statements
{elsif condition then
sequence_of_statements}
[else
sequence_of_statements]
end if;
21
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.
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