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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.
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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
The
context-free syntax of the language is described using a simple variant
of Backus-Naur Form. In particular:
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- Lower
case words in a sans-serif font, some containing embedded underlines,
are used to denote syntactic categories, for example:
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case_statement
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- Boldface
words are used to denote reserved words, for example:
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array
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- 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;
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- 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.
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term ::= factor {multiplying_operator factor}
term ::= factor | term multiplying_operator factor
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- A vertical
line separates alternative items unless it occurs immediately after an
opening curly bracket, in which case it stands for itself:
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constraint ::= scalar_constraint | composite_constraint
discrete_choice_list ::= discrete_choice {| discrete_choice}
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- 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.
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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.
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A
construct is a piece
of text (explicit or implicit) that is an instance of a syntactic category
defined under ``Syntax.''
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A
constituent of a
construct is the construct itself, or any construct appearing within
it.
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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.
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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:
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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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