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.
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:
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, for example:
For symbols used
in this notation (square brackets, curly brackets, and the vertical line),
the symbols when surrounded by ' represent themselves, for example:
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
are both equivalent to name
The delimiters, compound delimiters, reserved
words, and numeric_literal
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 document (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”.
When this document 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 Clause 2 of ISO/IEC 10646:2020.
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
is a piece
of text (explicit or implicit) that is an instance of a syntactic category
defined under “Syntax”.
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
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 runtime checks, the evaluations, conversions,
and checks may be arbitrarily interspersed, so long as each expression
is evaluated before converting or checking its value.
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.
NOTE 1 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:
NOTE 2 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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe