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 unless it occurs immediately after
an opening curly bracket, in which case it stands for itself
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
grammar given in this document 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.
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”.]
Discussion: This just means that programs
can be written in plain ASCII characters; no characters outside of the
7-bit range are required.
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 the note in
Clause 2 1
of ISO/IEC 10646:2020 2011
The “documents referenced” means Unicode,
Chapter 4 (specifically, section 4.2 — Case). Machine-readable
versions of Simple Uppercase Mapping and Simple Lowercase Mapping can
be found in http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt.
Data for older Unicode versions can be found on this site as well; start
and find the appropriate version number. Simple Uppercase Mapping is
the 12th field in this file (the 13th element of each line, since Unicode
counts from 0); the Simple Lowercase Mapping is the 13th field in this
file. In both cases, if no character is present in the field, the character
maps to itself.
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”.
Term entry: construct
— piece of text (explicit or implicit) that is an instance of a
syntactic category defined under Syntax
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
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.]
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
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.
Wording Changes from Ada 95
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.
We now explicitly define what the Reference Manual means by upper case,
as there are many possibilities for ISO 10646 characters.
The example for square brackets has been changed as there is no longer
Wording Changes from Ada 2005
Upper case is defined by "simple upper case mapping",
because "full case folding" is a mapping (mostly) to lower
Wording Changes from Ada 2012
Changed the syntax description to quote symbols
used in the notation when they appear as themselves. We made this change
to allow square brackets in Ada syntax, but the change also allowed us
to get rid of an old hack which allowed vertical lines to stand for themselves
in limited circumstances.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe