Ada Reference Manual (Ada 2022) — Legal Information
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3.5.1 Enumeration Types

1
An enumeration_type_definition defines an enumeration type. 

Syntax

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enumeration_type_definition ::= 
   (enumeration_literal_specification {, enumeration_literal_specification})
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enumeration_literal_specification ::= 
    defining_identifier | defining_character_literal
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defining_character_literal ::= character_literal

Legality Rules

5/3
The defining_identifiers in upper case and the defining_character_literals listed in an enumeration_type_definition shall be distinct. 

Static Semantics

6/3
Each enumeration_literal_specification is the explicit declaration of the corresponding enumeration literal: it declares a parameterless function, whose defining name is the defining_identifier or defining_character_literal, and whose result subtype is the base subtype of the enumeration type. 
7
Each enumeration literal corresponds to a distinct value of the enumeration type, and to a distinct position number. The position number of the value of the first listed enumeration literal is zero; the position number of the value of each subsequent enumeration literal is one more than that of its predecessor in the list.
8
The predefined order relations between values of the enumeration type follow the order of corresponding position numbers.
9
If the same defining_identifier or defining_character_literal is specified in more than one enumeration_type_definition, the corresponding enumeration literals are said to be overloaded. At any place where an overloaded enumeration literal occurs in the text of a program, the type of the enumeration literal has to be determinable from the context (see 8.6). 

Dynamic Semantics

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The elaboration of an enumeration_type_definition creates the enumeration type and its first subtype, which is constrained to the base range of the type. 
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When called, the parameterless function associated with an enumeration literal returns the corresponding value of the enumeration type. 
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NOTE   If an enumeration literal occurs in a context that does not otherwise suffice to determine the type of the literal, then qualification by the name of the enumeration type is one way to resolve the ambiguity (see 4.7).

Examples

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Examples of enumeration types and subtypes: 
14/5
type Day        is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
type Month_Name is (January, February, March, April, May, June, July,
                    August, September, October, November, December);
type Suit       is (Clubs, Diamonds, Hearts, Spades);
type Gender     is (M, F);
type Level      is (Low, Medium, Urgent);
type Color      is (White, Red, Yellow, Green, Blue, Brown, Black);
type Light      is (Red, Amber, Green); -- Red and Green are overloaded
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type Hexa       is ('A', 'B', 'C', 'D', 'E', 'F');
type Mixed      is ('A', 'B', '*', B, None, '?', '%');
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subtype Weekday is Day   range Mon .. Fri;
subtype Major   is Suit  range Hearts .. Spades;
subtype Rainbow is Color range Red .. Blue;
                               -- the Color Red, not the Light
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