An enumeration type is defined as a list of possible values:
Like for numeric types, where e.g. 1 is an integer literal, Red, Green and Blue are called the literals of this type. There are no other values assignable to objects of this type.
Operators and attributes
Apart from equality ("="), the only operators on enumeration types are the ordering operators: "<", "<=", "=", "/=", ">=", ">", where the order relation is given implicitly by the sequence of literals: Each literal has a position, starting with 0 for the first, incremented by one for each successor. This position can be queried via the 'Pos attribute; the inverse is 'Val, which returns the corresponding literal. In our example:
These attributes are important for simple IO (there are more elaborate IO facilities in Ada.Text_IO for enumeration types). Note that, since Ada is case-insensitive, the string given to 'Value can be in any case.
Literals are overloadable, i.e. you can have another type with the same literals.
Overload resolution within the context of use of a literal normally resolves which Red is meant. Only if you have an unresolvable overloading conflict, you can qualify with special syntax which Red is meant:
Like many other declarative items, enumeration literals can be renamed. In fact, such a literal is a actually function, so it has to be renamed as such:
Here, Primary_Color is assumed to be defined in package P, which is visible at the place of the renaming declaration. Renaming makes Red directly visible without necessity to resort the use-clause.
Note that redeclaration as a function does not affect the staticness of the literal.
Characters as enumeration literals
Rather unique to Ada is the use of character literals as enumeration literals:
This literal 'A' has nothing in common with the literal 'A' of the predefined type Character (or Wide_Character).
Every type that has at least one character literal is a character type. For every character type, string literals and the concatenation operator "&" are also implicitly defined.
type My_Character is (No_Character, 'a', Literal, 'z'); type My_String is array (Positive range <>) of My_Character; S: My_String := "aa" & Literal & "za" & 'z'; T: My_String := ('a', 'a', Literal, 'z', 'a', 'z');
In this example, S and T have the same value.
Ada's Character type is defined that way. See Ada Programming/Libraries/Standard.
Booleans as enumeration literals
Also Booleans are defined as enumeration types:
There is special semantics implied with this declaration in that objects and expressions of this type can be used as conditions. Note that the literals False and True are not Ada keywords.
Thus it is not sufficient to declare a type with these literals and then hope objects of this type can be used like so:
If you need your own Booleans (perhaps with special size requirements), you have to derive from the predefined Boolean:
You can use range to subtype an enumeration type:
type Day_Of_Week is (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday); subtype Working_Day is Day_Of_Week range Monday .. Friday;