10.3.5 Variables

A variable may be declared at the project file level, or in a package. Here are some examples of variable declarations:

        This_OS : OS := external ("OS"); --  a typed variable declaration
        That_OS := "Linux";              --  an untyped variable declaration

A typed variable declaration includes the variable name, followed by a colon, followed by the name of a string type, followed by :=, followed by a simple string expression.

An untyped variable declaration includes the variable name, followed by :=, followed by an expression. Note that, despite the terminology, this form of "declaration" resembles more an assignment than a declaration in Ada. It is a declaration in several senses:

• The variable name does not need to be defined previously
• The declaration establishes the kind (string versus string list) of the variable, and later declarations of the same variable need to be consistent with this

A string variable declaration (typed or untyped) declares a variable whose value is a string. This variable may be used as a string expression.

        File_Name       := "readme.txt";
        Saved_File_Name := File_Name & ".saved";

A string list variable declaration declares a variable whose value is a list of strings. The list may contain any number (zero or more) of strings.

        Empty_List := ();
        List_With_One_Element := ("-gnaty");
        List_With_Two_Elements := List_With_One_Element & "-gnatg";
        Long_List := ("main.ada", "pack1_.ada", "pack1.ada", "pack2_.ada"
                      "pack2.ada", "util_.ada", "util.ada");

The same typed variable may not be declared more than once at project level, and it may not be declared more than once in any package; it is in effect a constant or a readonly variable.

The same untyped variable may be declared several times. In this case, the new value replaces the old one, and any subsequent reference to the variable uses the new value. However, as noted above, if a variable has been declared as a string, all subsequent declarations must give it a string value. Similarly, if a variable has been declared as a string list, all subsequent declarations must give it a string list value.

A variable reference may take several forms:

• The simple variable name, for a variable in the current package (if any) or in the current project
• A context name, followed by a dot, followed by the variable name.

A context may be one of the following:

• The name of an existing package in the current project
• The name of an imported project of the current project
• The name of an ancestor project (i.e., a project extended by the current project, either directly or indirectly)
• An imported/parent project name, followed by a dot, followed by a package name

A variable reference may be used in an expression.