11.9.8 Variables

Variables store values (strings or list of strings) and can appear as part of an expression. The declaration of a variable creates the variable and assigns the value of the expression to it. The name of the variable is available immediately after the assignment symbol, if you need to reuse its old value to compute the new value. Before the completion of its first declaration, the value of a variable defaults to the empty string ("").

A typed variable can be used as part of a case expression to compute the value, but it can only be declared once in the project file, so that all case constructions see the same value for the variable. This provides more consistency and makes the project easier to understand. The syntax for its declaration is identical to the Ada syntax for an object declaration. In effect, a typed variable acts as a constant.

An untyped variable can be declared and overridden multiple times within the same project. It is declared implicitly through an Ada assignment. The first declaration establishes the kind of the variable (string or list of strings) and successive declarations must respect the initial kind. Assignments are executed in the order in which they appear, so the new value replaces the old one and any subsequent reference to the variable uses the new value.

A variable may be declared at the project file level, or within a package.

     typed_variable_declaration ::=
       <typed_variable_>simple_name : <typed_string_>name := string_expression;
     variable_declaration ::= <variable_>simple_name := expression;

Here are some examples of variable declarations:

        This_OS : OS := external ("OS"); --  a typed variable declaration
        That_OS := "GNU/Linux";          --  an untyped variable declaration
     
        Name      := "readme.txt";
        Save_Name := Name & ".saved";
     
        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");

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
• An expanded name, whose prefix is a context 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 expanded name whose prefix is an imported/parent project name, and whose selector is a package name in that project.