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11.3.9 case Constructions

A case construction is used in a project file to effect conditional behavior. Here is a typical example:

     project MyProj is
        type OS_Type is ("GNU/Linux", "Unix", "NT", "VMS");
        OS : OS_Type := external ("OS", "GNU/Linux");
        package Compiler is
          case OS is
            when "GNU/Linux" | "Unix" =>
              for Default_Switches ("Ada")
                  use ("-gnath");
            when "NT" =>
              for Default_Switches ("Ada")
                  use ("-gnatP");
            when others =>
          end case;
        end Compiler;
     end MyProj;

The syntax of a case construction is based on the Ada case statement (although there is no null construction for empty alternatives).

The case expression must be a typed string variable. Each alternative comprises the reserved word when, either a list of literal strings separated by the "|" character or the reserved word others, and the "=>" token. Each literal string must belong to the string type that is the type of the case variable. An others alternative, if present, must occur last.

After each =>, there are zero or more constructions. The only constructions allowed in a case construction are other case constructions, attribute declarations and variable declarations. String type declarations and package declarations are not allowed. Variable declarations are restricted to variables that have already been declared before the case construction.

The value of the case variable is often given by an external reference (see External References in Project Files).