guideline

Avoid declaring variables in package specifications.

This is part of a compiler. Both the package handling error messages and the package containing the code generator need to know the current line number. Rather than storing this in a shared variable of type Natural, the information is stored in a package that hides the details of how such information is represented and makes it available with access routines:

-------------------------------------------------------------------------
package Compilation_Status is
   type Line_Number is range 1 .. 2_500_000;
   function Source_Line_Number return Line_Number;
end Compilation_Status;
-------------------------------------------------------------------------
with Compilation_Status;
package Error_Message_Processing is
   -- Handle compile-time diagnostic.
end Error_Message_Processing;
-------------------------------------------------------------------------
with Compilation_Status;

package Code_Generation is
   -- Operations for code generation.
end Code_Generation;
-------------------------------------------------------------------------

rationale

Strongly coupled program units can be difficult to debug and very difficult to maintain. By protecting shared data with access functions, the coupling is lessened. This prevents dependence on the data structure, and access to the data can be controlled.

notes