Why does an Ada compiler let range violations pass? Why is my type declaration a runtime entity?

Question

Why does Ada compiler let range violations pass? It does give warning, but why does it let it pass if it is an error in any case? Is there a practical scenario in which this

Answer 1

This comp.lang.ada message suggests that you need at least -gnato -fstack-check command line options for Gnat to be a compliant Ada compiler.

However that's not the issue here : the compiler does warn about the range error; with Gnat I get:

gnatmake -gnato -fstack-check question
question.adb:3:35: warning: static value out of range of type "Standard.Natural"
question.adb:3:35: warning: "Constraint_Error" will be raised at run time

and the obvious error at run time.

In this case because the range is static the compiler could have caught the error; but as you are guessing, in general the type cannot be fully determined until runtime, as in the following example.

with Ada.Text_IO;
with Ada.Command_Line;

procedure question is
   subtype Arguments is Natural range 1 .. Ada.Command_Line.Argument_Count;
begin
   for i in Arguments loop
      Ada.Text_IO.Put_Line ("Argument " & integer'image(i) & 
                            " is " & Ada.Command_Line.Argument(i)); 
      declare
         OneArg : constant String := Ada.Command_Line.Argument(i);
         subtype Characters is Natural range OneArg'range;
      begin
         null;  -- process the string here
      end;   
   end loop;
end;

Here neither subtype is known until you run the program.

The declare block shows a related pattern I find very useful, that allows not just variable [sub]types, but allocates variable sized objects on the stack so that they are automatically reclaimed and re-sized on each loop iteration. (You could allocate with "new" and free with "unchecked_deallocation" as in other languages but very often, as here, there is simply no need)