Ada: Adding an exception in a separate procedure when reading a file - file

Question

This is a follow-up of this question: Ada: reading from a file .
I would like to add an exception that checks if the file that I'm opening actually exists or not. I have made a separate procedure to avoid code clutter.
Here is the main code test_read.adb:
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Long_Float_Text_IO;
with Ada.Float_Text_IO;
procedure Test_Read is
Input_File : File_Type;
Value : Long_Float;
procedure Open_Data (File : in Ada.Text_IO.File_Type; Name : in String) is separate;
begin
Ada.Text_IO.Open (File => Input_File, Mode => Ada.Text_IO.In_File, Name => "fx.txt");
while not End_Of_File (Input_File) loop
Ada.Long_Float_Text_IO.Get (File => Input_File, Item => Value);
Ada.Long_Float_Text_IO.Put (Item => value, Fore => 3, Aft => 5, Exp => 0);
Ada.Text_IO.New_Line;
end loop;
Ada.Text_IO.Close (File => Input_File);
end Test_Read;
And here is the separate body test_read-open_data.adb of the procedure Open_Data:
separate(test_read)
procedure Open_Data (File : in out Ada.Text_IO.File_Type;
Name : in String) is
--this procedure prepares a file for reading
begin
begin
Ada.Text_IO.Open
(File => File,
Mode => Ada.Text_IO.In_File,
Name => Name);
exception
when Ada.Text_IO.Name_Error =>
Ada.Text_IO.Put(File => Standard_Error, Item => "File not found.");
end;
end Open_Data;
On compilation I get an error message in the separate body test_read-open_data.adb:
actual for "File" must be a variable
How to fix this?
Thanks a lot...
Update:
I have now made the following corrections.
In test_read.adb, I now have procedure Open_Data (File : in out Ada.Text_IO.File_Type; Name : in String) is separate;
Updated the definition of the same Open_Data procedure in test_read-open_data.adb.
The program compiles well though I do not see it catch the exception say if I renamed the file fx.txt to fy.txt. The error message I get is just
raised ADA.IO_EXCEPTIONS.NAME_ERROR : fx.txt: No such file or directory
So I do not get my own error message :File not found.
What is still wrong?

Answer 1

The File parameter of Open_Data needs to be an in out parameter (as in, for example, Ada.Text_IO.Create), because you want the opened file to be accessible within Test_Read.
You are getting actual for "File" must be a variable because an in parameter is read-only.
procedure Open_Data (File : in out Ada.Text_IO.File_Type;
Name : in String) is
(Personally I rarely type the in mode, because it’s the default).
But in any case, it looks as though the reason for the observed behaviour is that Test_Read doesn’t actually call Open_Data!
(edited to make the recommended mode in out & to suggest calling Open_Data)

Answer 2

if your goal is to simply check if the file exists, consider using Ada.Directories.Exists

Answer 3

IIRC: Standard_Error is not a file, but a Stream.

Answer 4

I suspect that the reason you are not seeing your error message is that you are using Put rather than Put_Line. Different implementations/platforms treat output to the user's display differently. To be extra sure you will see the message, follow the Put_Line with a Get_Line. The Get_Line generally forces the output of the Put_Line.