Ada: Pragma List

Question

Can anybody tell me what Pragma List does (specifically, what is "listing of the compilation")? I don't understand the description from the LRM (2.8.25)

A pragma List takes one of the identifiers On or Off as the single argument. This pragma is allowed anywhere a pragma is allowed. It specifies that listing of the compilation is to be continued or suspended until a List pragma with the opposite argument is given within the same compilation. The pragma itself is always listed if the compiler is producing a listing.

score 3 · Accepted Answer · edited May 23 '17 at 10:29

A compiler can output, i.e. “list” its input, together with any messages that it will generate, such as error messages. This is useful when you want a clear, verbose view of what a message is about, in context. IDEs will normally link message to code, but even today, considering Jacob's hint at computing history, a list can literally point out. Using a pragma List, the programmer can exclude what does not need to be listed, if he or she knows. Or, exclude what should never be listed, for reasons of secrecy.

Listing first, then the original program text, with pragma List:

Compiling: /some/path/some_proc.adb
Source file time stamp: 2017-01-30 08:30:40
Compiled at: 2017-01-30 09:30:42

     1. procedure Some_Proc is
     2.     procedure Inner;
     3.     --  Does this and that...
     4.
     5.     pragma List (Off);
    10. pragma List (On);
    11.
    12. begin
    13.     Inner (42);
            |
        >>> too many arguments in call to "Inner"

    14. end Some_Proc;

 14 lines: 1 error
gprbuild: *** compilation phase failed

(If your compiler is GNAT, specify -gnatl among the switches, for listing, and compile:)

procedure Some_Proc is
    procedure Inner;
    --  Does this and that...

    pragma List (Off);
    procedure Inner is
    begin
        null;
    end Inner;
    pragma List (On);

begin
    Inner (42);
end Some_Proc;

score 2 · Answer 2 · answered Jan 30 '17 at 06:33

You should think back to how compilers worked and were used in the late 1970'ies. I'm pretty sure the meaning is as simple as it is written (substitute "is producing" with "outputs", to get a more modern wording).

with Ada.Text_IO;
--  Now you see me.
pragma List (Off);
--  Now you do not.
private with Some_Secret_Package;
pragma List (On);

package Hello_World is
   ...

Ada: Pragma List

2 Answers2