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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
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Subject: Re: What is the difference?
Date: Sat, 10 Aug 2019 17:24:35 +0200
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Date: 2019-08-10T17:24:35+02:00
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On 8/10/19 3:59 PM, Gilbert Gosseyn wrote:
> 
> It works in both versions.

It doesn't work at all:

$ cat gilbert_gosseyn.adb
procedure Gilbert_Gosseyn is
    function F1 return Integer is (23);

    procedure Gg1 is new Gg (F => F1);
begin
    Gg1;
end Gilbert_Gosseyn;
$ gnatmake gilbert_gosseyn.adb
x86_64-linux-gnu-gcc-8 -c gilbert_gosseyn.adb
gilbert_gosseyn.adb:4:25: "Gg" is undefined
gnatmake: "gilbert_gosseyn.adb" compilation error

If I clean it up, it compiles (which I presume is what you mean by "work"):

$ cat gilbert_gosseyn.adb
with G;

procedure Gilbert_Gosseyn is
    function F1 return Integer is (23);

    procedure Gg1 is new G.Gg (F => F1);
begin
    Gg1;
end Gilbert_Gosseyn;
$ gnatmake gilbert_gosseyn.adb
x86_64-linux-gnu-gcc-8 -c gilbert_gosseyn.adb
x86_64-linux-gnu-gcc-8 -c g.adb
x86_64-linux-gnu-gnatbind-8 -x gilbert_gosseyn.ali
x86_64-linux-gnu-gnatlink-8 gilbert_gosseyn.ali

> What is here the meaning of: is <>?

It means that if there is function named F visible at the point of instantiation 
with the same parameter-and-return-type profile as the formal function F, and no 
actual parameter is supplied for the formal function F, then the visible 
function F will be used by default:

$ cat gilbert_gosseyn.adb
with G;

procedure Gilbert_Gosseyn is
    function F return Integer is (23);

    procedure Gg1 is new G.Gg;
begin
    Gg1;
end Gilbert_Gosseyn;
$ gnatmake gilbert_gosseyn.adb
x86_64-linux-gnu-gcc-8 -c gilbert_gosseyn.adb
x86_64-linux-gnu-gnatbind-8 -x gilbert_gosseyn.ali
x86_64-linux-gnu-gnatlink-8 gilbert_gosseyn.ali

If Gg actually uses F:

$ cat g.adb
with Ada.Text_IO;

package body G is
    procedure Gg is
    begin
       Ada.Text_IO.Put_Line (Item => F'Image);
    end Gg;
end G;

then we can see that it calls the implicitly provided function F:

$ ./gilbert_gosseyn
  23

-- 
Jeff Carter
"I did not rob a bank. If I'd robbed a bank, everything
would be great. I tried to rob a bank, is what happened,
and they got me."
Take the Money and Run
139