HAC is perhaps the first open-source (albeit very partial) Ada compiler fully programmed in Ada itself.

Complete description in: doc/hac.txt

If you are impatient: in Alire (https://alire.ada.dev/), do "alr get hac", then "alr run" from the hac* directory.

Alternatively: have GNAT installed (https://www.adacore.com/download), then, on your preferred command-line interpreter:

  gnatmake -P hac

  cd exm
  ../hac gallery.adb

(if gnatmake doesn't work, try gprbuild; for Windows, '\' is meant in place of '/' )

Or, if you don't want to touch the command-line at all for playing with HAC, you can download and use LEA (http://l-e-a.sf.net/).

You'll find hundreds of examples in the exm directory and its subdirectories.

Here is the famous Hello World! File hello.adb:

with HAT;

procedure Hello is
begin
  HAT.Put ("Hello world!");
end Hello;

Another classic example (file fibo.adb):

with HAT;

procedure Fibo is

  function Fibonacci (P : Natural) return Positive is
  begin
    if P <= 2 then
      return 1;
    else
      return Fibonacci (P - 1) + Fibonacci (P - 2);
    end if;
  end Fibonacci;

  use HAT;

begin
  for i in 1 .. 22 loop
    Put_Line (Fibonacci (i));
  end loop;
end Fibo;

Enjoy!

HAC is free, open-source and released under the MIT license.