Peter Hermann writes
: Are there any portable means for conditional compilation in Ada?
: How many?
: (e.g. to circumvent Ada95 constructs)

   You  can  use a   boolean or   enumeration  constant defined   in a
"portability"  (hum, macros are not so  far ;-) package, together with
"if"  or "case" statements  and let the  optimization to the compiler.
That's the   cleanest way  I  know  of doing  it (preserves   the full
semantic of Ada, macros are far away ;-).

   If your "construct" is  a compilation unit, you  can use the pragma
Source_File_Name  which is a GNAT specific  pragma  (for now), and the
gnat.adc file (the place for configuration pragmas  with GNAT, I don't
know for other compilers), and provide different bodies. Example :

---------  X11 support
 
--  If you want to have X11 support, and to use the -w, --window command
--  line option, uncomment the following pragma. You'll also need to check
--  the documentation of DART about the X-Binding before compiling.
 
--  pragma Source_File_Name                         --%x11
--    (Unit_Name      => Images.Display,            --%x11
--     Body_File_Name => "x11.images-display.adb"); --%x11

   (The  "--%x11"   is here  for  Makefile easy-auto-configuration-sed
hacks.)  Just  provide  a  dummy  images-display.adb that   raises  an
"Unimplemented"  exception when  a routine  is  called.  We used  this
method for X11/no  X11 stuff and tasking/no  tasking stuff in the DART
project (not released yet).

   But  it     can  easily   be generalized,   and     if the   pragma
Source_File_Name  is adopted by  other vendors  (I  hope so),  it will
provide a  clean  solution (together with boolean/enumerates)  to most
configuration  problems *staying in  the   Ada 95 language*,   without
specific Makefiles everywhere (horrors most   of the time, if  there's
more than one target).

   Ada   can be a  portable language  without  too much external tools
(like preprocessors ands Makefiles) ;-).

   Hope this helps,

-- 
--  Laurent Guerby, student at Telecom Bretagne (France), Team Ada
--  "Use the Source, Luke. The Source will be with you, always (GPL)"
--  http://www-eleves.enst-bretagne.fr/~guerby/ (GATO Project)
--  Try GNAT, the GNU Ada 95 compiler (ftp://cs.nyu.edu/pub/gnat)