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From: Mats Karlssohn <mats@mida.se>
Newsgroups: comp.lang.ada
Subject: Compile time executed functions
Date: Tue, 27 Mar 2001 09:10:06 +0200
Organization: MIDA Systemutveckling AB
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Xref: supernews.google.com comp.lang.ada:6101
Date: 2001-03-27T07:10:31+00:00
List-Id: <comp.lang.ada>

After following the thread on C style preprocessing, an old wish of
mine popped up in my brain (or whatever is inside the scull :).

Sometimes, especially when doing embedded programming, I feel a great
need to execute a function to get literal initialization a constant.
That is I'd like to have the compiler to execute a function for me (at
compiletime!) and use the return of that function to initialize the
constant.

Example (pleas, this is just an example, it hasn't been compiled):

type Byte is mod 2**8;
type Word is mod 2**16;
type CRC_Lookup_Table is array(0..255) of Byte;

constant CRC_Polynom := 4711; -- replace with a 'correct' polunom

function Generate_CRC_Table(Polynom : in Word) return CRC_Lookup_Table
is
begin
   -- Implementation
end Generate_CRC_Table;

package Package_Using_CRC is
   constant CRC_Lookup_Table := Generate_CRC_Table(CRC16_Polynom);

   -- 
end Package_Using_CRC;


- The constant is really a constant an should therefore be placed in
ROM.
- Therefore the function Generate_CRC_Table isn't needed in the output
  at all and since it may be quite large and/or time consuming it would
  be nice to do the job once, at compile time.


The way I'm currently doing this is by building a small external program
that generates an appropriate code fragment, that gets pasted into the
right place. This gives both maintenance and configuration management
headaches.

I must admit that I havn't researched enough to say that I can't get the
same effect using generice and/or other techniques in Ada.

Now, first I wonder have somebody out there solved this issue in a way
that complies with the first to points above ?

I realize that a lot of limitations must be put on the
Generate_CRC_Table function, but, have something along these lines been
considered by the language designers.

I have hoped to get the time to either build a small preprocessor to do
this kind of execution/substitution trick, or to add the same trick into
GNAT using some pragma(Compiler_Executed, func). Sorry to say, I don't
see that ammount of time in the near future.


Finally, as usual, I beg you to bear with my english.

-- 
Mats Karlssohn, developer                         mailto:mats@mida.se  
Mida Systemutveckling AB                          http://www.mida.se
Box 64, S-732 22 ARBOGA, SWEDEN
Phone: +46-(0)589-89808   Fax: +46-(0)589-89809