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From: Lutz Donnerhacke <lutz@iks-jena.de>
Newsgroups: comp.lang.ada
Subject: Re: Ideas for Ada 200X
Date: Fri, 30 May 2003 13:27:25 +0000 (UTC)
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* Robert A Duff wrote:
> Correctly?  I guess it depends on what you want it to do.
> My example:
> 
>     Some_Array[Get_Next_Position] := Some_Array[Get_Next_Position] + 1;
> 
> is highly unlikely to be correct, since it evaluates Get_Next_Position
> twice (which is probably not what you want) and it does it in either
> order (which is almost certainly not what you want).  More likely, it's
> just a mistake, and the programmer wanted the semantics provided by your
> renaming above (or, more concisely and readably, C's += operator).

Ack.

with Ada.Text_IO;

procedure t is
   type offset is range 1 .. 9;
   type base is range 1 .. 19;
   type field is array(offset) of base;
   
   f : field := (1, 2, 3, 4, 5, 6, 7, 8, 9);
   o : offset := f'First;
   t : offset;

   function getnext return offset is
   begin
      o := o + 1;
      return o;
   end getnext;
   
   procedure print is
   begin
      Ada.Text_IO.Put_Line("f(" & offset'Image(o) & " ) =" & base'Image(f(o)));
   end print;
   procedure printall is
   begin
      for i in f'Range loop
	 Ada.Text_IO.Put(base'Image(f(i)));
      end loop;
      Ada.Text_IO.New_Line;
   end printall;
begin
   printall;
   
   t := getnext;
   print;
   
   f(getnext) := f(getnext) + 10;
   print; printall;
   
   declare
      x : base renames f(getnext);
   begin
      x := x + 10;
   end;
   print; printall;
   
   declare
      procedure Inc(x : in out base) is
      begin
	 x := x + 10;
      end Inc;
   begin
      Inc(f(getnext));
   end;
   print; printall;
end;

Results in:
 1 2 3 4 5 6 7 8 9
f( 2 ) = 2
f( 4 ) = 4
 1 2 14 4 5 6 7 8 9
f( 5 ) = 15
 1 2 14 4 15 6 7 8 9
f( 6 ) = 16
 1 2 14 4 15 16 7 8 9

Renaming and Procedural Increment has the required behavior. Doubling the
side effect argument does not.

The corresponding C-Source is:
#include <stdio.h>

int o = 0, f[10] = {1,2,3,4,5,6,7,8,9};

int getnext(void) {
   return ++o;
}

void print(void) {
   printf("f(%d) = %d\n", o, f[o]);
}

void printall(void) {
   int i;
   for(i = 0; i < sizeof(f)/sizeof(*f); i++) {
      printf("%d ", f[i]);
   }
   puts("");
}

void inc(int * x) {
   *x = *x + 10;
}

int main(void) {
   int t;
   printall();
   
   t = getnext();
   print();
   
   f[getnext()] = f[getnext()] + 10;
   print(); printall();
   
   f[getnext()] += 10;
   print(); printall();
   
   inc(&f[getnext()]);
   print(); printall();
}

You find it ugly to pass an "in out" parameter using a pointer?
You find it ugly to rename a expression in order to prevent reevaluation?

Yes. Either language has some common idioms you will find ugly.