Re: Update-in-place assignment

[Jacob Sparre Andersen <sparre@nbi.dk>]

Bill Findlay wrote:

> There is a case to be made for being able to specify that a component of an
> expression being assigned is the same as the destination of the assignment.
[...]
> E.g. we might have  i := idem -1;  or i := 1 - idem;
> or even  x := 0.5 * (1.0 + a*idem);

I like this idea, but we should (as Rosen points out) also
look at which opportunities it gives the programmer for
creating the corresponding implementations.

The least ugly construction I can come up with right now is
to allow the use of "idem" on an argument to specify that
even though it is formulated as a function, it is actually
just a procedure where the "idem" argument is "in out".

   function "*" (Left  : idem Matrix;
                 Right : in   Float) return Matrix;

should thus on the implementation side of things correspond
to

   procedure "+" (Left  : in out Matrix;
                  Right : in     Float);

But this doesn't contain any of the beauty of your
suggestion. Maybe it is best to leave it as an exercise for
the compiler to translate the expressions to the optimal
code.

Jacob
-- 
"There is a slight error in the exponent" - quantum vaacum
 mass is 10^35 times the total dark matter mass