Re: scope and/or parameters (beginner)

[Robert A Duff <bobduff@world.std.com>]

fluffy_dong@dsuper.net writes:

> I know that and I am curious as to why the people who designed Ada
> made it this way, as opposed to the way that I described.  If there is
> no OUT then it would/should not matter what is done to the value
> passed because it's not coming back.  It would/should be treated as an
> idependent copy of the variable passed.  I know that is not the way it
> works.  Why not? (Don't answer. It's been answered below.)

Even if all parameters were always passed by copy, the language
designers would *still* have made 'in' parameters constant inside the
called procedure.  Suppose I wanted an 'in out' parameter, but I forgot
to say 'in out'.  The Ada rule saves me, because when I assign the new
value to the parameter, I get a compilation error.  Note that it's
irrelevant whether the formal parameter is assigned by a direct
assignment statement, or by calling another procedure which in turn has
an 'in out' parameter.  (Constant means constant, and, like Robert, I'm
surprised that you would think passing it to another procedure would
somehow turn that off.)

Furthermore, constants generally make the code more readable, because if
you know something is constant, you don't have to worry about it
changing.  That's true of local variables, and it's just as true of
formal parameters.  So of course formal parameters should default to
being constant.  (If I had designed the language, I would have done the
same for local variables -- "X: Integer := 10;" would declare a
constant, and "X: var Integer := 10;" would declare a variable.)

And furthermore, the need to modify a parameter as a temp inside the
procedure is rare -- if you need to do that, it's no big deal to declare
a local variable and initialize it to the value of the parameter.  And
doing that makes it clear what's going on.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.