Re: Attempting to modify a function result

[Keith Thompson <kst@cts.com>]

"Robert I. Eachus" <eachus@mitre.org> writes:
> "Raymond C. Rawa" wrote:
> > I've encountered a "feature" of Ada83 or the VADS compiler that I don't
> > understand.
> 
>    It is a feature of Ada, not just VADS.
> 
> >    ...Display.a := 5; -- I mean't to type: The_Display.a := 5;
>  
>    Display is a function that returns an object of type Display_Type. 
> Display.a is a compontent of that object of type Integer, which gets
> assigned the value 5.  What is hard about that?

That doesn't mean you can legally assign to it.  RM95-6.4(12) says

    A function_call denotes a constant, as defined in 6.5; the nominal
    subtype of the constant is given by the result subtype of the
    function.

I couldn't find quite as clear a statement in RM83, but RM83-6.5 says

    A function is a subprogram that returns a value (the result of the
    function call).

As I understand it, a value is not an object.

Now, if the Display function returned an access value,
"Display.a := 5" could implicitly dereference the result and assign a
value to the component "a", but that's not what's happening here.

Looks like a bug in VADS to me.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center           <*>  <http://www.sdsc.edu/~kst>
One of the great tragedies of ancient history is that Helen of Troy
lived before the invention of the champagne bottle.