Re: formal array types and default values

["Randy Brukardt" <randy@rrsoftware.com>]

"Mehdi Saada" <00120260a@gmail.com> wrote in message 
news:5c75fcdd-c965-4d03-9161-3576212e674d@googlegroups.com...
> Let's make it a proposal for the next norm then. I'm half serious: 
> depending on the difficulty,
> which I know nothing of, I am ok to help, however I can.
> Such a nice (and logical) feature, would a shame not to do.

You mean a useless feature. :-)  (See my response to Niklas about the 
difficulty.)

The primary purpose of Default_Value is to detect errors. Specifically, most 
enumeration types have (or could have) a value that means Unknown or 
Uninitialized. For instance:

    type My_Color is (Unknown, Black, Red, Green, Blue, White)
        with Default_Value => Unknown;

With this declaration, any programming errors that fail to set an object of 
My_Color will show up as a value of "Unknown". Which can be reliably 
detected.

Other uses of default values are evil, because they hide an operation that 
should always be explicit. For instance:
     type Evil_Integer is new Integer
         with Default_Value => 0;

Since 0 is a legal, useful value of type Evil_Natural, some code might work 
by accident when an uninitialized object happened to get the right value by 
default. That sort of thing will not help future readers.

                                   Randy.