Re: Surprise in array concatenation

[Robert A Duff <bobduff@shell01.TheWorld.com>]

tmoran@acm.org writes:

> >I completely agree with the marginal utility of other-than-1 least
> >array indices.
>    I strongly disagree.  While not dirt common, other 'firsts in
> declarations do occur.  -n ..  +n comes to mind.

Heh?  "Marginal utility" means the same as "not dirt common"!

My suggestion was that the language should _allow_ the programmer to fix
the lower bound for an array type.  And/or the upper bound.  In Ada, you
can fix both or neither -- but not one or the other.

Anyway, you'd still be allowed to do your -n..+n thing, in the rare
cases where that's useful.

>...  If you are going to
> handle slices, you have to assume 'first /= 1 anyway,...

No, I don't.  ;-)

Slices should slide to the lower bound.  The Ada rule breaks
abstraction:

    procedure P(X: String) is
    begin
        ...
    end P;

    Y: String := "Hello, world!";

    P(Y(3..4));

Inside the body of P, X is just a String -- we don't (or shouldn't) know
that it's a substring of Y.  So we can't possibly make any sense (inside
P) of the fact that X'First = 3.  Index 3 from what?

If I ran the circus, X'First would be 1.

>... so you're not losing
> much by allowing it in declarations.  Using 'first instead of 1 also makes
> it simple to change between integer and enumeration value indexes.  The
> way Ada.Text_IO.Get_Line (et al) returns Last, which works even if you
> passed it a slice, has surely prevented many an error as compared to other
> languages which would likely return a count (since "all arrays start at
> x") and depend on the programmer to do any arithmetic needed to turn it
> into an index.  And if you started all arrays at 1,

I wouldn't insist on starting _all_ arrays at 1, but I think it makes
sense for _many_ arrays, including String.

>... you probably wouldn't
> allow the idiom
>   subtype cards is string(1 .. 80);
>   subtype sequence is range 73 .. 80;
>   ...
>   if card(sequence) = (sequence=>' ') then

Why not?  In Ada, "=" doesn't care about the bounds -- just the length.

- Bob