Re: how to do things like procedure foo(A: string) is bar: string := A; begin; ...;end;

[Robert Eachus <rieachus@comcast.net>]

On Saturday, December 16, 2017 at 3:56:45 PM UTC-5, Mehdi Saada wrote:
> DATA being strings components of the records LEFT and RIGHT, is the expression (LEFT.DATA & RIGHT.DATA)'Range possible ?
> I try to implement something like
> procedure foo(A: string) is
>    bar: string := A;
> begin
> ...
> end;
> but A'Length isn't known at compilation. In fact I want to use bar only like a macro, to avoid puting things like LEFT.DATA & RIGHT.DATA everywhere and not being able to use attributes (Range, length & co).

There is no problem with your example.  There is a potential (mental) gotcha if you concatenate a character or a null string literal to the beginning of another string literal.  A bit of thinking should tell you that such strings begin at 1, not Integer'First, and the last is Left.Data'First + Right.Data'Length.

If your subprogram Foo has an unconstrained String parameter (yours does) then the bounds of A are the bounds of the actual argument, and can be different on every call.  If you have a constrained String (i.e. String(6..15) or whatever) then inside Foo the bounds are those of the formal parameter.  If the LENGTHS of (constrained) formal and actual don't match, Constraint_Error will be raised.

As I said, there is not a problem in Ada--but everyone will eventually run into a couple of bugs because they unthinkingly assumed that the actual for the unconstrained formal starts at 1.

So now if you see something like:

   function Foo(S: String) return String is
   begin
     if S'Length < 80
     then return "" & S;
     else...

You will have a clue what is going on. ;-)