Re: constraint error question for language lawyers

[collard@software.org (David Collard)]

In article <1802@software.software.org> blakemor@software.org (Alex Blakemore) writes:
> OK, language lawyers of the world.
> This one has stumped everyone I've asked locally,  including several 
> people who have worked with Ada since there were compilers for it.
> 
> Vax Ada 2.1 and two versions of Verdix (5.x and 6.0.3) all behave in the same
> (unreasonable) manner - so there must be some obscure rule at work here.
> Does anyone
>   a. know what that rule might be ?
>   b. profess to have a reasonable explanation for its existence.
> 
> The Question:
>   Why does this procedure raise constraint error?
>   It happens on the second assignment to dummy, but
>   doesnt happen if dummy.len > 0 and appears to have 
>   something to do with the function call "&"
> 
> ----------------  cut here  -----------------
> 
> with text_io;
> use text_io;
> 
> procedure bozo is
>   
>   type short is range 0 .. 10;
> 
>   type data is array (short range <>) of character;
> 
>   type var_text (len : short := 0) is
>     record
>       text : data (1 .. len);
>     end record;
> 
>   dummy : var_text; -- unconstrained
> 
>   procedure do_nothing (d : data) is 
>   begin
>     null;
>   end do_nothing;
> 
> begin
> 
>   put_line ("before");
>   dummy := (len => 0, text => "");
>   put_line ("during");
>   dummy := (len  => dummy.len + 1,
>             text => dummy.text & 'a');
>   put_line ("after");
> 
> exception
> 
>   when constraint_error =>
>     put_line ("constraint_error raised");
> 
> end bozo;
> 
> ---------------- cut here -----------------
> 
> Here is the output:
> 
> before
> during
> constraint_error raised
> 
> --------------------------------------------------------------------
> Alex Blakemore                       CSNET:   blakemore@software.org
> Software Productivity Consortium     
> 2214 Rock Hill Road Herndon, VA 22070 (703) 742-7125

--This is really just another case of another weirdness about ada, which has
--been discussed here before.  It involves the bounds of an array cannot be
--changed during an aggregate assignment, as shown below:

procedure aggassign is
  type short is new integer range 0..100;
  type intarray is array(short range <>) of integer;
  type rec(a:short := 0) is record
    b : intarray(1..a);
  end record;

  x : rec;
  c : intarray(8..8) := (others => 0);  
  d : intarray(1..1) := (others => 0);
begin
  x := (a => 1, b => d);    -- will NOT raise constraint error
  x := (a => 1, b => c);    -- will raise constraint error

end aggassign;


--The reason your program is the same is because of LRM 4.5.3-5 which
--states that during catenation, when one operand is an element rather
--than an array, it is treated as if it were the same as an array with
--its bounds being the lower  bound of the subtype of the index of
--the array.  In your case this is zero. so your assignment is
--equivalent to:
--
--  dummy := (len => dummy.len + 1,
--            text => dummy.text & (0 => 'a'));
--
--which, as shown in the example above, will raise constraint error.
--
--instead, you can do what you want by explicitly stating the bounds
--of the array of one character as:
--
--  dummy := (len => dummy.len + 1,
--            text => dummy.text & (1 => 'a'));
--
--which works just fine:

with Text_IO; use Text_IO;
procedure bozo is

  type short is range 0..10;
  type data is array(short range <>) of character;
  type var_test(len : short := 0) is
    record
      text : data(1..len);
    end record;

  dummy : var_test;

begin
  put_line("before");
  dummy := (len => 0, text => "");
  put_line("during");

  dummy := (len => dummy.len + 1,                    -- this WILL work
            text => dummy.text & (1 => 'a'));

--  dummy := (len => dummy.len + 1,                  -- this WILL NOT work!
--            text => dummy.text & 'a');
--
--  dummy := (len => dummy.len + 1,                  -- this is equivalent to the
--            text => dummy.text & (0 => 'a'));      -- last one and WILL NOT work!
--
--  dummy := (len => dummy.len + 1,                  -- this WILL NOT work either!
--            text => dummy.text & (5 => 'a'));

  put_line("after");
end bozo;



--

-----------------------------------------------------------------------
D. Thor Collard                      Internet: collard@software.org
Software Productivity Consortium     UUNET:    ...!uunet!software!collard
2214 Rock Hill Rd, Herndon VA 22070