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Date: Tue, 02 Dec 2003 19:06:42 -0500
From: "Robert I. Eachus" <rieachus@comcast.net>
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Subject: Re: discriminant in constraint must appear alone
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Date: 2003-12-02T19:06:42-05:00
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Vincent Smeets wrote:

> Are there other possiblities?

There are always other possibilities. ;-)

In this case the rule (RM 3.8(12) is clear on how discriminants can 
appear in a record type.  You could try:

    type R (D2 : Positive) is
       record
          A1 : String (1 .. D);
          A2 : String (1 .. D);
          B  : String (1 .. D);
       end record;

Where A = A1 & A2, and D2 = D/2, or you could use aliasing to map a 
string on top of an array (1..D) of Integer_16 elements.  But I think 
the only reasonable approach for now is to have two discriminants.

Incidently I have tripped over this problem in the past, and I think it 
would be wonderful to add three possible uses of discriminants in array 
bounds:  -D, D + n and n * D, where n is an integer literal.  Except for 
the first case, a compiler has to be able to generate these types of 
expressions and evaluate them at run-time.  For example, where does the 
value of B start in the type above?  Probably at 2*D + 4 bytes from the 
start of an object of type R.  (Your milage may vary, I am assuming that 
for this type the bounds of the arrays are implicit, but that D2 is 
stored at the beginning of any objects.)


-- 
                                           Robert I. Eachus

100% Ada, no bugs--the only way to create software.