Re: What's wrong with this program?

[Jeffrey Carter <jrcarter@acm.org>]

Allison Greene wrote:
> 
> procedure Test_Discriminant is
>    type Base is tagged null record;
> 
>    type Tower is new Base with
>       record
>          I : Integer;
>       end record;
> 
>    Instance: Tower (I => 5);
> begin
>    null;
> end Test_Discriminant;
> 
> Result of compiling with GNAT:
> 
> test_discriminant.adb:13:20: invalid constraint: type has no
> discriminant

The error message is quite clear: You are attempting to provide a
discriminant constraint to a type that has no discriminant.

The declaration

type Tower (I : Integer) is record
   F : Float;
end record;

declares a type with a discriminant (I). You would declare an object of
this type with

Instance : Tower (I => 5);

which is exactly the syntax you used. For your declaration of type
Tower, this is illegal, since it does not have a discriminant. Perhaps
what you wanted was

Instance : Tower := (I => 5);

which creates the object and initializes it with the value to the right
of ":=". Since Tower is a record type, that value is a record aggregate.

-- 
Jeff Carter
"Your mother was a hamster and your father smelt of elderberries."
Monty Python & the Holy Grail