Re: Adding "()" operator to Ada 200X

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Mário Amado Alves wrote:

>> >    function "()" ( Source : in Array_Like_Type;
>> >                    Index  : in Index_Type )
>> >       return Component_Type;
>>
>>  To go along with this, a way to define a slice of an
>> Array_Like_Type would be nice.
> 
> Great idea!
> 
> function "()"
>   (Source : in Array_Like_Type;
>    From : in Index_Type;
>    To : in Index_Type)
>   return Array_Like_Type;
> 
> Or "(..)".

IMO one should first add a new set of types: "index" types. With some set of 
predefined operations. Then you define sets over index types. "range" is an 
example of such a set. More interesting would be "column", "row" for a 
multi-dimensional index. (Ada does not have slices of multi-dimensional 
arrays). Then a square subset (specifies submatrix). Then a diagonal set 
... ad infinum. (:-))

Only when you have proper index types, you can start to play with operations 
like slices and enumeration (in for-statement).

If you have true index types and objects, you will need only one ()!

function "()"
(  Source : in Array_Like_Type;
   Index  : Index_Like_Type
)  return ?

The problem is that, what the function returns, depends on how the index 
argument is constrained.

This brings back a very important point of dealing with type constraints in 
a more general way than Ada presently does. For instance to allow () 
described above, one should treat:

type Element is ...
type Vector is array (Index_1) of Element;
type Matrix is array (Index_1, Index_2) of Element;

as differenty constrained subtypes of the same type Matrix. Both 
discriminants and tags can be considered type constraints. Then there are 
two major problems:

1. There has to be a way to remove statically known constraints.

2. There has to be a way to evaluate constraints in an independent body 
(from the original procedure body).

> Yes, Matthew, it's just syntax. So are the (already) redefineable
> operators. But it's really useful, no? Compare e.g.
>   ...
>   +"Marius",
>   +"Matthew",
>   ...
> to
>   ...
>   new String' ("Marius"),
>   new String' ("Matthew"),
>   ...
> 
> Actually the fact that it's just syntax might facilitate getting it
> approved :-)
> 
> BTW, I think that if a standard containers library is included in 200X
> then Ada.Strings.Unbounded and the like should be revised in terms of
> it. It would drastically reduce the size of the specs. It would
> suffice to say something like
> 
>   Unbounded_String is equivalent to the container type
>   of an instantiation of Ada.Containers.Arrays.Unbounded
>   with the type Character.

Oh that's for sure. For example:

   type X is tagged something;

      is an equivalent to

   type X is something;
   pragma Embedded_Tag (X);
---
   type X is limited something;

      is an equivalent to

   type X is something;
   procedure ":=" (L : in out X; R : Y) is null;
---
   ...

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de