Re: null unconstrained arrays vs generic formal types

[haven.umd.edu!cs.umd.edu!skates.gsfc.nasa.gov!bambam.gsfc.nasa.gov!nbssal @uunet.uu.net (Stephe Leake)]

In article <EACHUS.93Sep23185154@spectre.mitre.org>, eachus@spectre.mitre.org (
Robert I. Eachus) writes...
>In article <23SEP199311043613@bambam.gsfc.nasa.gov> nbssal@bambam.gsfc.nasa.go
v (Stephe Leake) writes:
> 
> 
>   > So, does Ada 9x fix this by relaxing LRM 12.3.4(5)?  Does anyone
>   > have an alternate solution?
> 
> 	... stuff deleted ...
>
>   In Ada 9X you could make the array type a generic formal derived
>type, but the real solution is to reorganize things so that you have a
>generic formal package:
> 
>   generic
>      type Index_Base_Type is range <>;
>      type Element_Type is range <>; -- why not private?
>      Index_Last: Index_Base_Type;
>   package Element_Arrays is
>       subtype ZERO_INDEX_TYPE is range 0 .. Index_Last;
>       subtype INDEX_TYPE is ZERO_INDEX_TYPE range 1 .. Index_Last;
>       type INDEX_ARRAY_ELEMENT_TYPE is array (INDEX_TYPE range <>)
>	   of ELEMENT_TYPE;
>        procedure Element_Get
>	   (Item : out ELEMENT_TYPE; Width : in Text_IO.FIELD);
>        procedure Element_Put (
>	   Item : in ELEMENT_TYPE;
>	   Width : in Text_IO.FIELD;
>	   Base  : in Text_IO.NUMBER_BASE);
>        ...
>    end Element_Arrays;
> 
>    package Int_30_Arrays is new Element_Arrays(Integer, Integer, 30);
>    -- for example...
> 
>    Now your generic array IO package looks like:
> 
>   with Text_IO;
>   generic
>      with package Elem_Arrays is new Element_Arrays(<>);
>   package Unconstrained_Integer_1D is
>       use Elem_Arrays; -- depending on your religion. :-)
>       procedure Get (Item : out INDEX_ARRAY_ELEMENT_TYPE; Last : out ZERO_IND
EX_TYPE);
>       procedure Put (Item : in INDEX_ARRAY_ELEMENT_TYPE);
>   end Unconstrained_Integer_1D;
> 
>   Of course you could put Element_Get and Element_Put elsewhere, but
>the principle is that you can pass two subtypes of the same type as
>generic parameters in Ada 9X by wrapping them in a generic package
>parameter.

That's really cool! I would put the Element_Get and Element_Put in a daughter
package, so programs that don't need them can't see them. But the ability to
group subtypes (and other stuff) in a generic formal package looks really
powerful. I guess I'll have to take a hard-copy of the Ada 9X manual home for
entertainment reading :-)

>--
> 
>					Robert I. Eachus
> 

Stephen Leake	NASA Goddard Robotics Lab
internet : nbssal@robots.gsfc.nasa.gov