Re: Full view of a private partial view cannot be a subtype

[Jere <jhb.chat@gmail.com>]

On Sunday, December 3, 2017 at 9:34:51 AM UTC-5, Jeffrey R. Carter wrote:
> On 12/03/2017 02:33 PM, Jere wrote:
> > Thanks!  I didn't even think about nesting it like that.  That'll work.
> 
> Many people seem to think that as much as possible should be done through type 
> extension. They act as though the only tool they have is the hammer of type 
> extension, and so view every problem as a nail. In my experience type extension 
> is usually best avoided whenever possible.
> 

Well, I think either form is a bit too polar.  I've always been taught that
you use the tool that makes the most sense given the context.  If a type
has an "is a" relationship, you favor extension.  But if it has a "has a"
relationship, you favor composition.  If the situation is unique enough,
then you do something outside the norm.

In this case I wasn't using extension because I thought it was the best
method.  I was using it because Ada doesn't provide me a good way to
rename a type as part of the full view.  Normally I would use subtype, 
but I can't do that with a private declaration (I don't want the client
to have access to the "subtypedness", but I want to leverage it under
the hood).  Extension was my first stab at it because of the natural
flow of type declarations in Ada.

I did run into a snag with using composition.  Some of my functions
return numeric or access types that are created within the generic
that I am trying to use.  I'll still have to do conversions for those
since I want them to retain their numeric and access properties (so
a private nested type won't do).