Re: derived formal types and known discriminants

["Randy Brukardt" <randy@rrsoftware.com>]

<sbelmont700@gmail.com> wrote in message 
news:a1bd4695-00ff-49a8-a07a-84ebdb043202@googlegroups.com...
On Friday, June 30, 2017 at 2:10:01 PM UTC-4, Randy Brukardt wrote:
>> Sure, but they're very inflexible in that usage (as bad or worse than the
>> original unconstrained types). So I tend to view these as an either/or
>> proposition: untagged, mutable types with discriminants, or tagged types
>> without discriminants. Again, YMMV.

>At the risk of having some of the other thread's shade thrown in here, 
>aren't
>tagged discriminated records the de facto way of doing initialization? 
>That is,
>extending a Controlled type and then using discriminants in place of ctor 
>arguments?

I've heard of people trying to use that sort of thing, but I find it too 
inflexible (in that you have to declare an object at exactly the right 
place). Thus most of my types default-initialize to an "invalid" state, and 
there is one or more "Create" or "Open" routines to make a usable version. A 
bit more complicated, but it allows the initialization to occur anywhere 
(not necessarily where the object is declared, which usually is picked to 
give the proper lifetime for the object - that is, the destruction point) 
and the initialization can have arbitrary parameters.

You do need a check (or precondition) for validity for most operations, but 
that should be optimizable (if written as a precondition with an appropriate 
stable property [in Ada 202x]). So it shouldn't cost much if anything at 
runtime.

This after all is the model of file objects in Ada 83, and it works well for 
them and many other things.

                                             Randy.