Re: Instantiations of a generic with often encountered arguments

[Shark8 <onewingedshark@gmail.com>]

On Sunday, November 26, 2017 at 7:02:23 PM UTC-7, Victor Porton wrote:
> 
> I am now in analysis-paralysis of whether to define instantiations for often 
> encountered types in advance (globally) or immediately before use.

Ok, I completely understand that problem -- the right way will undoubtedly influence your codebase and you don't want to have to rewrite everything.

Unfortunately I can't give you a hard rule on how you should proceed because, like so many other things, it depends on what you're doing. (e.g. do you want to make SOME_TYPE limited & private, just private, just limited, or neither? It really depends, if you're passing around a handle where there's some sort of reference-counting going on, you'll want limited private to ensure that you're not accidentally mucking about on the internal count. OTOH, if you're implementing a parallel solver, you probably want your data non-limited so you can copy it and feed these copies to your worker-tasks freely.)

For generics, I've found that making a "global instantiation" generic is best when you're either (a) working on an entire [sub-]system that is generic [e.g. a parser that you're going to use multiple times with different syntax-sets] or (b) when the functions themselves are anticipated to be used widely in your code [e.g. the Ada.Numerics.Elementary_Functions example].

One last option is leveraging Ada's visibility/scoping rules to "mix and match" your options; example you have a generic package GENERIC_AUDIO_OPERATIONS containing subprograms that are going to be used all over the audio portion of an application:

With
Application.Generics.GENERIC_AUDIO_OPERATIONS;

Package Application.Subsystems.Audio is

Private
  Package Operations is New GENERIC_AUDIO_OPERATIONS( [[parameters]] );
  Use Operations;
  -- The instantiation is now directly visible/usable to the private and
  -- (IIRC) implementation portions of child packages.
End Application.Subsystems.Audio;