Version declaration proposal

[Don]

Nick Sabalausky wrote:
> "Don" <nospam at nospam.com> wrote in message 
> news:gmvg64$1v99$1 at digitalmars.com...
>> Steve Schveighoffer wrote:
>>> On Wed, 11 Feb 2009 09:01:29 +0100, Don wrote:
>>>
>>>> I think I have a solution to the versioning problems. My belief is that
>>>> version statements are fine, and Walter's 'rats nest' argument is valid.
>>>> Instead, I believe that version declarations limited to "version = XXX;"
>>>> are too weak. They also have syntax which is subtly different to the
>>>> rest of the language. (version = A; version = B; looks as though you're
>>>> changing the value of the 'version' variable).
>>>> My proposal is therefore:
>>>>
>>>> --------
>>>> * Keep version statements unchanged.
>>>> * Change version declarations to make them almost identical to bool
>>>> declarations:
>>>>
>>>> version _versionidentifier_ = _versionexpression_;
>>>>
>>>
>>> I like this idea.  I had one similar last night, but you beat me to it ;)
>>>
>>> My idea was going to be something that is similar to the current version 
>>> statements:
>>>
>>> version = _expression_ ? _identifier_ [ : _elseidentifier_ ]
>>>
>>> Where the [...] is optional.
>>>
>>> But your way looks cleaner.
>>>
>>> Not in love with the version x = extern notation ...
>> Me neither. Of the two options I presented, I've decided I prefer
>> extern version x;
> 
> But there should be a way to specify a default value. 

But only existence can be set on the command line, exactly as now.
So default is always false.
For an extern identifier,
true = it was specified on command line, false = it was not specified.

Considering your worst case:
 > // FeatureX *can* be specified on cmd line, default is complex.
 > extern version FeatureX = (FeatureA || !FeatureB) && Windows;
 > version(FeatureX)... // Ok

You could only do this with TWO extern features, one to enable it, one 
to disable.

extern version EnableFeatureX;
extern version DisableFeatureX;
version FeatureX = (((FeatureA || !FeatureB) && Windows) || 
EnableFeatureX ) && ! DisableFeatureX;

With what you're proposing, you'd need to change the command line syntax 
too.