Default value for date/time

[John Rose]

On May 8, 2018, at 5:37 PM, Stephen Colebourne <scolebourne at joda.org> wrote:
> 
> On 9 May 2018 at 01:13, John Rose <john.r.rose at oracle.com> wrote:
>>> I worry that while the key principle has got the project a long way,
>>> it is too hard line when it comes to some potential value types -
>>> Currency or Money being other examples where a default is silly, but
>>> they are otherwise fully value-like.
>> 
>> Any value type measuring a scaled amount is going to have a
>> zero right?  That's the default, probably.  It's affine amounts
>> like times that don't have a good default.
> 
> Whats the default Currency? USD is a not an acceptable answer. Money
> is the same as it can't exist without a currency.
> 
> What about a Distance object? Is it measured in Miles or Kilometers by default?

Thanks, I get it now.

> 
>> class ObserveTheDefault {
>>   LocalDateValue x;
>>   LocalDateValue[] a = new LocalDateValue[1];
>>   void test() {
>>     System.out.println(x);
>>     assert(LocalDateValue.default.equals(x));
>>     System.out.println(a[0]);
>>     assert(LocalDateValue.default.equals(a[0]));
>>   }
>>   public static void main(String… av) { new ObserveTheDefault().test(); }
>> }
> 
> My suggestion was that code does not compile as `LocalDateValue x`
> wasn't assigned in the constructor (not "works like an int", but then
> its not like an int). Anything else risks allowing a "null-like"
> LocalDate to escape and head off causing havoc to real code - just
> like null does...
> 
> Anyway, I'm not expecting firm/final answers right now - value types
> has too far to go. Consider my points to be concerns that might be
> worth considering.

We'll certainly come back around to this when we've booted up
the pure "works like an int".  You are asking for a certain kind of
mashup between objects and primitives, which is understandable,
but we won't be providing every conceivable point on the spectrum
between objects and primitives (too hard).  That's why I keep talking
about trade-offs.  OTOH, maybe what you are asking for is such
a sweet spot that we'll end up having to do it.  Can't tell right now,
but… don't hold your breath.  "works like an int" is difficult all by
itself.

— John