DateTime is not a Date?

ruby-talk
1

So, I was playing around a little in Ruby and noticed that |DateTime| is a subclass of |Date|, yet the method |today| is not defined for it.

I can certainly see how it doesn't make sense to have that method for that class, but I'm wondering why a Date isn't a DateTime? I suppose then we'd be removing the time element, so we'd get the same problem... but it was just striking is all.

2

It seems that on line 1261 of date.rb (inside the DateTime class), the
'today' method is undefined. For more information on Date, DateTime,
and Time, you may want to check out Sam Smoot's article on the subject:
http://substantiality.net/articles/2006/10/16/date-time-and-datetime-in-ruby

-Scott

Sammy Larbi wrote:

···

So, I was playing around a little in Ruby and noticed that |DateTime| is
a subclass of |Date|, yet the method |today| is not defined for it.

I can certainly see how it doesn't make sense to have that method for
that class, but I'm wondering why a Date isn't a DateTime? I suppose
then we'd be removing the time element, so we'd get the same problem...
but it was just striking is all.

3

So, I was playing around a little in Ruby and noticed that |DateTime| is a subclass of |Date|, yet the method |today| is not defined for it.

First of all this method is a singleton method of instance Date and these are not inherited the same way as instance methods are. DateTime.today does not make much sense - after all, what time would you expect? Instead there is DateTime.now:

>> Date.methods - DateTime.methods
=> ["today"]
>> DateTime.methods - Date.methods
=> ["now", "valid_time?"]

I can certainly see how it doesn't make sense to have that method for that class, but I'm wondering why a Date isn't a DateTime? I suppose then we'd be removing the time element, so we'd get the same problem... but it was just striking is all.

A Date cannot be a DateTime because it lacks time. Consequently DateTime inherits Date:

>> Date.ancestors
=> [Date, Comparable, Object, Kernel]
>> DateTime.ancestors
=> [DateTime, Date, Comparable, Object, Kernel]

Once can certainly argue whether DateTime *is a* Date or rather *has a* Date. But it is definitively clear that Date *is not* a DateTime simply because it does not provide the same set of information that DateTime provides (date *and* time).

Kind regards

  robert

···

On 20.11.2006 00:16, Sammy Larbi wrote:

4

Scott wrote, On 11/19/2006 9:50 PM:

It seems that on line 1261 of date.rb (inside the DateTime class), the
'today' method is undefined. For more information on Date, DateTime,
and Time, you may want to check out Sam Smoot's article on the subject:
http://substantiality.net/articles/2006/10/16/date-time-and-datetime-in-ruby

Thanks for that link... Someone else brought that line to my attention as well.

···

-Scott

Sammy Larbi wrote:
  

So, I was playing around a little in Ruby and noticed that |DateTime| is
a subclass of |Date|, yet the method |today| is not defined for it.

I can certainly see how it doesn't make sense to have that method for
that class, but I'm wondering why a Date isn't a DateTime? I suppose
then we'd be removing the time element, so we'd get the same problem...
but it was just striking is all.