[Tutor] Fwd: Difference between types

[Mark Lawrence]

On 25/05/2013 15:56, Dave Angel wrote:
> On 05/25/2013 08:54 AM, Albert-Jan Roskam wrote:
>>
>>
>>> From: Dave Angel <davea at davea.name>
>>>
>       <SNIP>
>>>
>>> The empty tuple is specified with ().  But for any tuple with one or
>>> more members, it's the commas that turn it into a tuple.  The parens are
>>> not necessarily needed unless the statement is complex enough that we
>>> need them for precedence.
>>>
>>> So   x = 3,4
>>>
>>> makes a one-tuple out of 3 and 4.  If you want a one-tuple (which is NOT
>>> a singleton), you need a silly-looking comma to specify it:
>>
>> So you say the term singleton is reserved to one-item sets? I looked
>> it up and: "In mathematics, a singleton, also known as a unit set, is
>> a set with exactly one element. For example, the set {0} is a
>> singleton.The term is also used for a 1-tuple (a sequence with one
>> element)".(http://en.wikipedia.org/wiki/Singleton_%28mathematics%29).
>>
>> <snip>
>>
>>
>
> Thanks for that reference.  I had forgotten the usage of singleton in
> pure mathematics.  In computer software the only way I can recall it
> used (in about 45 years) is the design pattern singleton.  For example
> None is a singleton.  You cannot create another instance of NoneType.
>
> http://en.wikipedia.org/wiki/Singleton_pattern
>
> Now even that is a little different than I've seen used.  I've seen True
> and False considered singletons, even though they're the same class. But
> each value (of two legal values) of that class can only be instantiated
> once.
>
> If the term is also used to represent one-tuples or one-item sets in
> software, that seems to me to be very confusing and misleading.  So I'm
> not saying you're wrong, only that I hope you are.
>
> (Flash:) I just spotted Steven's post.  Thanks for the correction about
> 3,4  which was just a typo.  And yes, I was referring to meanings two
> and three in your list.  And indirectly to the Gang of Four book,
>      http://en.wikipedia.org/wiki/Design_Patterns
>
>

IIRC Alex Martelli argued that you don't need a singleton, you need a 
borg.  I'll leave those interested to look this up :)

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Mark Lawrence