Re: More on lists and sets

From: Marshall Spight <marshall.spight_at_gmail.com>
Date: 26 Mar 2006 18:28:10 -0800
Message-ID: <1143426490.400045.161370@z34g2000cwc.googlegroups.com>

David Cressey wrote:

> You are right, that a list doesn't inherently possess the "no duplicates
> allowed" rule, and therefore is like a bag rather than a set.

Hmmm, I see what you're saying, but that "bag" word I don't like much.

A list is a mapping from nat -> element_type. Like the dependent set of attributes of any other relation (and this *is* a relation) it is possible
to have duplicates in the dependent set.

We can also considere an ordered set, with is a bidirection mapping nat <-> element type. I like to think of this as a list with an additional
functional dependency.

> But look at
> it this other way: if the ordinal position in the list were taken as a data
> item, even if it isn't explicitly represented as data, then the elements
> of a list do form a set.

Yes!

Marshall Received on Sun Mar 26 2006 - 20:28:10 CST