Re: Sets and Lists, again

David Cressey wrote:

> Recently, in a thread on implementing both threads and lists in a
> programming language, the example of lists or sets of Presidents arose. I
> mentioned that in a list of presidents, Grover Cleveland would appear once,
> but in a list of presidencies, he would appear twice.
>
> Bob Badour asked what purppose would be served by a list of presidents, or
> words to that effect. I'm interested.
>
> If one could have a set of presidents, why would one ever want a list? In
> general, if a language implements sets, why would the same language need
> to also implement lists? What does it buy you?
>
> I'm thinking of Lisp, which implemented lists, but not sets. MDL (aka
> Muddle) implemented arrays, and that's one step closer to implementing sets,
> but not all the way.
>
> SQL implemented sets, but not lists. Although local extensions of SQL do
> implement lists, e.g. "Segmented Strings" in DEC Rdb (aka Oracle/Rdb),
> it's not really part of the language as such.
>
> I'm also thinking of Pascal, which implemented sets, (as bitmaps), and also
> lists, albeit implicitly. What I mean is that you can combine the concepts
> of "record" and "pointer" in Pascal to construct dynamic linked lists of
> whatevers. But Pascal was primarily for teaching and learning programming,
> and may have implemented both for precisely that purpose.
>
> So, if you have sets, why do you need lists?

Good question. When considering answers, consider that if one has a set of presidents and a set of terms of office, one also has a set of presidencies simply by joining the two.

Do lists provide anything similar? Received on Fri May 19 2006 - 18:28:49 CEST