Re: More on lists and sets

Marshall Spight wrote:
> vc wrote:
> > Marshall Spight wrote:
> > > vc wrote:
> > > > paul c wrote:
> > > >
> > > > The usual list intersection opeartion (L = L1 intersect L2) is defined
> > > > as
> > > >
> > > > L = [x | x <- L1, x in L2]
> > >
> > > You speak as if "list intersection" is a well-defined, well-understood
> > > term with no significant disagreement.
> >
> > It's been so defined and understood starting at least with Lisp as well
> > as in languages like Scheme, Haskell and ML.
>
> I asked a Lisp programmer, and he said, no, that's not what
> means, and provided supporting common lisp documentation
> to show it having another meaning.

What other meaning did he have in mind ?

> I asked an SML programmer
> and he said it wasn't a term he was familiar with, but he would
> assume it meant the operation you described. I would have
> liked to have asked a Haskell programmer, but there are no
> working Haskell programmers in the USA (kidding) so I had
> to look it up in "Haskell: The Craft of Functional Programming"
> instead. No entries under "list intersection" and "intersection, list".

A Haskell standard library excerpt:

"17.2 "Set" operations
There are a number of "set" operations defined over the List type. nub (meaning "essence") removes duplicates elements from a list. delete, (\\), union and intersect (and their By variants) preserve the invariant that their result does not contain duplicates, provided that their first argument contains no duplicates.

nub removes duplicate elements from a list. For example:

  nub [1,3,1,4,3,3] = [1,3,4]

delete x removes the first occurrence of x from its list argument, e.g.,

  delete 'a' "banana" == "bnana"

(\\) is list difference (non-associative). In the result of xs \\ ys, the first occurrence of each element of ys in turn (if any) has been removed from xs. Thus, (xs ++ ys) \\ xs == ys.

union is list union, e.g.,

  "dog" `union` "cow" == "dogcw"

intersect is list intersection, e.g.,

  [1,2,3,4] `intersect` [2,4,6,8] == [2,4]

" Received on Tue Mar 28 2006 - 13:46:45 CEST