Why not do what python does?
drop -2 -- drops the last 2 elements from the list
take -2 -- grabs the last 2 elements from the list
take n list | n<0 = drop (length list + n) list
drop n list | n<0 = take (length list + n) list
If the list is an infinite list, the behavior is equivalent to B.
If the list is finite, these are really useful primitives, like chop() in
perl.
I think this solution also satisfies Chris Okasaki's:
take n xs ++ drop n xs == xs
(except where the list is infinite)
-Alex-
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On Mon, 24 Jan 2000, Jon Fairbairn wrote:
> > > Take and drop
> > > [..]
> > > I can see three alternatives:
> > >
> > > (A) Make them defined for any n. If n < 0, do something reasonable:
> > > take: give empty list
> > > drop: give whole list
> > >
> > > (B) Make them defined for n > length xs, but fail for n < 0.
> > >
> > > (C) Status quo
> > >
> > > PROPOSAL: Use alternative (A)
> >
> > I vote for (B).
>
> Ditto, though I would say that the restriction is that the
> argument should be a natural number, and its simply a
> failing of the type system that it cannot express this.
>
> --
> J�n Fairbairn [EMAIL PROTECTED]
> 18 Kimberley Road [EMAIL PROTECTED]
> Cambridge CB4 1HH +44 1223 570179 (pm only, please)
>
>
>
