Re: Negative nth index

[Mattias Engdegård]

25 dec. 2024 kl. 11.56 skrev Pip Cet <pipcet@protonmail.com>:

> s/nth/nthcdr/g in my email, then.  Of course the two should behave
> analogously, the question is whether they should both throw an error (my
> opinion) or shouldn't.

The behaviour of nthcdr for negative N is well-established and turns out to be 
somewhat useful in practice.

If compatibility with existing code were completely unimportant, I'd recommend 
that negative arguments cause nth to signal but not nthcdr. But it isn't, so I 
don't.

> If it were merely undocumented, I might agree, but the current behavior
> doesn't match the documentation,

Yes, the manual and doc strings could certainly be improved.

> because taking cdr -1 times is
> obviously an impossible thing to do.

Well now, we could define

 (nthcdr N L) = (cons nil (nthcdr (1+ N) L)

for N<0. (No, we shouldn't.)

> This isn't about performance.

Of course it's about performance. In some respect it always is!