Re: [Chicken-users] Cyclic lists and the Interpreter

[Graham Fawcett]

On Mon, Apr 7, 2008 at 3:24 PM, Alex Rozenshteyn <address@hidden> wrote:
> (define a (list 'a))
> (set-cdr! a a)
> ;a is now a cyclic "list"
> (pair? a) ; -> #t
> (list? a) ; -> #f
> (length a)
>
> and now the interpreter gets stuck.
> control-c does not break (this is because I have the readline egg
> installed).
>
>
> I have two questions:
> 1) How do I get C-c to break?
> 2) Is the interpreter supposed to freeze if length is passed a cyclic list?

Yes, it's correct. See length and length+ in SRFI-1:
http://srfi.schemers.org/srfi-1/srfi-1.html#Miscellaneous

length+ will give you sane behaviour with cyclic lists.

> Other scheme implementations generate an error.

Yes, but that's implementation-specifc. (length) may "diverge or
signal an error" according to SRFI-1, and in Chicken's case it
diverges.

Graham