Re: Predicate for true lists

["Basil L. Contovounesios" <contovob@tcd.ie>]

Drew Adams <drew.adams@oracle.com> writes:

> 2. You say this in the doc string of `proper-list-length':
>
>    A proper list is neither circular nor dotted (i.e., its
>    last cdr is nil).
>
> And your entry in the manual for that function:
>
>   In addition to satisfying @code{listp}, a proper list is
>   neither circular nor dotted.
>
> Fair enough, but it depends on what is meant by a "circular"
> list.  Does it mean only a list whose last cdr shares list
> structure with some other of its cdr's?  If so then what you
> say holds.  And yes, I guess that's what most of us think of.
>
> But what of a list that has a finite number of elements,
> so it last cdr is nil, but one or more of whose elements
> is itself a circular list in the above sense?  Is it in
> some sense "circular" because of its circular-list element?

No, because "proper", "circular", and "dotted" all refer to list
structure (CDRs/links), not contents (CARs).

> It should be enough to say that a "true", or "proper"
> list is one that has nil as its last cdr, without
> adding that this means non-"circular".

I think that both "last cdr is nil" and "neither dotted nor circular"
completely qualify a list as being proper, so I would prefer to be
explicit and state both definitions, say, in the docstring.  If nothing
else, it makes the jargon "dotted" and "circular" more discoverable.
Unless my reasoning/understanding is mistaken, that is.

-- 
Basil