RE: [External] : Closures - do you understand them well?

[Drew Adams <drew.adams@oracle.com>]

> But yes, like explicitly specifying a nil binding (X nil) instead of (X)
> this is bout readability and personal preferences.  (X '()) is my
> hint to the reader that X will be used as a list type accumulator in the
> BODY.  I prefer that over (X ()), (X nil) and (X).  It's a stylistic
> habit.
> 
> Why?  Because I want to make clear that the variable is initialized with
> an empty list, compared to what the evaluation of an empty list aka nil
> returns (which accidentally happens to be the same again in Lisp).

I have _exactly_ the same concern as you:
human-reader readability (and in most cases
that's me).

I use (let ((xs    ())
            (foop  nil)
            (bar   nil)
            (yy))

to tell me that:

1. xs and foop are bound initially.
   yy gets its meaningful value only from
   the `let' body - its initial nil value
   is (should be) meaningless/ignored.

2. xs has a list value (at least initially).
   IOW, I use () for the same reason you use
   '().

3. foop and bar have non-(non-nil) values
   (at least initially).

4. foop is used as a Boolean (true/false).

   bar is likely not used just as a Boolean.
   (bar might have several possible values,
   where nil might mean some base case,
   which could, but need not, mean false.

Wrt #4: I use the suffix code convention
"p"/"-p"  for variable names, as well as for
function names.  I find this _very_ helpful.
Throughout the body, I'm reminded that foop
is Boolean.
___

You say, "which accidentally happens to be
the same again in Lisp".  To me it's not an
accident; it's by design.  It's arguable,
and it's been argued, but to me this design
is (1) clever/elegant and (2) _very_ handy.
Is #1 important?  Maybe not, but to me it's
lovely.