From: Tamas Papp
Subject: array element-type nil
Date: 
Message-ID: <87myvyfox6.fsf@pu100877.student.princeton.edu>
I can't figure this out from the Hyperspec, so I thought I would ask:
is nil a valid type specifier for the element-type of an array?

In SBCL,

CL-USER> (defparameter *a* (make-array 5 :element-type nil))
*A*
CL-USER> (array-element-type *a*)
NIL
CL-USER> *a*
An attempt to access an array of element-type NIL was made.  Congratulations!
   [Condition of type SB-KERNEL:NIL-ARRAY-ACCESSED-ERROR]See also:
  Common Lisp Hyperspec, UPGRADED-ARRAY-ELEMENT-TYPE [function]
  Common Lisp Hyperspec, 15.1.2.1 [section]
  Common Lisp Hyperspec, 15.1.2.2 [section]

Maybe the answer is in the sections referred there, but I couldn't
find it.  Is an array with element-type nil a valid array?  What is
its purpose?  Is it some strange beast like an array with dimension
that is zero?

Thanks,

Tamas
From: Willem Broekema
Subject: Re: array element-type nil
Date: 
Message-ID: <1189182180.983771.283680@w3g2000hsg.googlegroups.com>
On Sep 7, 5:58 pm, Tamas Papp <······@gmail.com> wrote:
> I can't figure this out from the Hyperspec, so I thought I would ask:
> is nil a valid type specifier for the element-type of an array?

The type nil is the empty type; there can not be values of that type.
(The value nil is of type null, not type nil.) Nevertheless it is a
valid type specifier for arrays!

In 2003 there was an interesting discussion about these strange
arrays, in particular about what its upgraded-array-element-type
should be:

http://groups.google.com/group/comp.lang.lisp/browse_frm/thread/309e1e34d56e7010/6324a50d14eb6858?lnk=gst&rnum=2#6324a50d14eb6858


- Willem
comp.lang.lisp / 2007 / sep