[heresy-devel] null/ bug.

[Matt Lamari]

Would this do the job?  Basically adding null and "t" types to the 
typecase. . .   Or am I missing a problem here

(defun null/ (list)
  "Returns nil if list has contents, a value otherwise.  Will only 
traverse a single element for lazy-lists"
  (etypecase list
    (null t)
    (list (null list))
    (sequence (zerop (length list)))
    (lazy-list-list-based (null (get-list-head list)))
    (lazy-list-known-empty t)
    (lazy-list (with-traversal-result (value next) (resolved 
(get-traversal-result (get-call-for-first list))) (not (resolved next))))
    (t nil)))

Robin Lee Powell wrote:
> On Tue, Sep 02, 2008 at 01:37:46PM -0700, Robin Lee Powell wrote:
>   
>> IMO, membership predicates should never except, but:
>>
>> * (null/ 'e)
>>
>> debugger invoked on a SB-KERNEL:CASE-FAILURE in thread #<THREAD
>> "initial thread" RUNNING {100266AC71}>: E fell through ETYPECASE
>> expression. Wanted one of (LAZY-LIST HERESY::LAZY-LIST-KNOWN-EMPTY
>> HERESY::LAZY-LIST-LIST-BASED SEQUENCE LIST).
>>     
>
> This turns out to be a more serious problem than I thought;
>
> (atom (list/ ))
>
> returns t, so the obvious fix of
>
> (or (atom x) (non-null/ x))
>
> doesn't work.  On top of that, the "is a lazy list" predicate
> doesn't seem to be exported from Heresy.  I had to add (atom nil) to
> the etypecase for null/, but really I think that should be a
> typecase with (t nil) at the end.
>
> -Robin
>
>