Re: [Sbcl-devel] bug? seems like (special y) declaration has effect on (type list y)

[Christophe R. <cs...@ca...>]

"Attila Lendvai" <att...@gm...> writes:

> (let ((y))
>   (declare (type list y)
>            (special y)
>            (optimize speed))
>   (length y))
>
> it compiles with notes that the type of y is not certain. when
> commenting out the (special y) declaration there are no notes, so the
> type annotation seems to be processed in that case.

In this very particular case, there may be no need for a note.
However, in general, special declarations inhibit type propagation and
inference because they can be shared between threads, and their values
changed by other threads.  To alter your example slightly:

(defun foo ()
  (declare (type list y) (special y) (optimize speed))
  (length y))

In this situation, between performing a typecheck for LIST on Y and
taking the length, another thread could do 
  (setf (symbol-value 'y) #())
and then if the call to LENGTH has been compiled to a list-specific
version, your lisp will segfault or corrupt the heap.

If you need some kind of fast behaviour, then I suggest you bind the
value of your special to a lexical variable; because then changes to
the value of the special don't affect the remainder of the body.  For
example:

(let ((y))
  (declare (special y) (optimize speed))
  (let ((yy y))
    (declare (type list yy))
    (length yy)))

Best,

Christophe