[clisp-list] Re: Strange backquote bug, or so I think.

[Kaz K. <ka...@as...>]

On Sat, 8 Mar 2003, Kaz Kylheku wrote:

> > I've known about this issue quite some time. It's either a bug,
> > or there is some subtlety about backquotes that I don't understand.
> 
> I have more information that may be useful. Firstly, here is a smaller
> piece of code that reproduces the problem:
> 
>  (let ((x 3)) ``,,`,x) 
> 
>  ==> `,NIL

Even smaller, doh:

 ``,,`,3

 ==> `,NIL

Now for some really strange stuff: what is hiding behind the above
`,NIL form? Here is a trick: if we coerce it to a vector, the guts
become visible:

  (coerce ``,,`,3 'vector)

  ==> #(SYSTEM::BACKQUOTE (SYSTEM::UNQUOTE NIL) 3)

The third element *is* there, but the unquote form is screwed! So this
actually works:

  (eval ``,,`,3)
  ==> 3

Whee! (eval `,NIL) is 3.