Subject: Re: bug#33998: 27.0.50; cl-delete does not delete the first list element

From: Deus Max <[email-masked]>
Date: Tue, 08 Jan 2019 00:46:28 +0200

On Mon, Jan 07 2019, João Távora wrote:

> Emacs's cl-delete attempts to emulate Common Lisp's CL:DELETE.  In Emacs
> it behaves, in this regard, no different from delete or delq.  It is
> "potentially destructive" because it *may* modify the *structure* of the
> sequence *value* that you pass to it, be it a linked list or a vector:
>
> 1) If it is a linked list, it does the trick of making the pointer
>    before the element you want to delete point to the element after it;
>
> 2) If it is a vector, it moves all the vector elements after the one you
>    want to delete back one position and readjusts the vector size.
>
> If you notice, for situation 2 you could theoretically affect the
> variable binding directly.  And curiously, this is where I found
> differences between Emacs's cl-delete and some CL's implementation of
> CL:DELETE.
>
> Emacs:
>
>     (setq bla (vector 1 2 3 4))
>     (delete 1 bla) => [2 3 4]
>     bla => [1 2 3 4]
>      
>     (setq bla (vector 1 2 3 4))
>     (cl-delete 1 bla) => [2 3 4]
>     bla => [1 2 3 4]
>
> Allegro common lisp and CMU common lisp:
>
>    (setq bla (vector 1 2 3 4))
>    (delete 1 bla) => #(2 3 4) 
>    bla => #(2 3 4)
>
> SBCL common lisp:
>
>    (setq bla (vector 1 2 3 4))
>    (delete 1 bla) => #(2 3 4)
>    bla => #(2 3 4 4)
>
> So, for vector sequences, CL:DELETE is apparently allowed to do
> whatever.  Reading the hyperspec, it seems that all these results are
> correct, even SBCL's.
>
> João

Wow! thanks for clarifying the points.
I had to fire up my SBCL to see it !