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bug#33998: 27.0.50; cl-delete does not delete the first list element
From: |
Deus Max |
Subject: |
bug#33998: 27.0.50; cl-delete does not delete the first list element |
Date: |
Tue, 08 Jan 2019 00:46:28 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) |
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 !
- bug#33998: 27.0.50; cl-delete does not delete the first list element, (continued)
- bug#33998: 27.0.50; cl-delete does not delete the first list element, João Távora, 2019/01/08
- bug#33998: 27.0.50; cl-delete does not delete the first list element, Drew Adams, 2019/01/08
- bug#33998: 27.0.50; cl-delete does not delete the first list element, João Távora, 2019/01/08
- bug#33998: 27.0.50; cl-delete does not delete the first list element, Drew Adams, 2019/01/08
- bug#33998: 27.0.50; cl-delete does not delete the first list element, João Távora, 2019/01/08
- bug#33998: 27.0.50; cl-delete does not delete the first list element, Drew Adams, 2019/01/08
bug#33998: 27.0.50; cl-delete does not delete the first list element, Deus Max, 2019/01/07