Re: Appending to a list

[steve-humphreys@gmx.com]

> Sent: Monday, December 14, 2020 at 5:50 AM
> From: "Emanuel Berg via Users list for the GNU Emacs text editor" <help-gnu-emacs@gnu.org>
> To: help-gnu-emacs@gnu.org
> Subject: Re: Appending to a list
>
> steve-humphreys wrote:
> 
> > I thought I could push element by element.
> 
> You can, you can push whatever, if that data structure makes it
> more involved solve it for onee pilot case, then write a small
> `defun', put it there, and ever after just pass the single
> element to that.
> 
> > I got inspiration from some discussions here that push is
> > more efficient and wanted to have a go. Yes, I do use push to
> > make the larger list. Have not thought about defvar before.
> > I have read that "setq" does not introduce a variable, but
> > I would need some explanation about that.
> 
> It works with `setq' only without `defvar' the byte-compiler,
> which should always be used BTW, without it the byte-compiler
> will complain.

Do you mean that by commenting the defvars and using "setq",
the compiler will complain because no variables were defined
by bird-list*.  

defvar can only be assigned if they have not been set and have
a documentation string.  But when do you decide betweenh setq 
and defvar?
 
> Comment-out the defvars in this
> 
> (require 'cl-lib)
> 
> (defvar bird-list-all)
> (defvar bird-list-1)
> (defvar bird-list-2)
> 
> (setq bird-list-all '())
> 
> (setq bird-list-1   '("Western jackdaw" "European goldfinch"))
> (setq bird-list-2   '("rook"))
> 
> (cl-pushnew bird-list-1 bird-list-all) ; (("Western jackdaw" "European goldfinch"))
> (cl-pushnew bird-list-2 bird-list-all) ; (("rook") ("Western jackdaw" "European goldfinch"))

Is the problem with the compiler occurring because of cl-pushnew?
 
> and the byte-compiler will say
> 
>   geh.el: 
>   In toplevel form:
>   geh.el:193:7: Warning: assignment to free variable ‘bird-list-all’
>   geh.el:195:7: Warning: assignment to free variable ‘bird-list-1’
>   geh.el:196:7: Warning: assignment to free variable ‘bird-list-2’
>   geh.el:198:13: Warning: reference to free variable ‘bird-list-1’
>   geh.el:198:25: Warning: reference to free variable ‘bird-list-all’
>   geh.el:199:13: Warning: reference to free variable ‘bird-list-2’
> 
> In more ambitious projects, like a package, e.g. [1], defvar
> should be used also to make available documentation, both in
> the code and for the on-line help which in practice is the same
> thing - Emacs is self-documenting, remember?
> 
> Heh, not really, right? But still, do it.
> 
> Anyway, don't rely too much on variables, defvar or no defvar.
> Better to rely on defuns and use `let' and `let*' there. IMO!
> 
> [1] https://dataswamp.org/~incal/emacs-init/buc.el
> 
> -- 
> underground experts united
> http://user.it.uu.se/~embe8573
> https://dataswamp.org/~incal
> 
> 
>