Re: Why doesn't nconc change my variable?

[Pascal J. Bourguignon]

Marcin Borkowski <mbork@wmi.amu.edu.pl> writes:

> Hi list,
>
> I don't get it.
>
> (setq my-list ())
> (nconc my-list '("wtf"))
>
> and my-list is still nil.
>
> If, OTOH, I do
>
> (setq my-list ())
> (setq my-list (nconc my-list '("wtf")))
>
> my-list is ("wtf").
>
> Why is that so?  I though nconc is supposed to change all its arguments
> but the last one.  Is the latter construct a correct way of adding an
> element at the end of the list?

() is actually read as the symbol nil.
Symbols are symbols, and not cons cells.  
You cannot transform a symbol into a cons cell.
It is impossible for any function to transform nil into a cons cell.

And vice-versa, it is impossible to transform a cons cell into a symbol
such as nil.  This is the reason why you should also use setf when
deleting elements from a list:

   (setf list (delete e list))


-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
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