"Sebastien Vauban" <wxhgmqzgwmuf@spammotel.com> writes:

> Hi Eric,
>
> Eric Schulte wrote:
>> "Sebastien Vauban" <wxhgmqzgwmuf@spammotel.com> writes:
>>> Eric Schulte wrote:
>>>> Emacs Lisp is an exception in terms of colname processing, it has default
>>>> header arguments set to pass column names through to the code block, where
>>>> the processing may be done trivially in Emacs Lisp.
>>>
>>> OK, but I don't understand the precedence of header arguments. I thought
>>> that a header argument given on the code block preempted all the other
>>> values (system-wide default for all languages, language defaults, file-wide
>>> arguments, and subtree arguments).
>>>
>>> Why isn't this true here as well?
>>
>> That is what is happening here, although combinations of :hlines and
>> :colnames can be tricky. Especially weird, is that if you want to *unset* a
>> header argument which is set at a higher level, you need to set it to '(),
>> as in ":colnames '()".
>
> Much clearer, but not yet crystal-clear for me...
>
> Let me explain. AFAICT, there were 5 possibles values of the ":colnames"
> header argument:
>
> - no header argument :: (default for all languages but Emacs Lisp)
> - ":colnames no" :: (default for Emacs Lisp code blocks)
> - ":colnames yes" :: Tells Org Babel that your first row contains column
>   names.
> - ":colnames <LIST>" :: Specifies to use <LIST> as column names.
> - ":colnames nil" :: Same as ":colnames yes".
>
>
> Right?
>

Almost, values 1 (none) and 5 (nil) are the same.

>
> Now, indeed, your trick with ":colnames '()" (or even ":colnames
> ()"...) does work well for Emacs-Lisp...
>
> Though, I thought that "()" was equivalent to "nil", but it seems not
> to be the case, then. Is it because of some sort of type coercion,
> that would convert nil as a string or something along such lines?
>

See "Emacs Lisp evaluation of variables" in (info "(org)var").  We could
add nil as a special exception, but that might be surprising to some
people.

>
> Extra question: when do we have to use such a trick?  When the value can be a
> list of things?  If yes, why are you talking of ":hlines" -- there is no list
> argument there?
>

Whenever you want to "unset" a header argument, which has a value set at
some higher level.