Re: [O] [babel] Bugs for Emacs Lisp code blocks

[Sebastien Vauban]

Hi Eric,

Eric Schulte wrote:
> "Sebastien Vauban" <address@hidden> writes:
>> Eric Schulte wrote:
>>> "Sebastien Vauban" <address@hidden> 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.

I don't share your view about this last statement.

** Input table

#+name: unset-colnames-example-input
| a | b |
|---+---|
| 1 | 2 |
| 3 | 4 |

** Having no =:colnames= header argument (case 1)

Same results for R and sh code blocks (first good news ;-)) -- I'm avoiding,
on purpose, testing with Emacs Lisp...

#+begin_src R :var data=unset-colnames-example-input
  data
#+end_src

#+results:
| 1 | 2 |
| 3 | 4 |

#+begin_src sh :var data=unset-colnames-example-input
  echo "$data"
#+end_src

#+results:
| 1 | 2 |
| 3 | 4 |

** Using =:colnames nil= header argument (case 5)

Once again, R and sh blocks do produce the same results...

#+begin_src R :var data=unset-colnames-example-input :colnames nil
  data
#+end_src

#+results:
| a | b |
|---+---|
| 1 | 2 |
| 3 | 4 |

#+begin_src sh :var data=unset-colnames-example-input :colnames nil
  echo "$data"
#+end_src

#+results:
| a | b |
|---+---|
| 1 | 2 |
| 3 | 4 |

... but they are _not_ equivalent to the "no header argument" (case 1).

** Using =:colnames yes= header argument (case 3)

On the contrary, case 5 is equivalent to the case 3: same results as
":colnames yes".

#+begin_src R :var data=unset-colnames-example-input :colnames yes
  data
#+end_src

#+results:
| a | b |
|---+---|
| 1 | 2 |
| 3 | 4 |

#+begin_src sh :var data=unset-colnames-example-input :colnames yes
  echo "$data"
#+end_src

#+results:
| a | b |
|---+---|
| 1 | 2 |
| 3 | 4 |

>> 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").

Not sure to find what you want me to read in that page...

> We could add nil as a special exception, but that might be surprising to
> some people.

As far as the Lisp interpreter is concerned, () and nil are the same, right?
Then, why do you talk of adding a special exception?  Maybe, I don't
understand your point because I'm missing the context info you wanted me to
read just above?

>> 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.
>
> ** unset the colnames header argument
> #+name: unset-colnames-example-input
> | a | b |
> |---+---|
> | 1 | 2 |
> | 3 | 4 |
>
> Unlike most code blocks, Emacs Lisp has colnames set to "yes" in its
> default header arguments. [...]
> If we wanted to unset this value, we could do the following.
>
> #+begin_src emacs-lisp :var data=unset-colnames-example-input :colnames ()
>   data
> #+end_src
>
> #+RESULTS:
> | 1 | 2 |
> | 3 | 4 |

This is clear -- thanks! -- but it does not unset the header argument as long
as case 1 and 5 are not the same in the above given example (for R and sh
blocks).

Best regards,
  Seb

-- 
Sebastien Vauban