[Orgmode] [babel] silent code block evaluation on export

[Erik Iverson]

Hello! I have pasted an org-mode file with my question, it's
easies to explain by copying the below file and exporting it
to HTML.

Thanks,
--Erik

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* Silent code block evaluation on export.

The goal is to /run/ a code block on export for some side effect,
without any evidence in the exported file that this happened. For
example, loading some R package, reading in a dataset, or setting R
options. All this is presumably done with :session in mind, even
though I don't use it here.

In Sweave, we can specify the code block options echo = FALSE, results
= hide to faciliate this.

However, there are no command line arguments alone in org-mode that
can replicate such behavior.  Below are several examples and how they
fail.  There is also an example of a 'trick' that does work to
generate the output (or lack thereof) that we want.

/:results value/ tries to coerce its return value to a data.frame, so when
I reference 'non-conformable object' below, I mean something for which
the as.data.frame function generates an error.


* /:results value/ (the default for code blocks)

** with a simple object on the last line, a vector

This prints the return object, OK.

/:exports results :results value/
#+begin_src R :exports results :results value
2 + 2
#+end_src


** with a non-conformable object as the final value

This produces an error, as expected, and no output is produced, but
only by accident.

/:exports results :results value/
#+begin_src R :exports results :results value
summary(lm(1 ~ 1))
#+end_src

** using NULL as the last line

This does what we want, but requires a 'trick'. OK.

/:exports results :results value/
#+begin_src R :exports results :results value
summary(lm(1 ~ 1))
NULL
#+end_src


* /:results silent/

** with a simple object on the last line, a vector

This still prints the return object, it may be debateable if it
should.  The combination of /:exports results/ and
/:results silent/ does not seem inherently self-contradictory.

/:exports results :results silent/
#+begin_src R :exports results :results silent
2 + 2
#+end_src

** with a non-conformable object as the final value

However, with a non-conformable object on last line, an error is
produced when the code is evaluated in the buffer. I'm wondering if
/:results silent/ is specified, why the cast to data.frame is
occurring? It could be happening for good reason that I'm not aware
of. The lack of output here is due to an error occurring.

/:exports results :results silent/
#+begin_src R :exports results :results silent
summary(lm(1 ~ 1))
#+end_src

** using NULL as the last line

This does as we want, using the NULL trick.

/:exports results :results silent/
#+begin_src R :exports results :results silent
summary(lm(1 ~ 1))
NULL
#+end_src


* Ideas

Since the NULL trick is R-only, how could we specify command line
options to faciliate this use case?

Possible solutions:
0) There already exists a solution that I don't list here.

1) Make /:exports results :results silent/ do it, but note the results
   above with a non-conformable object!

2) Make /:exports none/ still evaluate the code block on
   export. Probably counter-intuitive.

3) Make export use the /:eval/ argument combined with /:exports/,
   maybe something like: /:exports none :eval yes/

4) Another better solution!