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On Thu, 18 Feb 2021 at 16:52, Rodrigo Morales <moralesrodrigo1...@gmail.com>
wrote:
>
> This message contains my thoughts on a feature request which I think
> would be useful: The =:pre= header argument, it would be used to
> execute a code block before the code block at point is executed. It
> would be similar to the behavior of the =:post= header argument.
>
> Here's a simple example that shows how =:pre= could be used
>
> #+NAME: clean-path-experiments
> #+begin_src dash
> if [ ! -z "$my__experiments" ] && [ -d "$my__experiments" ]; then
> find ~/e -mindepth 1 -maxdepth 1 -exec rm -rf {} +
> fi
> #+end_src
>
> #+NAME: tree-command
> #+begin_src dash
> tree -a --noreport
> #+end_src
>
> #+NAME: create-dir-for-minimal-reproducible-example
> #+HEADER: :pre clean-path-experiments()
> #+HEADER: :post tree-command()
> #+begin_src python
> import os
>
> [os.makedirs(_) for _ in ["a/foo", "a/bar", "b"]]
> [os.mknod(_) for _ in ["a/1.txt", "a/2.txt", "a/foo/b.txt", "a/bar/b.txt",
> "b/b.txt"]]
> #+end_src
>
> #+RESULTS: create-dir-for-minimal-reproducible-example
> #+begin_example
> .
> ├── a
> │ ├── 1.txt
> │ ├── 2.txt
> │ ├── bar
> │ │ └── b.txt
> │ └── foo
> │ └── b.txt
> └── b
> └── b.txt
> #+end_example
>
> I think that such header argument would be useful because of two
> main reasons:
>
> * It would improve readability of code blocks by explicitly expressing
> dependency
>
> By having a header argument for the sole purpose of expressing
> dependencies between code blocks, the readability of header arguments
> would be improved. Recall that it is currently possible to express
> such dependency by calling a code block through a =:var <<name>>=
> header argument but I think that =:var= header argument must only be
> used for defining variables (be it from results obtained from
> different code blocks or literals).
>
> The first code block shown below show the differences between using
> =:var= and =:pre= for the same scenario. =first-code-block= uses the
> =:var= header argument while the =second-code-block= uses the =:pre=
> header argument.
>
> #+NAME: first-code-block
> #+begin_src org
> For our experimentation, let's start with an empty directory and let's
> execute the first step.
>
> ,#+NAME: first-step
> ,#+HEADER: :results silent
> ,#+HEADER: :var e=clean-path-experiments
> ,#+begin_src dash
> touch first-step.txt
> ,#+end_src
>
> We know execute the second step.
>
> ,#+NAME: second-step
> ,#+HEADER: :results silent
> ,#+HEADER: :var e=first-step
> ,#+begin_src dash
> touch second-step.txt
> ,#+end_src
>
> Finally, we execute the third step.
>
> ,#+NAME: third-step
> ,#+HEADER: :results silent
> ,#+HEADER: :var e=second-step
> ,#+begin_src dash
> touch third-step.txt
> ,#+end_src
> #+end_src
>
> #+NAME: second-code-block
> #+begin_src org
> For our experimentation, let's start with an empty directory and let's
> execute the first step.
>
> ,#+NAME: first-step
> ,#+HEADER: :results silent
> ,#+HEADER: :pre clean-path-experiments()
> ,#+begin_src dash
> touch first-step.txt
> ,#+end_src
>
> We know execute the second step.
>
> ,#+NAME: second-step
> ,#+HEADER: :results silent
> ,#+HEADER: :pre first-step()
> ,#+begin_src dash
> touch second-step.txt
> ,#+end_src
>
> Finally, we execute the third step.
>
> ,#+NAME: third-step
> ,#+HEADER: :results silent
> ,#+HEADER: :pre second-step()
> ,#+begin_src dash
> touch third-step.txt
> ,#+end_src
> #+end_src
>
> In my opinion, the second code block looks more readable than the
> first one.
>
> * it would add importance to the =:post= header argument
>
> The =:post= header argument can be used in Org Mode 9.3 to execute a
> given code block after the code block at point is executed; having a
> header argument that does the opposite of the =:post= header argument
> would give relevance to the =:post= header argument.
>
> --
> Greetings,
> Rodrigo Morales.
>
> IRC: rdrg109 (freenode)
>
