[O] [RFC] new :post header argument for post-processing of code block results

[Eric Schulte]

Hi,

I've been wanting to add the ability to post-process the results of a
code block for some time, and some recent threads (e.g., [1] and [2])
could both have benefited from post-processing of code block output.

The attached patch [3] adds a new :post header argument, which may be
used to specify a code block reference to use to post-process the
results of the current code block.  The following example illustrates
how this header argument could be used to add attr_latex lines to table
code block results.

#+Title: Proposed =post= Header Argument
#+Author: Eric Schulte

* a code block which does the post processing
This code block adds latex width attributes to whatever =data= is
passed to it.
#+name: add-attr
#+begin_src sh :var data="" :results output drawer :var width="\textwidth"
  echo "#+attr_latex: width=$width"
  echo "$data"
#+end_src

* two code blocks which use the post processing
The following two code blocks demonstrate the use of the new =post=
header argument, whcih we could add if it seems generally useful and
not too confusing.

This works by lexically binding the =*this*= variable to the results
of the current code block, and then calling the =add-attr= code block
to post-process these results.

#+begin_src sh :results output wrap :post add-attr(width="5cm",data=(identity 
*this*))
cat <<EOF
| 1 | 2 |
| 3 | 4 |
EOF
#+end_src

#+RESULTS:
:RESULTS:
#+attr_latex: width=5cm
| 1 | 2 |
| 3 | 4 |
:END:

This code block does the same thing as the previous one, but using
different values and arguments.

#+header: :post add-attr(width="0.5\\textwidth",data=(identity *this*))
#+begin_src sh :results output wrap
cat <<EOF
| 5 | 6 |
| 7 | 8 |
EOF
#+end_src

#+RESULTS:
:RESULTS:
#+attr_latex: width=0.5\textwidth
| 5 | 6 |
| 7 | 8 |
:END:

* another unrelated example
Say we only care about the size of our results, then we could use the
following.
#+name: wc
#+begin_src sh data=""
  echo "$data"|wc
#+end_src

This code block checks the size of some Emacs Lisp data (in this case
my =load-path= variable).
#+begin_src emacs-lisp :post wc(data=(identity *this*))
  load-path
#+end_src

#+RESULTS:
: 160     160    6783

Does this new header argument seem useful?  Any suggestions for better
syntax which don't add too much conceptual or code complexity?

Thanks,

Footnotes: 
[1]  http://thread.gmane.org/gmane.emacs.orgmode/69339

[2]  http://thread.gmane.org/gmane.emacs.orgmode/68700

[3]

>From f83f31d82c0c660c74a2af7114d4e23c9b37c095 Mon Sep 17 00:00:00 2001
From: Eric Schulte <address@hidden>
Date: Sun, 31 Mar 2013 19:02:11 -0600
Subject: [PATCH] :post header arg post-processes code block results

* lisp/ob-core.el (org-babel-common-header-args-w-values): Add :post to
  the list of header arguments.
  (org-babel-execute-src-block): Post process results when the :post
  header argument has been supplied.
---
 lisp/ob-core.el | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lisp/ob-core.el b/lisp/ob-core.el
index 723aa9d..e1321eb 100644
--- a/lisp/ob-core.el
+++ b/lisp/ob-core.el
@@ -456,6 +456,7 @@ then run `org-babel-pop-to-session'."
     (noweb-ref . :any)
     (noweb-sep  . :any)
     (padline   . ((yes no)))
+    (post       . :any)
     (results   . ((file list vector table scalar verbatim)
                   (raw html latex org code pp drawer)
                   (replace silent none append prepend)
@@ -625,6 +626,11 @@ block."
                                      (not (listp result)))
                                 (list (list result)) result))
                           (funcall cmd body params)))
+                   ;; possibly perform post process provided its appropriate
+                   (when (cdr (assoc :post params))
+                     (let ((*this* result))
+                       (setq result (org-babel-ref-resolve
+                                     (cdr (assoc :post params))))))
                    ;; if non-empty result and :file then write to :file
                    (when (cdr (assoc :file params))
                      (when result
-- 
1.8.2


-- 
Eric Schulte
http://cs.unm.edu/~eschulte