[Orgmode] [babel] Execute checks before trying to load Babel languages

[Sébastien Vauban]

Hi,

I am sharing my .emacs file, and this is beginning to cause problems with
people who don't have the same environment as I do. In particular with the
following:

#+begin_src emacs-lisp
          (org-babel-do-load-languages
           'org-babel-load-languages
           '(
             (C . t)
             (R . t)
             (calc . t)
             (ditaa . t)
             (dot . t)
             (emacs-lisp . t)
             (gnuplot . t)
             (haskell . nil)
             (latex . t)
             (ledger . t)
             (ocaml . nil)
             (octave . nil)
             (org . t)
             (perl . t)
             (python . t)
             (ruby . t)
             (screen . t)
             (sh . t)
             (sql . t)
             (sqlite . t)))
#+end_src

Someone got:

--8<---------------cut here---------------start------------->8---
    Debugger entered--Lisp error: (file-error "Cannot open load file" "ob-calc")
--8<---------------cut here---------------end--------------->8---

In fact, this is much more general than checking the Org-version: it addresses
as well installed packages such as

- Rterm (R and ess-mode)
- ditaa (requires =sudo aptitude install openjdk-6-jre=)
- gnuplot (requires gnuplot-mode)
- ledger (requires that =ob-ledger= is found...)
- org (requires that =ob-org= is found...)
- ruby

Do I have to check myself for such executables in a way similar to:

#+begin_src emacs-lisp
(when (find-exec "Rterm")
  ... add R language ...
  )

(when (file-exists-p "ob-ledger")
  ... add ledger language ...
  )
#+end_src

or could we come up with some more generic and compact solution that would
easily be usable by all of us?

Maybe such a check should be made, by default, in every language file?

Best regards,
  Seb

-- 
Sébastien Vauban