[R] require() but for source()d files

[Ivan Calandra]

Dear useRs,

I'm looking for something like require() but which will work on 
source()d files.

I have a .R file with lots of functions and I'm writing a new function 
(say, 'foo') that depends on the functions from this file.

Until now, I have always source()d the .R file before running 'foo'.
But I would like to include some code in 'foo' to source() the file only 
if it hasn't been done yet, and do nothing if the file has already been 
source()d in the session.
So, the same behavior as require() with packages, but with a .R file.

Does it make sense? I could provide an example if you think it would help.

Thank you in advance,
Ivan

-- 
Ivan Calandra
University of Franche-Comté
Laboratoire Chrono-Environnement
Bureau ATER -107L
16, Route de Gray
25030 Besançon Cedex, France
ivan.calandra at univ-fcomte.fr
+33 (0) 381 66 20 60
http://chrono-environnement.univ-fcomte.fr/spip.php?article1830