I'm writing several resources for webmachine that share lots of same functions. So I'm wondering if it's possible to write the common functions in a separate module, then somehow include it, and have its exports automatically exported by the resource (without having to export them from every resource explicitly).
Yes you can use the mixer library, https://github.com/opscode/mixer it is a compiler parse transform that will do exactly what you want. I use it all over the place for webmachine callbacks