If you use a good "IDE" setup, it'll tell you what extensions you should enable for some code to compile. Intero (on Emacs) allows you to hit C-c C-r to insert the relevant incantation at the top of the file.

About what extensions people consider kosher, Stephen Diehl's "What I Wish I Knew When Learning Haskell" has a great list that sorts them into "common", "uhh", and "you obviously know what you're doing".