I second this. Learning less popular languages, you drive off a cliff when you get to the intermediate stage. I wasted a lot of time because I didn't which language extensions (developed over last the 20 years but never added to the base language) and modules are generally seen as essential. Books and web tutorials didn't talk much about them, but the real world was different. I was like I was thrown out the tutorial-door into a wide world of questionable documentation and implicit knowledge known only by wizards in IRC, if ye dare idle to find 'em.
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".
