Incidentally, the original depends on one more thing that @Potato44 didn't mention: GHC's NondecreasingIndentation extension, which is what allows you to start the second line without indentation.
I wish there were better resources for golfing. I have trouble reading all the real formal stuff and everything else assumes you want to write good code not short code.
It would be really nice if there were approachable descriptions of the syntax.