Thanks for the tips! I guess some of my frustration was that I was trying to get haste running with stack, but it needs ghc v7-something, but I'm getting ghc from pacman, so stack kept asking me to manually resolve libraries... there were other dramas and confusions too, but mostly I have little patience for making package managers play nicely, especially when I'm not very familiar with them in the first place and am new to the language! I get distracted and go off to play piano or something that's more straightforward to me :)
which gets you the version of GHC you need to be able to run $whatever_command into ~/.stack if you don't have it. Much less brittle than a system-wide installation, and perfect for working with multiple GHCs. Anecdotally: I setup the Arch Linux machine I'm typing this on yesterday. I installed[1] Stack/GHC before xorg :)