I need to downgrade xmonad because after the latest upgrade it crashes when I run a specific program (fldigi specifically). Contrary to question How do you downgrade multiple packages in arch linux?, I need to downgrade packages recursively with their dependencies.
In order to downgrade xmonad I need to downgrade haskell-x11 which depends on haskell-... which depends on ~20 other packages, but only ~20. In this case the set of packages does not spiral out of control, though it's enough that working through it one at a time is going to be tedious.
Once I have verified that downgrading fixes the problem I intend to roll forward again and help the people who actually know Haskell to debug this problem.