If and when updates of haskell or Xmonad come in….

Recompile Xmonad after installation – do not reboot before recompiling.

or

Super + Shift + R

IF you did not recompile Xmonad after updates of haskell then you can not login into Xmonad since the application is not correctly build and it will throw you back to login screen.

Basically we can not login as lightdm can not start anything.

With right CTRL and F2 or F3 in VirtualBox etc you can go to a TTY and run your commands mentioned above or CTRL + ALT + F2 on bare metal.