Every time I try to run flycheck-haskell-setup I get an error:

Reading Haskell configuration failed with exit code 1 and ouput:
get-cabal-configuration.hs: user error (mzero)

That is the error generated by the MonadPlus instance of IO when mzero is evaluated, it seems it must be coming from an imported function that is partial.

I'm trying to use this package through a Nix shell so that's probably part of the problem, but it is difficult to figure out what is going wrong without knowing where that error is coming from.