I run Ghcmod-vim on Windows with gvim. Run :GhcModCheck it passes file for Ghc-mod utility and outputs in a quickfix error like:
G:\Haskell\G:\Haskell\huffman.hs|5 col 1 warning| The import of foldl' from module `Data.List' is redundant
As you can see, it includes path to file two times.