I was writing code similar to the following and compiling with "-Wall -Werror." Clearly I *am* using Data.ByteString for the function signatures. {-# OPTIONS_GHC -fwarn-unused-imports -Werror #-} import qualified Data.ByteString as B import qualified Data.ByteString.UTF8 as BU toString :: B.ByteString -> String toString = BU.toString fromString :: String -> B.ByteString fromString = BU.fromString main :: IO () main = putStrLn $ toString $ fromString "Hello World!" The warning I get is: Unused.hs:2:0: Warning: Module `Data.ByteString' is imported, but nothing from it is used, except perhaps instances visible in `Data.ByteString' To suppress this warning, use: import Data.ByteString() Am I doing something wrong or is this a bug in GHC? Thanks, Michael