Created attachment 517868 [details] build.log dev-haskell/transformers-0.4.3.0 fails to build with dev-lang/ghc-8.0.2 with the attached error message. Control/Monad/Trans/Error.hs:69:10: error: Duplicate instance declarations: instance MonadPlus IO -- Defined at Control/Monad/Trans/Error.hs:69:10 instance MonadPlus IO -- Defined in ‘GHC.Base’ Control/Monad/Trans/Error.hs:73:10: error: Duplicate instance declarations: instance Alternative IO -- Defined at Control/Monad/Trans/Error.hs:73:10 instance Alternative IO -- Defined in ‘GHC.Base’
ghc-8.0.2 comes with transformers-0.5.2.0. Your system should use those. Do you know what holds old transformers back for you?