Control.Monad.State found in multiple packages haskell

Question

While evaluating the line "import Control.Monad.State" in a Haskell module, GHC gives me the following error:

Could not find module `Control.Monad.State':
  it was found in multiple packages: monads-fd-0.0.0.1 mtl-1.1.0.2
Failed, modules loaded: none.

How do I resolve this conflict?

Answer 1

Both packages implement Control.Monad.State and GHC does not know which implementation it should prefer, so you need to hide one of the packages from GHC. Seems like the -ignore-package <name> GHC flag might help you here.

Answer 2

You have several options. Either:

• ghc-pkg hide monads-fd. This will cause GHC and GHCi to ignore the presence of the monads-fd by default until you later ghc-pkg expose monads-fd, but software installed by Cabal will still be able to build against it.
• Use the {-# LANGUAGE PackageImports #-} pragma, and change your import statement to import "mtl" Control.Monad.State.
• Use Cabal to build your project, and specify mtl in the Build-depends line.

The first is best for casual hacking, and the last is best for production builds.

These all assume you want the mtl module and not the monads-fd module; otherwise swap them.