Control.Monad.Writer found in multiple packages haskell
cabal, ghc, hackage, haskell
Solution
Yes, this is "standard". The reason is a historic refactoring of the `mtl` package to accomodate two different styles of handling type classes with multiple parameters, like the `MonadWriter` class : type families and functional dependencies.
The `mtl` package retains the older functional dependencies approach, whereas `monads-tf` has the newer type families approach.
As there are now libraries in the Haskell ecosystem that use `monads-tf`, it's inevitable that both will end up in the package database on typical installations - I've personally been experiencing it for several weeks.
Problem
I tried to import the `Control.Monad.Writer` module like this: ``` import Control.Monad.Writer ``` `ghc` version 7.4.1 gives the following error: ``` Ambiguous module name `Control.Monad.Writer': it was found in multiple packages: monads-tf-0.1.0.1 mtl-2.1.1 ``` There is a question with a similar problem and a workaround here. Despite the solution given in this thread my questions are: Is this the standard configuration of cabal and ghc? - If so: is there a reason for the module to be in two packages by default? - If not: what (might have) happened and can it be undone? If that matters: I'm working on Debian 7.3 wheezy and installed the `haskell-platform` package. Further, I installed some packages using `cabal install`. Many thanks in advance!