On Tue, 10 Mar 2009, ariep wrote:
==== Problem instance ====
In my code, I use some monad transformers. I used to use the "mtl" package,
but I recently switched to the combination "transformers"/"monads-tf"
(mainly for the Applicative instances).
The same code also uses the "haskeline" library, for line reading.
Haskeline allows to edit the user's history (previously entered lines), but
only through a Control.Monad.State interface, and that happens to be the
Control.Monad.State from "mtl". So now, my package also needs to import
"mtl".
This combination of package dependencies gives import conflicts: whenever I
import some Control.Monad.{State,Error,Cont,...} module, there is more than
one package providing such a module.
I think 'transformers' exports Control.Monad.Trans.State. This should not
conflict with MTL. However, MTL's State type is different from
transformer's one. Is that your problem?
==== Solution? ====
For now, I resolve the ambiguity by using "Package imports", i.e. annotate
the ambiguous imports by the right package name.
Is this the best solution? The ghc manual states, that the package import
extension should not be necessary. I can think of another solution, by
creating a dedicated package, which imports mtl:Control.Monad.* and
re-exports them under another name, but that's even less elegant than
package imports.
Since the 'cabal' tool became better and better, today I use a cabal file
even for small programs. Cabal automatically calls GHC with the right
package exposing and hiding options.
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
