[Haskell-beginners] Trouble with liftIO in ReaderT

[Daniel Fischer]

On Sunday 30 January 2011 15:16:36, John Smith wrote:
> I'm trying to attach a handler to a Gtk2Hs event, and get the error:
>
> No instance for (Control.Monad.IO.Class.MonadIO
>                     (mtl-1.1.0.2:Control.Monad.Reader.ReaderT
> (GHC.Ptr.Ptr EAny) IO)) arising from a use of `liftIO'
> Possible fix:
>    add an instance declaration for
>    (Control.Monad.IO.Class.MonadIO
>       (mtl-1.1.0.2:Control.Monad.Reader.ReaderT (GHC.Ptr.Ptr EAny) IO))
>
> This occurs with the sample at
> http://www.haskell.org/haskellwiki/Gtk2Hs/Tutorials/Intro
>
> import Graphics.UI.Gtk
> import Control.Monad.Trans(liftIO)
>
> main = do
>    initGUI
>    window <- windowNew
>    window `on` deleteEvent $ liftIO mainQuit >> return False
>    -- i.e., on window deleteEvent (liftIO mainQuit >> return False)
>    widgetShow window
>    mainGUI
>
> Why can't GHC find the instance MonadIO m => MonadIO (ReaderT r m)?
>

GHC specifies the package the offending ReaderT comes from, mtl-1.1.0.2.
mtl-1.1.0.2's ReaderT is an instance of mtl-1.1.0.2's MonadIO class. That 
class was defined in Control.Monad.Trans.

Note that 
a) the package from which the MonadIO class in the error message comes is 
not specified,
b) it is qualified as Control.Monad.IO.Class.MonadIO, it comes from a 
module of the transformers package.

I suspect that your programme uses inconsistent dependencies, some part of 
it was built against mtl-1.1.0.2, some other part uses mtl-2.* (which is a 
wrapper around transformers, exporting mtl's old API [with a few 
differences]) or transformers directly.

Check which mtl-version gtk2hs was built against (
$ ghc-pkg describe gtk
, look at the depends field fairly low down), probably that was built 
against mtl-1.1.0.2.

To fix it you can
- maybe rebuild gtk2hs (gtk, glade, pango, ...) against mtl-2 [much 
compiling, may not work, but would save some headaches later if it does]
- build your programme against mtl-1.1.0.2 (either by writing a .cabal file 
specifying mtl-1 or by passing -package mtl-1.1.0.2 on the command line).

HTH,
Daniel