[Haskell-cafe] ANNOUNCE: monad-control-0.3

[Joey Hess]

Bas van Dijk wrote:
> You can use the following:
> 
> {-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies, MultiParamTypeClasses #-}
> 
> import Control.Applicative
> import Control.Monad
> import Control.Monad.Base
> import Control.Monad.Trans.Class
> import Control.Monad.Trans.Control
> import Control.Monad.Trans.State
> import Control.Monad.IO.Class
> 
> newtype Annex a = Annex { runAnnex :: StateT AnnexState IO a }
>        deriving (Applicative, Functor, Monad, MonadIO)
> 
> data AnnexState = AnnexState
> 
> instance MonadBase IO Annex where
>     liftBase = Annex . liftBase
> 
> instance MonadBaseControl IO Annex where
>     newtype StM Annex a = StAnnex (StM (StateT AnnexState IO) a)
>     liftBaseWith f = Annex $ liftBaseWith $ \runInIO ->
>                        f $ liftM StAnnex . runInIO . runAnnex
> 
> When I have some time I will add some better documentation to monad-control.

Hmm, very close. With -Wall, I get:

Annex.hs:54:10:
    Warning: No explicit method nor default method for `restoreM'
    In the instance declaration for `MonadBaseControl IO Annex'

And my program crashes at runtime (!)

No instance nor default method for class operation Control.Monad.Trans.Control.restoreM

-- 
see shy jo
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