On Tue, Dec 6, 2011 at 5:03 AM, Joey Hess
I'm trying to convert from 0.2 to 0.3, but in way over my head.
{-# LANGUAGE GeneralizedNewtypeDeriving #-} newtype Annex a = Annex { runAnnex :: StateT AnnexState IO a } deriving ( Monad, MonadIO, -- MonadControlIO MonadBaseControl IO )
I added that after seeing this when I changed some code to use the new liftBaseOp instead of liftIOOp. (They're equivilant, right?)
No instance for (MonadBaseControl IO Annex) arising from a use of `liftBaseOp'
But with ghc 7.0.4, the derivation fails:
Annex.hs:45:17: Can't make a derived instance of `MonadBaseControl IO Annex' (even with cunning newtype deriving): the class has associated types In the newtype declaration for `Annex'
The only way I can find to make my code compile is to lose the newtype. But of course that makes for some ugly type messages.
-- see shy jo
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Hi Joey, I just spent a fair amount of time yesterday upgrading packages to monad-control 0.3. What I had to do was add in the MonadTransControl and MonadBaseControl instances manually. It's actually not too difficult; just copy out the instance for StateT and make a few changes. Be warned that Bas used some tricky CPP stuff, however, which you'll have to unwind ;). Michael