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

[Michael Snoyman]

On Tue, Dec 6, 2011 at 5:03 AM, Joey Hess <joey at kitenet.net> wrote:
> 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