[Haskell-cafe] writing wizards in Yesod

[Henning Thielemann]

On Thu, 27 Aug 2020, Olaf Klinke wrote:

> {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
> import Control.Monad.Reader
> import Control.Monad.State.Class
> import Data.IORef
>
> class Monad m => Ref m var where
>    readRef   :: var a -> m a
>    writeRef  :: var a -> a -> m ()
>    modifyRef :: var a -> (a -> a) -> m ()
>    modifyRef v f = readRef v >>= (writeRef v . f)
>
> instance Ref IO IORef where
>    readRef   = readIORef
>    writeRef  = writeIORef
>    modifyRef = modifyIORef
>
> getRef :: Ref m var => ReaderT (var a) m a
> getRef = ReaderT readRef
>
> putRef :: Ref m var => a -> ReaderT (var a) m ()
> putRef = ReaderT . flip writeRef
>
> instance {-# OVERLAPPING #-} Ref m var =>
>    MonadState a (ReaderT (var a) m) where
>       get = getRef
>       put = putRef

I guess, with the "explicit dictionary" trick you do not need an 
(overlapping) instance at all:
    http://hackage.haskell.org/package/data-ref-0.0.2/docs/Data-Ref.html