[Haskell-cafe] writing wizards in Yesod

[Olaf Klinke]

On Thu, 2020-08-27 at 01:20 +0300, Georgi Lyubenov wrote:
> Hi!
> 
> I believe the canonical way to handle this in Yesod is the "reader
> pattern"
> (https://www.fpcomplete.com/blog/2017/06/readert-design-pattern):
> * it's by the same author
> 
I think the essence of the above blog post concerning state is the
following. The overlapping instance hints at why this is not in Yesod
in this generality. It is probably fine to declare such a MonadState
instance for any concrete reader monad, though. 

Olaf

{-# 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