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Implement MonadError for ContT e (Codensity m) #60

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Implement MonadError for ContT e (Codensity m) #60

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db70c4f
Implement MonadError for `ContT e (Codensity m)`
Atry Jan 18, 2020
b576025
Add a forwarder instance of MonadError
Atry Jan 18, 2020
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13 changes: 13 additions & 0 deletions src/Control/Monad/Codensity.hs
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Expand Up @@ -42,10 +42,12 @@ import Control.Applicative
import Control.Monad (MonadPlus(..))
import qualified Control.Monad.Fail as Fail
import Control.Monad.Free
import Control.Monad.Error.Class
import Control.Monad.IO.Class
import Control.Monad.Reader.Class
import Control.Monad.State.Class
import Control.Monad.Trans.Class
import Control.Monad.Trans.Cont
import Data.Functor.Adjunction
import Data.Functor.Apply
import Data.Functor.Kan.Ran
Expand Down Expand Up @@ -248,6 +250,17 @@ instance (Functor f, MonadFree f m) => MonadFree f (Codensity m) where
wrap t = Codensity (\h -> wrap (fmap (\p -> runCodensity p h) t))
{-# INLINE wrap #-}

instance {-# OVERLAPPING #-} MonadError e (ContT e (Codensity m)) where
throwError e = ContT (const (pure e))
catchError c recover = ContT $ \successHandler -> Codensity $ \errorHandler ->
runCodensity (runContT c successHandler)
$ \e -> runCodensity (runContT (recover e) successHandler) errorHandler

instance MonadError e m => MonadError e (Codensity m) where
throwError e = Codensity (const (throwError e))
catchError c recover =
Codensity $ \k -> catchError (runCodensity c k) $ \e -> runCodensity (recover e) k

instance MonadReader r m => MonadState r (Codensity m) where
get = Codensity (ask >>=)
{-# INLINE get #-}
Expand Down