Control.Monad.Trans.Control.Identity

MonadTransControlIdentity

class MonadTransControl t => MonadTransControlIdentity t where

The MonadTransControlIdentity type class is a stronger version of MonadTransControl:

MonadTransControl instances are aware of the monadic state of the transformer and allow to save and restore this state. MonadTransControlIdentity instances on the other hand exist only for exactly those transformers, that don't have any monadic state.

So for any instance of this class this should hold:

forall a. StT t a ~ a

This can't be given as a constraint to the class due to limitations regarding the TypeFamilies extension.

Methods

liftWithIdentity :: Monad m => ((forall x. t m x -> m x) -> m a) -> t m a

Instances

MonadTransControlIdentity (IdentityT :: (Type -> Type) -> Type -> Type)

Defined in Control.Monad.Trans.Control.Identity

Methods

liftWithIdentity :: Monad m => ((forall x. IdentityT m x -> m x) -> m a) -> IdentityT m a

MonadTransControlIdentity (ReaderT r :: (Type -> Type) -> Type -> Type)

Defined in Control.Monad.Trans.Control.Identity

Methods

liftWithIdentity :: Monad m => ((forall x. ReaderT r m x -> m x) -> m a) -> ReaderT r m a

MonadTransControlIdentity instances can easily be created for monad transformers, because of the superclass MonadTransControl:

newtype ExampleT = ...
  deriving (Monad, MonadTrans)

instance MonadTransControl ExampleT where
  ...

instance MonadTransControlIdentity ExampleT where
  liftWithIdentity = defaultLiftWithIdentity

defaultLiftWithIdentity :: (Monad m, MonadTransControl t) => ((forall x. StT t x ~ x => t m x -> m x) -> m a) -> t m a

MonadBaseControlIdentity

Regarding the IO base monad this can be seen as an alternative way to implement MonadUnliftIO.

class MonadBaseControl b m => MonadBaseControlIdentity b m | m -> b where

The MonadBaseControlIdentity type class is a stronger version of MonadBaseControl.

Just like MonadTransControlIdentity instances of MonadBaseControlIdentity hold no monadic state:

forall a. StM m a ~ a

Methods

liftBaseWithIdentity :: ((forall x. m x -> b x) -> b a) -> m a

Instances

MonadBaseControl b b => MonadBaseControlIdentity b b

Defined in Control.Monad.Trans.Control.Identity

Methods

liftBaseWithIdentity :: ((forall x. b x -> b x) -> b a) -> b a

MonadBaseControlIdentity b m => MonadBaseControlIdentity b (IdentityT m)

Defined in Control.Monad.Trans.Control.Identity

Methods

liftBaseWithIdentity :: ((forall x. IdentityT m x -> b x) -> b a) -> IdentityT m a

MonadBaseControlIdentity b m => MonadBaseControlIdentity b (ReaderT r m)

Defined in Control.Monad.Trans.Control.Identity

Methods

liftBaseWithIdentity :: ((forall x. ReaderT r m x -> b x) -> b a) -> ReaderT r m a

Just like MonadTransControlIdentity, MonadBaseControlIdentity instances can easily be created for monad transformers:

instance MonadBaseControlIdentity b m => MonadBaseControlIdentity b (ExampleT m) where
  liftBaseWithIdentity = defaultLiftBaseWithIdentity

defaultLiftBaseWithIdentity :: (MonadBaseControlIdentity b m, MonadTransControlIdentity t) => ((forall x. t m x -> b x) -> b a) -> t m a