Control.Monad.Reader.Class

Computation type:

Computations which read values from a shared environment.

Binding strategy:

Monad values are functions from the environment to a value. The bound function is applied to the bound value, and both have access to the shared environment.

Useful for:

Maintaining variable bindings, or other shared environment.

Zero and plus:

None.

Example type:

Reader [(String,Value)] a

The Reader monad (also called the Environment monad). Represents a computation, which can read values from a shared environment, pass values from function to function, and execute sub-computations in a modified environment. Using Reader monad for such computations is often clearer and easier than using the State monad.

Inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones (http://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.

Documentation

class Monad m => MonadReader m where

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Minimal complete definition

ask, local

Associated Types

type EnvType m

Methods

ask :: m (EnvType m)

Retrieves the monad environment.

local :: (EnvType m -> EnvType m) -> m a -> m a

Executes a computation in a modified environment. Parameters:

• The function to modify the environment.
• Reader to run.
• The resulting Reader.

Instances

(Monoid w, MonadReader m) => MonadReader (WriterT w m)

Associated Types

type EnvType (WriterT w m :: * -> *) :: *

Methods

ask :: WriterT w m (EnvType (WriterT w m))

local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a

(Monoid w, MonadReader m) => MonadReader (WriterT w m)

Associated Types

type EnvType (WriterT w m :: * -> *) :: *

Methods

ask :: WriterT w m (EnvType (WriterT w m))

local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a

MonadReader m => MonadReader (StateT s m)

Associated Types

type EnvType (StateT s m :: * -> *) :: *

Methods

ask :: StateT s m (EnvType (StateT s m))

local :: (EnvType (StateT s m) -> EnvType (StateT s m)) -> StateT s m a -> StateT s m a

MonadReader m => MonadReader (StateT s m)

Associated Types

type EnvType (StateT s m :: * -> *) :: *

Methods

ask :: StateT s m (EnvType (StateT s m))

local :: (EnvType (StateT s m) -> EnvType (StateT s m)) -> StateT s m a -> StateT s m a

MonadReader m => MonadReader (ExceptT e m)

Associated Types

type EnvType (ExceptT e m :: * -> *) :: *

Methods

ask :: ExceptT e m (EnvType (ExceptT e m))

local :: (EnvType (ExceptT e m) -> EnvType (ExceptT e m)) -> ExceptT e m a -> ExceptT e m a

MonadReader ((->) LiftedRep LiftedRep r)

Associated Types

type EnvType ((LiftedRep -> LiftedRep) r :: * -> *) :: *

Methods

ask :: (LiftedRep -> LiftedRep) r (EnvType ((LiftedRep -> LiftedRep) r))

local :: (EnvType ((LiftedRep -> LiftedRep) r) -> EnvType ((LiftedRep -> LiftedRep) r)) -> (LiftedRep -> LiftedRep) r a -> (LiftedRep -> LiftedRep) r a

Monad m => MonadReader (ReaderT * r m)

Associated Types

type EnvType (ReaderT * r m :: * -> *) :: *

Methods

ask :: ReaderT * r m (EnvType (ReaderT * r m))

local :: (EnvType (ReaderT * r m) -> EnvType (ReaderT * r m)) -> ReaderT * r m a -> ReaderT * r m a

MonadReader m => MonadReader (ContT * r m)

Associated Types

type EnvType (ContT * r m :: * -> *) :: *

Methods

ask :: ContT * r m (EnvType (ContT * r m))

local :: (EnvType (ContT * r m) -> EnvType (ContT * r m)) -> ContT * r m a -> ContT * r m a

(Monoid w, Monad m) => MonadReader (RWST r w s m)

Associated Types

type EnvType (RWST r w s m :: * -> *) :: *

Methods

ask :: RWST r w s m (EnvType (RWST r w s m))

local :: (EnvType (RWST r w s m) -> EnvType (RWST r w s m)) -> RWST r w s m a -> RWST r w s m a

(Monoid w, Monad m) => MonadReader (RWST r w s m)

Associated Types

type EnvType (RWST r w s m :: * -> *) :: *

Methods

ask :: RWST r w s m (EnvType (RWST r w s m))

local :: (EnvType (RWST r w s m) -> EnvType (RWST r w s m)) -> RWST r w s m a -> RWST r w s m a

asks :: MonadReader m => (EnvType m -> a) -> m a

Retrieves a function of the current environment. Parameters:

• The selector function to apply to the environment.

See an example in Control.Monad.Reader.