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://web.cecs.pdx.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.

Instances

MonadReader ((->) r)

Associated Types

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

Methods

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

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

MonadReader m => MonadReader (MaybeT m)

Associated Types

type EnvType (MaybeT m :: * -> *) :: *

Methods

ask :: MaybeT m (EnvType (MaybeT m))

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

MonadReader m => MonadReader (ListT m)

Associated Types

type EnvType (ListT m :: * -> *) :: *

Methods

ask :: ListT m (EnvType (ListT m))

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

(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 (IdentityT * m)

Associated Types

type EnvType (IdentityT * m :: * -> *) :: *

Methods

ask :: IdentityT * m (EnvType (IdentityT * m))

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

(Error e, MonadReader m) => MonadReader (ErrorT e m)

Associated Types

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

Methods

ask :: ErrorT e m (EnvType (ErrorT e m))

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

Arguments

:: MonadReader m

=> (EnvType m -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.