monads-tf-0.1.0.3: Monad classes, using type families

Copyright(c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
(c) Jeff Newbern 2003-2007
(c) Andriy Palamarchuk 2007
LicenseBSD-style (see the file LICENSE)
Maintainerross@soi.city.ac.uk
Stabilityexperimental
Portabilitynon-portable (type families)
Safe HaskellSafe
LanguageHaskell98

Control.Monad.Reader.Class

Description

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.

Synopsis

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 #

Arguments

:: (EnvType m -> EnvType m)

The function to modify the environment.

-> m a

Reader to run in the modified environment.

-> m a 

Executes a computation in a modified environment.

Instances
MonadReader m => MonadReader (MaybeT m) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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 #

MonadReader ((->) r :: * -> *) # 
Instance details

Defined in Control.Monad.Reader.Class

Associated Types

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

Methods

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

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

Monad m => MonadReader (ReaderT r m) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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) # 
Instance details

Defined in Control.Monad.Reader.Class

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.