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.

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

Monad ((->) r) => MonadReader ((->) r)

(Monad (MaybeT m), MonadReader m) => MonadReader (MaybeT m)

(Monad (ListT m), MonadReader m) => MonadReader (ListT m)

(Monad (IdentityT m), MonadReader m) => MonadReader (IdentityT m)

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

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

(Monad (StateT s m), MonadReader m) => MonadReader (StateT s m)

(Monad (StateT s m), MonadReader m) => MonadReader (StateT s m)

(Monad (ReaderT r m), Monad m) => MonadReader (ReaderT r m)

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

(Monad (ContT r m), MonadReader m) => MonadReader (ContT r m)

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

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

asks

Arguments

:: MonadReader m

=> (EnvType m -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.