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 Control.Monad.State.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.

1 class

1 value

classclass Monad m => MonadReader r (m :: Type -> Type) | m -> r where

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

Methods

ask :: m r
Retrieves the monad environment.
local :: (r -> r) -> m a -> m a
Executes a computation in a modified environment.
reader :: (r -> a) -> m a
Retrieves a function of the current environment.

Instances16MonadReader, …

MonadReader r m => MonadReader r (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
Monad m => MonadReader r (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r m => MonadReader r (ExceptT e m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r m => MonadReader r (IdentityT m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r m => MonadReader r (StateT s m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r m => MonadReader r (StateT s m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r' m => MonadReader r' (SelectT r m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monoid w, MonadReader r m) => MonadReader r (AccumT w m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r ((->) r)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadReader r' m => MonadReader r' (ContT r m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monad m, Monoid w) => MonadReader r (RWST r w s m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monad m, Monoid w) => MonadReader r (RWST r w s m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monad m, Monoid w) => MonadReader r (RWST r w s m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class