Portabilitynon-portable (multi-param classes, functional dependencies)
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:
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 ( Advanced School of Functional Programming, 1995.

class Monad m => MonadReader r m | m -> r where
ask :: m r
local :: (r -> r) -> m a -> m a
asks :: MonadReader r m => (r -> a) -> m a
class Monad m => MonadReader r m | m -> r where
Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below. See examples in Control.Monad.Reader.
ask :: m r
Retrieves the monad environment.
local :: (r -> r) -> m a -> m a

Executes a computation in a modified environment. Parameters:

  • The function to modify the environment.
  • Reader to run.
  • The resulting Reader.
show/hide Instances
MonadReader r (Reader r)
MonadReader r ((->) r)
(Error e, MonadReader r m) => MonadReader r (ErrorT e m)
Monad m => MonadReader r (ReaderT r m)
MonadReader r' m => MonadReader r' (ContT r m)
asks :: MonadReader r m => (r -> 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.

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