reader package:mtl

Retrieves a function of the current environment.

• 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.

The parameterizable reader monad. Computations are functions of a shared environment. The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

The reader monad transformer, which adds a read-only environment to the given monad. The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

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

Transform the value returned by a Reader.

• runReader (mapReader f m) = f .
  runReader m

Transform the computation inside a ReaderT.

• runReaderT (mapReaderT f m) = f .
  runReaderT m

Runs a Reader and extracts the final value from it. (The inverse of reader.)

Execute a computation in a modified environment (a specialization of withReaderT).

• runReader (withReader f m) = runReader m
  . f

Execute a computation in a modified environment (a more general version of local).

• runReaderT (withReaderT f m) =
  runReaderT m . f