monads-tf- Monad classes, using type families

Portabilitynon-portable (type families)
Safe HaskellSafe-Inferred



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 m whereSource

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 Source


ask :: m (EnvType m)Source

Retrieves the monad environment.



:: (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.



:: MonadReader m 
=> (EnvType m -> a)

The selector function to apply to the environment.

-> m a 

Retrieves a function of the current environment.