monads-tf- Monad classes, using type familiesSource codeContentsIndex
Portabilitynon-portable (type families)
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 Control.Monad.State.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 where
type EnvType m
ask :: m (EnvType m)
local :: (EnvType m -> EnvType m) -> m a -> m a
asks :: MonadReader m => (EnvType m -> a) -> m a
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 mReader to run in the modified environment.
-> m a
-> m a
Executes a computation in a modified environment.
show/hide Instances
:: MonadReader m
=> EnvType m -> a
-> m a
Retrieves a function of the current environment.
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