Portability | non-portable (multi-param classes, functional dependencies) |
---|---|
Stability | experimental |
Maintainer | libraries@haskell.org |
Control.Monad.Reader.Class
Description
- 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.
- class Monad m => MonadReader r m | m -> r where
- asks :: MonadReader r m => (r -> a) -> m a
Documentation
class Monad m => MonadReader r m | m -> r whereSource
See examples in Control.Monad.Reader.
Note, the partially applied function type (->) r
is a simple reader monad.
See the instance
declaration below.
Arguments
:: MonadReader r m | |
=> (r -> a) | The selector function to apply to the environment. |
-> m a |
Retrieves a function of the current environment.