| 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://www.cse.ogi.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.
Methods
Retrieves the monad environment.
local :: (r -> r) -> m a -> m aSource
Executes a computation in a modified environment. Parameters:
- The function to modify the environment.
-
Readerto run. - The resulting
Reader.
Instances
| (Monad m, MonadTrans t, Monad (t (ReaderT r m)), Monad (t (StateT r m))) => MonadReader r (t (ReaderT r m)) | |
| (MonadTrans t, Monad (t (Reader r)), Monad (t (State r))) => MonadReader r (t (Reader r)) | |
| MonadReader r (Reader r) | |
| MonadReader r ((->) r) | |
| Monad m => MonadReader r (ReaderT r m) |
asks :: MonadReader r m => (r -> a) -> m aSource
Retrieves a function of the current environment. Parameters:
- The selector function to apply to the environment.
See an example in Control.Monad.Reader.