| Portability | non-portable (multi-param classes, functional dependencies) |
|---|---|
| Stability | experimental |
| Maintainer | libraries@haskell.org |
| Safe Haskell | Safe-Inferred |
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 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.
Methods
Retrieves the monad environment.
Arguments
| :: (r -> r) | The function to modify the environment. |
| -> m a |
|
| -> m a |
Executes a computation in a modified environment.
Arguments
| :: (r -> a) | The selector function to apply to the environment. |
| -> m a |
Retrieves a function of the current environment.
Instances
| MonadReader r m => MonadReader r (MaybeT m) | |
| MonadReader r m => MonadReader r (ListT m) | |
| MonadReader r m => MonadReader r (IdentityT m) | |
| MonadReader r ((->) r) | |
| (Monoid w, MonadReader r m) => MonadReader r (WriterT w m) | |
| (Monoid w, MonadReader r m) => MonadReader r (WriterT w m) | |
| MonadReader r m => MonadReader r (StateT s m) | |
| MonadReader r m => MonadReader r (StateT s m) | |
| (Error e, MonadReader r m) => MonadReader r (ErrorT e m) | |
| MonadReader r' m => MonadReader r' (ContT r m) | |
| Monad m => MonadReader r (ReaderT r m) | |
| (Monad m, Monoid w) => MonadReader r (RWST r w s m) | |
| (Monad m, Monoid w) => MonadReader r (RWST r w s m) |
Arguments
| :: MonadReader r m | |
| => (r -> a) | The selector function to apply to the environment. |
| -> m a |
Retrieves a function of the current environment.