Copyright	(c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001
License	BSD-style (see the file LICENSE)
Maintainer	R.Paterson@city.ac.uk
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

Control.Monad.Trans.Reader

Contents

The Reader monad
The ReaderT monad transformer
Reader operations
Lifting other operations

Description

Declaration of the ReaderT monad transformer, which adds a static environment to a given monad.

If the computation is to modify the stored information, use Control.Monad.Trans.State instead.

Synopsis

type Reader r = ReaderT r Identity
reader :: Monad m => (r -> a) -> ReaderT r m a
runReader :: Reader r a -> r -> a
mapReader :: (a -> b) -> Reader r a -> Reader r b
withReader :: (r' -> r) -> Reader r a -> Reader r' a
newtype ReaderT r m a = ReaderT {
- runReaderT :: r -> m a
}
mapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n b
withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
ask :: Monad m => ReaderT r m r
local :: (r -> r) -> ReaderT r m a -> ReaderT r m a
asks :: Monad m => (r -> a) -> ReaderT r m a
liftCallCC :: CallCC m a b -> CallCC (ReaderT r m) a b
liftCatch :: Catch e m a -> Catch e (ReaderT r m) a

The Reader monad

type Reader r = ReaderT r Identity Source #

The parameterizable reader monad.

Computations are functions of a shared environment.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

reader :: Monad m => (r -> a) -> ReaderT r m a Source #

Constructor for computations in the reader monad (equivalent to asks ).

runReader Source #

Arguments

:: Reader r a

A Reader to run.

-> r

An initial environment.

-> a

Runs a Reader and extracts the final value from it. (The inverse of reader .)

mapReader :: (a -> b) -> Reader r a -> Reader r b Source #

Transform the value returned by a Reader.

runReader  (mapReader  f m) = f . runReader  m

withReader Source #

Arguments

:: (r' -> r)

The function to modify the environment.

-> Reader r a

Computation to run in the modified environment.

-> Reader r' a

Execute a computation in a modified environment (a specialization of withReaderT ).

runReader  (withReader  f m) = runReader  m . f

The ReaderT monad transformer

newtype ReaderT r m a Source #

The reader monad transformer, which adds a read-only environment to the given monad.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

Constructors

ReaderT

Fields

runReaderT :: r -> m a

Instances

ReaderT r) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

lift :: Monad m => m a -> ReaderT r m a Source #

Monad m => Monad (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

(>>=) :: ReaderT r m a -> (a -> ReaderT r m b) -> ReaderT r m b #

(>>) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m b #

return :: a -> ReaderT r m a #

fail :: String -> ReaderT r m a #

Functor m => Functor (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

fmap :: (a -> b) -> ReaderT r m a -> ReaderT r m b #

(<$) :: a -> ReaderT r m b -> ReaderT r m a #

MonadFix m => MonadFix (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

mfix :: (a -> ReaderT r m a) -> ReaderT r m a #

MonadFail m => MonadFail (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

fail :: String -> ReaderT r m a #

Applicative m => Applicative (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

pure :: a -> ReaderT r m a #

(<*>) :: ReaderT r m (a -> b) -> ReaderT r m a -> ReaderT r m b #

liftA2 :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c #

(*>) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m b #

(<*) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m a #

Contravariant m => Contravariant (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

contramap :: (a -> b) -> ReaderT r m b -> ReaderT r m a #

(>$) :: b -> ReaderT r m b -> ReaderT r m a #

MonadZip m => MonadZip (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

mzip :: ReaderT r m a -> ReaderT r m b -> ReaderT r m (a, b) #

mzipWith :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c #

munzip :: ReaderT r m (a, b) -> (ReaderT r m a, ReaderT r m b) #

MonadIO m => MonadIO (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

liftIO :: IO a -> ReaderT r m a #

Alternative m => Alternative (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

empty :: ReaderT r m a #

(<|>) :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a #

some :: ReaderT r m a -> ReaderT r m [a] #

many :: ReaderT r m a -> ReaderT r m [a] #

MonadPlus m => MonadPlus (ReaderT r m) Source #

Instance details

Defined in Control.Monad.Trans.Reader

Methods

mzero :: ReaderT r m a #

mplus :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a #

mapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n b Source #

Transform the computation inside a ReaderT.

runReaderT  (mapReaderT  f m) = f . runReaderT  m

withReaderT Source #

Arguments

:: (r' -> r)

The function to modify the environment.

-> ReaderT r m a

Computation to run in the modified environment.

-> ReaderT r' m a

Execute a computation in a modified environment (a more general version of local ).

runReaderT  (withReaderT  f m) = runReaderT  m . f

Reader operations

ask :: Monad m => ReaderT r m r Source #

Fetch the value of the environment.

local Source #

Arguments

:: (r -> r)

The function to modify the environment.

-> ReaderT r m a

Computation to run in the modified environment.

-> ReaderT r m a

Execute a computation in a modified environment (a specialization of withReaderT ).

runReaderT  (local  f m) = runReaderT  m . f

asks Source #

Arguments

:: Monad m

=> (r -> a)

The selector function to apply to the environment.

-> ReaderT r m a

Retrieve a function of the current environment.

```
asks  f = liftM  f ask 
```

Lifting other operations

liftCallCC :: CallCC m a b -> CallCC (ReaderT r m) a b Source #

Lift a callCC operation to the new monad.

liftCatch :: Catch e m a -> Catch e (ReaderT r m) a Source #

Lift a catchE operation to the new monad.