Moduletransformers-0.6.3.0Haskell2010

Control.Monad.Trans.Reader

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.

Contents

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

Synopsis

type Reader
value reader
value runReader
value mapReader
value withReader
newtype ReaderT
value mapReaderT
value withReaderT
value ask
value local
value asks
value liftCallCC
value liftCatch

Packagetransformers-0.6.3.0
Exports13
LanguageHaskell2010
LicenceBSD-3-Clause
SourceReader.hs

The Reader monad

typetype Reader r = ReaderT r Identity

Source#

The parameterizable reader monad, which is non-strict.

Computations are functions of a shared environment.

The return function ignores the environment, while m >>= k passes the inherited environment to both subcomputations:

image: images/bind-ReaderT.svg

valuereader :: Monad m => (r -> a) -> ReaderT r m a

Source#

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

valuerunReader

:: Reader r a
A Reader to run.
-> r
An initial environment.
-> a

Source#

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

valuemapReader :: (a -> b) -> Reader r a -> Reader r b

Source#

Transform the value returned by a Reader.

runReader (mapReader f m) = f . runReader m

valuewithReader

:: (r' -> r)
The function to modify the environment.
-> Reader r a
Computation to run in the modified environment.
-> Reader r' a

Source#

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

runReader (withReader f m) = runReader m . f

The ReaderT monad transformer

newtypenewtype ReaderT r (m :: Type -> Type) a

Source#

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

The return function ignores the environment, while m >>= k passes the inherited environment to both subcomputations:

image: images/bind-ReaderT.svg

ReaderT r m is strict if and only if m is.

Constructors

ReaderT
- runReaderT :: r -> m a

Instances15Generic1, MonadTrans, Monad, Functor, MonadFix, MonadFail, …

Generic1 (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
MonadTrans (ReaderT r)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
Monad m => Monad (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
Functor m => Functor (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
MonadFix m => MonadFix (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
MonadFail m => MonadFail (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
Applicative m => Applicative (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
Alternative m => Alternative (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
MonadPlus m => MonadPlus (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
MonadIO m => MonadIO (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
MonadZip m => MonadZip (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
Contravariant m => Contravariant (ReaderT r m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
Generic (ReaderT r m a)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
type Rep (ReaderT r m a) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.3.0-34b4" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (r -> m a))))Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader
type Rep1 (ReaderT r m) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.3.0-34b4" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (FUN 'Many r :.: Rec1 m)))Defined in transformers-0.6.3.0 · Control.Monad.Trans.Reader

valuemapReaderT :: (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

valuewithReaderT

:: (r' -> r)
The function to modify the environment.
-> ReaderT r m a
Computation to run in the modified environment.
-> ReaderT r' m a

Source#

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

runReaderT (withReaderT f m) = runReaderT m . f

Reader operations

valueask :: Monad m => ReaderT r m r

Source#

Fetch the value of the environment.

valuelocal

:: (r -> r)
The function to modify the environment.
-> ReaderT r m a
Computation to run in the modified environment.
-> ReaderT r m a

Source#

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

runReaderT (local f m) = runReaderT m . f

valueasks

:: Monad m
=> (r -> a)
The selector function to apply to the environment.
-> ReaderT r m a

Source#

Retrieve a function of the current environment.

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

Lifting other operations

valueliftCallCC :: CallCC m a b -> CallCC (ReaderT r m) a b

Source#

Lift a callCC operation to the new monad.

valueliftCatch :: Catch e m a -> Catch e (ReaderT r m) a

Source#

Lift a catchE operation to the new monad.