Moduletransformers-0.6.1.1Haskell2010

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.

2 types
11 values

Packagetransformers-0.6.1.1
Exports13
LanguageHaskell2010
LicenceBSD-3-Clause
SourceReader.hs

The Reader monad

5 declarations

typetype Reader r = ReaderT r Identity

The parameterizable reader monad.

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

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

valuerunReader

:: 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.)

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

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

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

runReader (withReader f m) = runReader m . f

The ReaderT monad transformer

3 declarations

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

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

Constructors

ReaderT
- runReaderT :: r -> m a

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

Generic1 (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadTrans (ReaderT r)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
Monad m => Monad (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
Functor m => Functor (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadFix m => MonadFix (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadFail m => MonadFail (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
Applicative m => Applicative (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
Alternative m => Alternative (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadPlus m => MonadPlus (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadIO m => MonadIO (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadZip m => MonadZip (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
Contravariant m => Contravariant (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
Generic (ReaderT r m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
type Rep (ReaderT r m a) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (r -> m a))))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
type Rep1 (ReaderT r m) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (FUN 'Many r :.: Rec1 m)))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader

valuemapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n b

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

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

runReaderT (withReaderT f m) = runReaderT m . f

Reader operations

3 declarations

valueask :: Monad m => ReaderT r m r

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

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

Retrieve a function of the current environment.

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

Lifting other operations

2 declarations

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

Lift a callCC operation to the new monad.

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

Lift a catchE operation to the new monad.