Modulererebase-1.21.2Haskell2010

Control.Monad.Trans.Reader

2 types
11 values

Packagererebase-1.21.2
Exports13
LanguageHaskell2010
LicenceMIT
SourceReader.hs

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

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

Lift a callCC operation to the new monad.

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
```

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

Transform the value returned by a Reader.

runReader (mapReader f m) = f . runReader m

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

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.)

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

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

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

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

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

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

Instances53Generic1, MonadAccum, MonadError, MonadReader, MonadState, MonadWriter, …

Generic1 (ReaderT r m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
MonadAccum w m => MonadAccum w (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Accum
MonadError e m => MonadError e (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Error.Class
Monad m => MonadReader r (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadState s m => MonadState s (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.State.Class
MonadWriter w m => MonadWriter w (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadSelect r' m => MonadSelect r' (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Select
Provides a read-only environment of type r to the 'strategy' function. However, the 'ranking' function (or more accurately, representation) has no access to r. Put another way, you can influence what values get chosen by changing r, but not how solutions are ranked.
MonadBase b m => MonadBase b (ReaderT r m)Defined in transformers-base-0.4.6 · Control.Monad.Base
(Functor f, MonadFree f m) => MonadFree f (ReaderT e m)Defined in free-5.2 · Control.Monad.Free.Class
MonadTrans (ReaderT r)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Reader
BindTrans (ReaderT e)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Trans
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
MonadCont m => MonadCont (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Cont.Class
MonadCatch m => MonadCatch (ReaderT r m)Defined in exceptions-0.10.9 · Control.Monad.Catch
MonadMask m => MonadMask (ReaderT r m)Defined in exceptions-0.10.9 · Control.Monad.Catch
MonadThrow m => MonadThrow (ReaderT r m)Defined in exceptions-0.10.9 · Control.Monad.Catch
PrimMonad m => PrimMonad (ReaderT r m)Defined in primitive-0.9.1.0 · Control.Monad.Primitive
Distributive g => Distributive (ReaderT e g)Defined in distributive-0.6.2.1 · Data.Distributive
Decidable m => Decidable (ReaderT r m)Defined in contravariant-1.5.5 · Data.Functor.Contravariant.Divisible
Divisible m => Divisible (ReaderT r m)Defined in contravariant-1.5.5 · Data.Functor.Contravariant.Divisible
Alt f => Alt (ReaderT e f)Defined in semigroupoids-6.0.1 · Data.Functor.Alt
Apply m => Apply (ReaderT e m)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Class
Bind m => Bind (ReaderT e m)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Class
Conclude m => Conclude (ReaderT r m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Conclude
Decide m => Decide (ReaderT r m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Decide
Divise m => Divise (ReaderT r m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Divise
Plus f => Plus (ReaderT e f)Defined in semigroupoids-6.0.1 · Data.Functor.Plus
Representable m => Representable (ReaderT e m)Defined in adjunctions-4.4.3 · Data.Functor.Rep
Invariant m => Invariant (ReaderT r m)Defined in invariant-0.6.4 · Data.Functor.Invariant
from the transformers package
Selective f => Selective (ReaderT env f)Defined in selective-0.7.0.1 · Control.Selective
Indexable m => Indexable (ReaderT e m)Defined in keys-3.12.3 · Data.Key
Keyed m => Keyed (ReaderT e m)Defined in keys-3.12.3 · Data.Key
Lookup m => Lookup (ReaderT e m)Defined in keys-3.12.3 · Data.Key
Zip m => Zip (ReaderT e m)Defined in keys-3.12.3 · Data.Key
ZipWithKey m => ZipWithKey (ReaderT e m)Defined in keys-3.12.3 · Data.Key
Pointed m => Pointed (ReaderT r m)Defined in pointed-5.0.4 · Data.Pointed
FunctorWithIndex i m => FunctorWithIndex (e, i) (ReaderT e m)Defined in indexed-traversable-0.1.4 · WithIndex
Adjunction w m => Adjunction (EnvT e w) (ReaderT e m)Defined in adjunctions-4.4.3 · Data.Functor.Adjunction
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
type PrimState (ReaderT r m) = PrimState mDefined in primitive-0.9.1.0 · Control.Monad.Primitive
type Rep (ReaderT e m) = (e, Rep m)Defined in adjunctions-4.4.3 · Data.Functor.Rep
type Key (ReaderT e m) = (e, Key m)Defined in keys-3.12.3 · Data.Key

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

Lift a catchE operation to the new monad.