Control.Monad.Levels.Reader

Provides a shared environment for reading values and executing sub-computations in a modified environment.

Documentation

ask :: HasReader r m => m r

Obtain the reader environment.

asks :: HasReader r m => (r -> a) -> m a

Retrieve a function of the current environment. An alias of reader.

reader :: forall r m a. HasReader r m => (r -> a) -> m a

Retrieve a function of the current environment.

local :: forall r m a. HasReader r m => (r -> r) -> m a -> m a

Execute a computation in a modified environment.

newtype ReaderT r m a :: * -> (* -> *) -> * -> *

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

MonadTower m => IsReader r (ReaderT r m)

MonadTrans (ReaderT r)

MonadLevel m => ConstraintPassThrough (IsTransformer (ReaderT r)) (ListT m) True

MonadLevel m => ConstraintPassThrough (IsTransformer (ReaderT r)) (ContT c m) True

Alternative m => Alternative (ReaderT r m)

Monad m => Monad (ReaderT r m)

Functor m => Functor (ReaderT r m)

MonadFix m => MonadFix (ReaderT r m)

MonadPlus m => MonadPlus (ReaderT r m)

Applicative m => Applicative (ReaderT r m)

MonadIO m => MonadIO (ReaderT r m)

MonadTower m => MonadLevel_ (ReaderT r m)

MonadTower m => MonadTower_ (ReaderT r m)

type LowerMonad (ReaderT r m) = m

type WithLower_ (ReaderT r m) = AddIdent

type AllowOtherValues (ReaderT r m) = True

type DefaultAllowConstraints (ReaderT r m) = True

type BaseMonad (ReaderT r m) = BaseMonad m

type InnerValue (ReaderT r m) a = a

type HasReader r m = SatisfyConstraint (IsReader r) m

A monad stack containing a Reader environment of type r.

class MonadTower m => IsReader r m

The minimal definitions needed by a monad providing a Reader environment.

Instances

IsReader r ((->) r)

MonadTower m => IsReader r (ReaderT r m)

(MonadTower m, Monoid w) => IsReader r (RWST r w s m)

(MonadTower m, Monoid w) => IsReader r (RWST r w s m)

ValidConstraint (IsReader r)

MonadTower m => ConstraintPassThrough (IsReader r) (ListT m) True

MonadTower m => ConstraintPassThrough (IsReader r) (ContT c m) True

type ConstraintSatisfied (IsReader r) m