Control.Effect.Reader

An effect providing access to an immutable (but locally-modifiable) context value.

This effect is similar to the traditional MonadReader typeclass, though it allows the presence of multiple Reader t effects.

Predefined carriers:

• Control.Carrier.Reader.
• Control.Monad.Trans.Reader.
• Control.Monad.Trans.RWS.Lazy
• Control.Monad.Trans.RWS.Strict
• If Reader r is the last effect in a stack, it can be interpreted directly to (-> r) (a function taking an r).

Since: 0.1.0.0

Reader effect

data Reader r m k where

Since: 0.1.0.0

Constructors

Ask :: Reader r m r

Local :: (r -> r) -> m a -> Reader r m a

Instances

Algebra (Reader r) ((->) r)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n ((->) r) -> Reader r n a -> ctx () -> r -> ctx a

Algebra sig m => Algebra (Reader r :+: sig) (ReaderC r m)

Defined in Control.Carrier.Reader

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ReaderC r m) -> (Reader r :+: sig) n a -> ctx () -> ReaderC r m (ctx a)

Algebra sig m => Algebra (Reader r :+: sig) (ReaderT r m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ReaderT r m) -> (Reader r :+: sig) n a -> ctx () -> ReaderT r m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (RWST r w s m) -> (Reader r :+: (Writer w :+: (State s :+: sig))) n a -> ctx () -> RWST r w s m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (RWST r w s m) -> (Reader r :+: (Writer w :+: (State s :+: sig))) n a -> ctx () -> RWST r w s m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (RWST r w s m) -> (Reader r :+: (Writer w :+: (State s :+: sig))) n a -> ctx () -> RWST r w s m (ctx a)

ask :: Has (Reader r) sig m => m r

Retrieve the environment value.

runReader a (ask >>= k) = runReader a (k a)

Since: 0.1.0.0

asks :: Has (Reader r) sig m => (r -> a) -> m a

Project a function out of the current environment value.

asks f = fmap f ask

Since: 0.1.0.0

local :: Has (Reader r) sig m => (r -> r) -> m a -> m a

Run a computation with an environment value locally modified by the passed function.

runReader a (local f m) = runReader (f a) m

Since: 0.1.0.0

Re-exports

class Monad m => Algebra sig m | m -> sig

The class of carriers (results) for algebras (effect handlers) over signatures (effects), whose actions are given by the alg method.

Since: 1.0.0.0

Minimal complete definition

alg

Instances

Algebra Choose NonEmpty

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n NonEmpty -> Choose n a -> ctx () -> NonEmpty (ctx a)

Algebra Empty Maybe

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n Maybe -> Empty n a -> ctx () -> Maybe (ctx a)

Algebra NonDet []

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n [] -> NonDet n a -> ctx () -> [ctx a]

Algebra sig m => Algebra sig (Choosing m)

Defined in Control.Effect.Choose

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (Choosing m) -> sig n a -> ctx () -> Choosing m (ctx a)

Algebra sig m => Algebra sig (Ap m)

This instance permits effectful actions to be lifted into the Ap monad given a monoidal return type, which can provide clarity when chaining calls to mappend.

mappend <$> act1 <*> (mappend <$> act2 <*> act3)

is equivalent to

getAp (act1 <> act2 <> act3)

Since: 1.0.1.0

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (Ap m) -> sig n a -> ctx () -> Ap m (ctx a)

Algebra sig m => Algebra sig (Alt m)

This instance permits effectful actions to be lifted into the Alt monad, which eases the invocation of repeated alternation with <|>:

a <|> b <|> c <|> d

is equivalent to

getAlt (mconcat [a, b, c, d])

Since: 1.0.1.0

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (Alt m) -> sig n a -> ctx () -> Alt m (ctx a)

Algebra sig m => Algebra sig (IdentityT m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (IdentityT m) -> sig n a -> ctx () -> IdentityT m (ctx a)

Algebra (Lift Identity) Identity

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n Identity -> Lift Identity n a -> ctx () -> Identity (ctx a)

Algebra (Lift IO) IO

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n IO -> Lift IO n a -> ctx () -> IO (ctx a)

Algebra (Error e) (Either e)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (Either e) -> Error e n a -> ctx () -> Either e (ctx a)

Monad m => Algebra (Lift m) (LiftC m)

Defined in Control.Carrier.Lift

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (LiftC m) -> Lift m n a -> ctx () -> LiftC m (ctx a)

Monoid w => Algebra (Writer w) ((,) w)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n ((,) w) -> Writer w n a -> ctx () -> (w, ctx a)

Algebra (Reader r) ((->) r)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n ((->) r) -> Reader r n a -> ctx () -> r -> ctx a

Algebra sig m => Algebra (Choose :+: sig) (ChooseC m)

Defined in Control.Carrier.Choose.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ChooseC m) -> (Choose :+: sig) n a -> ctx () -> ChooseC m (ctx a)

Algebra sig m => Algebra (Cull :+: (NonDet :+: sig)) (CullC m)

Defined in Control.Carrier.Cull.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (CullC m) -> (Cull :+: (NonDet :+: sig)) n a -> ctx () -> CullC m (ctx a)

Algebra sig m => Algebra (Cut :+: (NonDet :+: sig)) (CutC m)

Defined in Control.Carrier.Cut.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (CutC m) -> (Cut :+: (NonDet :+: sig)) n a -> ctx () -> CutC m (ctx a)

Algebra sig m => Algebra (Empty :+: sig) (EmptyC m)

Defined in Control.Carrier.Empty.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (EmptyC m) -> (Empty :+: sig) n a -> ctx () -> EmptyC m (ctx a)

Algebra sig m => Algebra (Empty :+: sig) (EmptyC m)

Defined in Control.Carrier.Empty.Maybe

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (EmptyC m) -> (Empty :+: sig) n a -> ctx () -> EmptyC m (ctx a)

Algebra sig m => Algebra (Empty :+: sig) (MaybeT m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (MaybeT m) -> (Empty :+: sig) n a -> ctx () -> MaybeT m (ctx a)

Algebra sig m => Algebra (Fail :+: sig) (FailC m)

Defined in Control.Carrier.Fail.Either

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (FailC m) -> (Fail :+: sig) n a -> ctx () -> FailC m (ctx a)

Algebra sig m => Algebra (Fresh :+: sig) (FreshC m)

Defined in Control.Carrier.Fresh.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (FreshC m) -> (Fresh :+: sig) n a -> ctx () -> FreshC m (ctx a)

Algebra sig m => Algebra (Fresh :+: sig) (FreshC m)

Defined in Control.Carrier.Fresh.Strict

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (FreshC m) -> (Fresh :+: sig) n a -> ctx () -> FreshC m (ctx a)

Algebra sig m => Algebra (NonDet :+: sig) (NonDetC m)

Defined in Control.Carrier.NonDet.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (NonDetC m) -> (NonDet :+: sig) n a -> ctx () -> NonDetC m (ctx a)

Algebra sig m => Algebra (Trace :+: sig) (TraceC m)

Defined in Control.Carrier.Trace.Ignoring

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (TraceC m) -> (Trace :+: sig) n a -> ctx () -> TraceC m (ctx a)

(MonadIO m, Algebra sig m) => Algebra (Trace :+: sig) (TraceC m)

Defined in Control.Carrier.Trace.Printing

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (TraceC m) -> (Trace :+: sig) n a -> ctx () -> TraceC m (ctx a)

Algebra sig m => Algebra (Trace :+: sig) (TraceC m)

Defined in Control.Carrier.Trace.Returning

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (TraceC m) -> (Trace :+: sig) n a -> ctx () -> TraceC m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Accum w :+: sig) (AccumC w m)

Defined in Control.Carrier.Accum.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (AccumC w m) -> (Accum w :+: sig) n a -> ctx () -> AccumC w m (ctx a)

(Algebra sig m, Semigroup w, MonadIO m) => Algebra (Accum w :+: sig) (AccumC w m)

Defined in Control.Carrier.Accum.IORef

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (AccumC w m) -> (Accum w :+: sig) n a -> ctx () -> AccumC w m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Accum w :+: sig) (AccumC w m)

Defined in Control.Carrier.Accum.Strict

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (AccumC w m) -> (Accum w :+: sig) n a -> ctx () -> AccumC w m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Accum w :+: sig) (AccumT w m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (AccumT w m) -> (Accum w :+: sig) n a -> ctx () -> AccumT w m (ctx a)

Algebra sig m => Algebra (Error e :+: sig) (ErrorC e m)

Defined in Control.Carrier.Error.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ErrorC e m) -> (Error e :+: sig) n a -> ctx () -> ErrorC e m (ctx a)

Algebra sig m => Algebra (Error e :+: sig) (ErrorC e m)

Defined in Control.Carrier.Error.Either

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ErrorC e m) -> (Error e :+: sig) n a -> ctx () -> ErrorC e m (ctx a)

Algebra sig m => Algebra (Error e :+: sig) (ExceptT e m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ExceptT e m) -> (Error e :+: sig) n a -> ctx () -> ExceptT e m (ctx a)

Algebra sig m => Algebra (Reader r :+: sig) (ReaderC r m)

Defined in Control.Carrier.Reader

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ReaderC r m) -> (Reader r :+: sig) n a -> ctx () -> ReaderC r m (ctx a)

Algebra sig m => Algebra (Reader r :+: sig) (ReaderT r m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ReaderT r m) -> (Reader r :+: sig) n a -> ctx () -> ReaderT r m (ctx a)

Algebra sig m => Algebra (State s :+: sig) (StateC s m)

Defined in Control.Carrier.State.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (StateC s m) -> (State s :+: sig) n a -> ctx () -> StateC s m (ctx a)

(MonadIO m, Algebra sig m) => Algebra (State s :+: sig) (StateC s m)

Defined in Control.Carrier.State.IORef

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (StateC s m) -> (State s :+: sig) n a -> ctx () -> StateC s m (ctx a)

Algebra sig m => Algebra (State s :+: sig) (StateC s m)

Defined in Control.Carrier.State.Lazy

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (StateC s m) -> (State s :+: sig) n a -> ctx () -> StateC s m (ctx a)

Algebra sig m => Algebra (State s :+: sig) (StateC s m)

Defined in Control.Carrier.State.Strict

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (StateC s m) -> (State s :+: sig) n a -> ctx () -> StateC s m (ctx a)

Algebra sig m => Algebra (State s :+: sig) (StateT s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (StateT s m) -> (State s :+: sig) n a -> ctx () -> StateT s m (ctx a)

Algebra sig m => Algebra (State s :+: sig) (StateT s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (StateT s m) -> (State s :+: sig) n a -> ctx () -> StateT s m (ctx a)

Algebra sig m => Algebra (Throw e :+: sig) (ThrowC e m)

Defined in Control.Carrier.Throw.Either

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (ThrowC e m) -> (Throw e :+: sig) n a -> ctx () -> ThrowC e m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterC w m)

Defined in Control.Carrier.Writer.Church

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (WriterC w m) -> (Writer w :+: sig) n a -> ctx () -> WriterC w m (ctx a)

(Monoid w, Algebra sig m) => Algebra (Writer w :+: sig) (WriterC w m)

Defined in Control.Carrier.Writer.Strict

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (WriterC w m) -> (Writer w :+: sig) n a -> ctx () -> WriterC w m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (WriterT w m) -> (Writer w :+: sig) n a -> ctx () -> WriterT w m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (WriterT w m) -> (Writer w :+: sig) n a -> ctx () -> WriterT w m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (WriterT w m) -> (Writer w :+: sig) n a -> ctx () -> WriterT w m (ctx a)

(Reifies s (Interpreter eff m), Algebra sig m) => Algebra (eff :+: sig) (InterpretC s eff m)

Defined in Control.Carrier.Interpret

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (InterpretC s eff m) -> (eff :+: sig) n a -> ctx () -> InterpretC s eff m (ctx a)

Algebra (eff :+: sig) (sub m) => Algebra (Labelled label eff :+: sig) (Labelled label sub m)

Defined in Control.Effect.Labelled

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (Labelled label sub m) -> (Labelled label eff :+: sig) n a -> ctx () -> Labelled label sub m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (RWST r w s m) -> (Reader r :+: (Writer w :+: (State s :+: sig))) n a -> ctx () -> RWST r w s m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (RWST r w s m) -> (Reader r :+: (Writer w :+: (State s :+: sig))) n a -> ctx () -> RWST r w s m (ctx a)

(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)

Defined in Control.Algebra

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (RWST r w s m) -> (Reader r :+: (Writer w :+: (State s :+: sig))) n a -> ctx () -> RWST r w s m (ctx a)

(LabelledMember label sub sig, Algebra sig m) => Algebra (sub :+: sig) (UnderLabel label sub m)

Defined in Control.Effect.Labelled

Methods

alg :: forall ctx (n :: Type -> Type) a. Functor ctx => Handler ctx n (UnderLabel label sub m) -> (sub :+: sig) n a -> ctx () -> UnderLabel label sub m (ctx a)

type Has eff sig m = (Members eff sig, Algebra sig m)

m is a carrier for sig containing eff.

Note that if eff is a sum, it will be decomposed into multiple Member constraints. While this technically allows one to combine multiple unrelated effects into a single Has constraint, doing so has two significant drawbacks:

1. Due to a problem with recursive type families, this can lead to significantly slower compiles.
2. It defeats ghc’s warnings for redundant constraints, and thus can lead to a proliferation of redundant constraints as code is changed.

Since: 1.0.0.0

run :: Identity a -> a

Run an action exhausted of effects to produce its final result value.

Since: 1.0.0.0