Modulefused-effects-1.1.2.7Haskell2010

Control.Effect.Writer

An effect allowing writes to an accumulated quantity alongside a computed value. A Writer w effect keeps track of a monoidal datum of type w and strictly appends to that monoidal value with the tell effect. Writes to that value can be detected and intercepted with the listen and censor effects.

Predefined carriers:

Control.Carrier.Writer.Church
Control.Carrier.Writer.Strict. (A lazy carrier is not provided due to the inherent space leaks associated with lazy writer monads.)
Control.Monad.Trans.RWS.CPS
Control.Monad.Trans.RWS.Lazy
Control.Monad.Trans.RWS.Strict
Control.Monad.Trans.Writer.CPS
Control.Monad.Trans.Writer.Lazy
Control.Monad.Trans.Writer.Strict
If Writer w is the last effect in a stack, it can be interpreted to a tuple (w, a) given some result type a and the presence of a Monoid instance for w.

2 types
1 class
5 values

Packagefused-effects-1.1.2.7
Exports8
LanguageHaskell2010
LicenceBSD-3-Clause
SourceWriter.hs

Writer effect

5 declarations

datadata Writer w (m :: Type -> Type) k where

Constructors

Tell :: w -> Writer w m ()
Listen :: m a -> Writer w m (w, a)
Censor :: (w -> w) -> m a -> Writer w m k

Instances9Algebra, …

Monoid w => Algebra (Writer w) (Tuple2 w)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Writer.Church
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Monoid w, Algebra sig m) => Algebra (Writer w :+: sig) (WriterC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Writer.Strict
(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)Defined in fused-effects-1.1.2.7 · Control.Algebra

valuetell :: Has (Writer w) sig m => w -> m ()

Write a value to the log.

runWriter (tell w >> m) = first (mappend w) <$> runWriter m

valuelisten :: Has (Writer w) sig m => m a -> m (w, a)

Run a computation, returning the pair of its output and its result.

runWriter (listen m) = fmap (fst &&& id) (runWriter m)

valuelistens :: Has (Writer w) sig m => (w -> b) -> m a -> m (b, a)

Run a computation, applying a function to its output and returning the pair of the modified output and its result.

listens f m = fmap (first f) (listen m)

valuecensor :: Has (Writer w) sig m => (w -> w) -> m a -> m a

Run a computation, modifying its output with the passed function.

runWriter (censor f m) = fmap (first f) (runWriter m)

Re-exports

3 declarations

classclass Monad m => Algebra (sig :: (Type -> Type) -> Type -> Type) (m :: Type -> Type) | m -> sig where

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

Instances53Algebra, …

Algebra Choose NonEmptyDefined in fused-effects-1.1.2.7 · Control.Algebra
Algebra Empty MaybeDefined in fused-effects-1.1.2.7 · Control.Algebra
Algebra NonDet []Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra sig (Choosing m)Defined in fused-effects-1.1.2.7 · Control.Effect.Choose
Algebra sig m => Algebra sig (Ap m)Defined in fused-effects-1.1.2.7 · Control.Algebra
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)
```
Algebra sig m => Algebra sig (Alt m)Defined in fused-effects-1.1.2.7 · Control.Algebra
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])
```
Algebra sig m => Algebra sig (IdentityT m)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra (Lift Identity) IdentityDefined in fused-effects-1.1.2.7 · Control.Algebra
Algebra (Lift IO) IODefined in fused-effects-1.1.2.7 · Control.Algebra
Algebra (Error e) (Either e)Defined in fused-effects-1.1.2.7 · Control.Algebra
Monad m => Algebra (Lift m) (LiftC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Lift
Monoid w => Algebra (Writer w) (Tuple2 w)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra (Reader r) ((->) r)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra (Choose :+: sig) (ChooseC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Choose.Church
Algebra sig m => Algebra (Cull :+: (NonDet :+: sig)) (CullC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Cull.Church
Algebra sig m => Algebra (Cut :+: (NonDet :+: sig)) (CutC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Cut.Church
Algebra sig m => Algebra (Empty :+: sig) (EmptyC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Empty.Church
Algebra sig m => Algebra (Empty :+: sig) (EmptyC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Empty.Maybe
Algebra sig m => Algebra (Empty :+: sig) (MaybeT m)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra (Fail :+: sig) (FailC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Fail.Either
Algebra sig m => Algebra (Fresh :+: sig) (FreshC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Fresh.Church
Algebra sig m => Algebra (Fresh :+: sig) (FreshC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Fresh.Strict
Algebra sig m => Algebra (NonDet :+: sig) (NonDetC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.NonDet.Church
Algebra sig m => Algebra (Trace :+: sig) (TraceC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Trace.Ignoring
Algebra sig m => Algebra (Trace :+: sig) (TraceC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Trace.Returning
(MonadIO m, Algebra sig m) => Algebra (Trace :+: sig) (TraceC m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Trace.Printing
Algebra sig m => Algebra (Error e :+: sig) (ErrorC e m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Error.Church
Algebra sig m => Algebra (Error e :+: sig) (ErrorC e m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Error.Either
Algebra sig m => Algebra (Error e :+: sig) (ExceptT e m)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra (Reader r :+: sig) (ReaderC r m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Reader
Algebra sig m => Algebra (Reader r :+: sig) (ReaderT r m)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra (State s :+: sig) (StateC s m)Defined in fused-effects-1.1.2.7 · Control.Carrier.State.Church
Algebra sig m => Algebra (State s :+: sig) (StateC s m)Defined in fused-effects-1.1.2.7 · Control.Carrier.State.Lazy
Algebra sig m => Algebra (State s :+: sig) (StateC s m)Defined in fused-effects-1.1.2.7 · Control.Carrier.State.Strict
Algebra sig m => Algebra (State s :+: sig) (StateT s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra (State s :+: sig) (StateT s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
Algebra sig m => Algebra (Throw e :+: sig) (ThrowC e m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Throw.Either
(Algebra sig m, Monoid w) => Algebra (Accum w :+: sig) (AccumC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Accum.Church
(Algebra sig m, Monoid w) => Algebra (Accum w :+: sig) (AccumC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Accum.Strict
(Algebra sig m, Monoid w) => Algebra (Accum w :+: sig) (AccumT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Writer.Church
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (WriterT w m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Semigroup w, MonadIO m) => Algebra (Accum w :+: sig) (AccumC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Accum.IORef
(Monoid w, Algebra sig m) => Algebra (Writer w :+: sig) (WriterC w m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Writer.Strict
(MonadIO m, Algebra sig m) => Algebra (State s :+: sig) (StateC s m)Defined in fused-effects-1.1.2.7 · Control.Carrier.State.IORef
(Reifies s (Interpreter eff m), Algebra sig m) => Algebra (eff :+: sig) (InterpretC s eff m)Defined in fused-effects-1.1.2.7 · Control.Carrier.Interpret
Algebra (eff :+: sig) (sub m) => Algebra (Labelled label eff :+: sig) (Labelled label sub m)Defined in fused-effects-1.1.2.7 · Control.Effect.Labelled
(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(Algebra sig m, Monoid w) => Algebra (Reader r :+: (Writer w :+: (State s :+: sig))) (RWST r w s m)Defined in fused-effects-1.1.2.7 · Control.Algebra
(LabelledMember label sub sig, Algebra sig m) => Algebra (sub :+: sig) (UnderLabel label sub m)Defined in fused-effects-1.1.2.7 · Control.Effect.Labelled

type

type Has (eff :: (Type -> Type) -> Type -> Type) (sig :: (Type -> Type) -> Type -> Type) (m :: Type -> Type) = (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:

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

valuerun :: Identity a -> a

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