Control.Monad.Levels.Writer.Lazy

Documentation

writer :: forall w m a. HasWriter w m => (a, w) -> m a

Embed a simple writer action.

tell :: HasWriter w m => w -> m ()

An action that produces the output w.

type HasWriter w m = (Monoid w, SatisfyConstraint (IsWriter w) m)

listen :: forall w m a. CanListen w m a => m a -> m (a, w)

Execute the action m and add its output to the value of the computation.

type CanListen w m a = SatisfyConstraintF (IsWriter w) m a (ListenFn w)

type ListenFn w = Func MonadicValue (MkVarFnFrom (MonadicTuple w))

pass :: forall w m a. CanPass w m a => m (a, w -> w) -> m a

Execute the action m (which returns a value and a function) and returns the value, applying the function to the output.

type CanPass w m a = SatisfyConstraintF (IsWriter w) m a (PassFn w)

type PassFn w = MkVarFn (MonadicTuple (Endo w))

newtype WriterT w m a :: * -> (* -> *) -> * -> *

A writer monad parameterized by:

• w - the output to accumulate.
• m - The inner monad.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

Constructors

WriterT

Fields

runWriterT :: m (a, w)

Instances

(Monoid w, MonadTower m) => IsWriter w (WriterT w m)

Monoid w => MonadTrans (WriterT w)

(Monoid w, Alternative m) => Alternative (WriterT w m)

(Monoid w, Monad m) => Monad (WriterT w m)

Functor m => Functor (WriterT w m)

(Monoid w, MonadFix m) => MonadFix (WriterT w m)

(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)

(Monoid w, Applicative m) => Applicative (WriterT w m)

Foldable f => Foldable (WriterT w f)

Traversable f => Traversable (WriterT w f)

(Eq w, Eq1 m) => Eq1 (WriterT w m)

(Ord w, Ord1 m) => Ord1 (WriterT w m)

(Read w, Read1 m) => Read1 (WriterT w m)

(Show w, Show1 m) => Show1 (WriterT w m)

(Monoid w, MonadIO m) => MonadIO (WriterT w m)

(Monoid w, MonadTower m) => MonadLevel_ (WriterT w m)

(Monoid w, MonadTower m) => MonadTower_ (WriterT w m)

(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)

(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)

(Read w, Read1 m, Read a) => Read (WriterT w m a)

(Show w, Show1 m, Show a) => Show (WriterT w m a)

type LowerMonad (WriterT w m) = m

type WithLower_ (WriterT w m) = AddIG

type AllowOtherValues (WriterT w m) = True

type DefaultAllowConstraints (WriterT w m) = True

type BaseMonad (WriterT w m) = BaseMonad m

type InnerValue (WriterT w m) a = (a, w)

class (Monoid w, MonadTower m) => IsWriter w m

The minimal definition needed for a monad providing a writer environment.

Instances

(Monoid w, MonadTower m) => IsWriter w (WriterT w m)

(Monoid w, MonadTower m) => IsWriter w (WriterT w m)

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

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

Monoid w => ValidConstraint (IsWriter w)

type ConstraintSatisfied (IsWriter w) m