Control.Monad.Levels.Writer

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))

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