Control.Monad.Free

data Free f a

The free monad for a type constructor f.

Implemented in the spirit of Reflection without Remorse, the free monad is represented using a sequential data structure in order to overcome the quadratic complexity of left-associated binds and traversal through the free monad structure.

Instances

• (Functor f, Eq1 f, Eq a) => Eq (Free f a)
• (Functor f, Eq1 f) => Eq1 (Free f)
• (Functor f, Ord1 f, Ord a) => Ord (Free f a)
• (Functor f, Ord1 f, Ord a) => Ord1 (Free f)
• Functor (Free f)
• Bind (Free f)
• Applicative (Free f)
• Apply (Free f)
• Monad (Free f)
• MonadTrans Free
• MonadRec (Free f)
• (Functor f, Foldable f) => Foldable (Free f)
• (Traversable f) => Traversable (Free f)

liftF :: forall f. f ~> (Free f)

Lift an impure value described by the generating type constructor f into the free monad.

suspendF :: forall f. Applicative f => (Free f) ~> (Free f)

Suspend a value given the applicative functor f into the free monad.

hoistFree :: forall g f. (f ~> g) -> (Free f) ~> (Free g)

Use a natural transformation to change the generating type constructor of a free monad.

foldFree :: forall m f. MonadRec m => (f ~> m) -> (Free f) ~> m

Run a free monad with a natural transformation from the type constructor f to the tail-recursive monad m. See the MonadRec type class for more details.

substFree :: forall g f. (f ~> (Free g)) -> (Free f) ~> (Free g)

Like foldFree, but for folding into some other Free monad without the overhead that MonadRec incurs.

runFree :: forall a f. Functor f => (f (Free f a) -> Free f a) -> Free f a -> a

Run a free monad with a function that unwraps a single layer of the functor f at a time.

runFreeM :: forall a m f. Functor f => MonadRec m => (f (Free f a) -> m (Free f a)) -> Free f a -> m a

Run a free monad with a function mapping a functor f to a tail-recursive monad m. See the MonadRec type class for more details.

resume :: forall a f. Functor f => Free f a -> Either (f (Free f a)) a

Unwraps a single layer of the functor f.