Safe Haskell | Safe |
---|---|

Language | Haskell2010 |

Recursion schemes, also known as scary named folds...

## Synopsis

- cata :: Functor f => (f a -> a) -> Mu f -> a
- para :: Functor f => (f (Mu f, a) -> a) -> Mu f -> a
- para' :: Functor f => (Mu f -> f a -> a) -> Mu f -> a
- paraList :: (Functor f, Foldable f) => (Mu f -> [a] -> a) -> Mu f -> a
- ana :: Functor f => (a -> f a) -> a -> Mu f
- apo :: Functor f => (a -> f (Either (Mu f) a)) -> a -> Mu f
- hylo :: Functor f => (f a -> a) -> (b -> f b) -> b -> a
- zygo_ :: Functor f => (f b -> b) -> (f (b, a) -> a) -> Mu f -> a
- zygo :: Functor f => (f b -> b) -> (f (b, a) -> a) -> Mu f -> (b, a)
- histo :: Functor f => (f (Attr f a) -> a) -> Mu f -> a
- futu :: Functor f => (a -> f (CoAttr f a)) -> a -> Mu f
- cataM :: (Monad m, Traversable f) => (f a -> m a) -> Mu f -> m a
- cataM_ :: (Monad m, Traversable f) => (f a -> m a) -> Mu f -> m ()
- paraM :: (Monad m, Traversable f) => (f (Mu f, a) -> m a) -> Mu f -> m a
- paraM_ :: (Monad m, Traversable f) => (f (Mu f, a) -> m a) -> Mu f -> m ()
- paraM' :: (Monad m, Traversable f) => (Mu f -> f a -> m a) -> Mu f -> m a

# Classic ana/cata/para/hylo-morphisms

cata :: Functor f => (f a -> a) -> Mu f -> a Source #

A *catamorphism* is the generalization of right fold from lists to trees.

para :: Functor f => (f (Mu f, a) -> a) -> Mu f -> a Source #

A *paramorphism* is a more general version of the catamorphism.

para' :: Functor f => (Mu f -> f a -> a) -> Mu f -> a Source #

Another version of `para`

(a bit less natural in some sense).

paraList :: (Functor f, Foldable f) => (Mu f -> [a] -> a) -> Mu f -> a Source #

A list version of `para`

(compare with Uniplate)

ana :: Functor f => (a -> f a) -> a -> Mu f Source #

An *anamorphism* is simply an unfold. Probably not very useful in this context.

apo :: Functor f => (a -> f (Either (Mu f) a)) -> a -> Mu f Source #

An *apomorphism* is a generalization of the anamorphism.

hylo :: Functor f => (f a -> a) -> (b -> f b) -> b -> a Source #

A *hylomorphism* is the composition of a catamorphism and an anamorphism.

# Zygomorphisms

zygo_ :: Functor f => (f b -> b) -> (f (b, a) -> a) -> Mu f -> a Source #

A *zygomorphism* is a basically a catamorphism inside another catamorphism.
It could be implemented (somewhat wastefully) by first annotating each subtree
with the corresponding values of the inner catamorphism (`synthCata`

), then running
a paramorphims on the annotated tree:

zygo_ g h == para u . synthCata g where u = h . fmap (first attribute) . unAnn first f (x,y) = (f x, y)

# Futu- and histomorphisms

histo :: Functor f => (f (Attr f a) -> a) -> Mu f -> a Source #

Histomorphism. This is a kind of glorified cata/paramorphism, after all:

cata f == histo (f . fmap attribute) para f == histo (f . fmap (\t -> (forget t, attribute t)))

futu :: Functor f => (a -> f (CoAttr f a)) -> a -> Mu f Source #

Futumorphism. This is a more interesting unfold.