# Zygohistomorphic prepromorphisms

### From HaskellWiki

(Difference between revisions)

m (Error in the use of « à la » (« la » is more or less the French for « the »)) |
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import Control.Functor.Extras | import Control.Functor.Extras | ||

− | + | zygoHistoPrepro | |

− | + | :: (Unfoldable t, Foldable t) | |

+ | => (Base t b -> b) | ||

+ | -> (forall c. Base t c -> Base t c) | ||

+ | -> (Base t (EnvT b (Stream (Base t)) a) -> a) | ||

+ | -> t | ||

+ | -> a | ||

+ | zygoHistoPrepro f g t = gprepro (distZygoT f distHisto) g t | ||

-- unless you want a generalized zygomorphism. | -- unless you want a generalized zygomorphism. | ||

</pre> | </pre> |

## Latest revision as of 21:03, 20 February 2011

Used when you really need both semi-mutual recursion and history and to repeatedly apply a natural transformation as you get deeper into the functor. Zygo implements semi-mutual recursion like a zygomorphism. Para gives you access to your result à la paramorphism.

import Control.Morphism.Zygo import Control.Morphism.Prepro import Control.Morphism.Histo import Control.Functor.Algebra import Control.Functor.Extras zygoHistoPrepro :: (Unfoldable t, Foldable t) => (Base t b -> b) -> (forall c. Base t c -> Base t c) -> (Base t (EnvT b (Stream (Base t)) a) -> a) -> t -> a zygoHistoPrepro f g t = gprepro (distZygoT f distHisto) g t -- unless you want a generalized zygomorphism.