# 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 |
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− | zygohistomorphic_prepromorphism :: Functor f => Algebra f b -> GAlgebra f (Cofree f) a -> (f :~> f) -> FixF f -> a |
+ | zygoHistoPrepro |

− | zygohistomorphic_prepromorphism f = g_prepro (distZygoT (liftAlgebra f) (distHisto id)) |
+ | :: (Unfoldable t, Foldable t) |

+ | => (Base t b -> b) |
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+ | -> (forall c. Base t c -> Base t c) |
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+ | -> (Base t (EnvT b (Stream (Base t)) a) -> a) |
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+ | -> t |
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+ | -> a |
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+ | zygoHistoPrepro f g t = gprepro (distZygoT f distHisto) g t |
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-- unless you want a generalized zygomorphism. |
-- unless you want a generalized zygomorphism. |
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</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.