Andy Melnikov (nponeccop) wrote,
Andy Melnikov

Хуеморфизмы to the rescue

Я тут изобрёл ужасно генерическую функцию reprocess2, которая кмк решит все проблемы:
{-# LANGUAGE DeriveFunctor #-}
import Data.Functor.Fixedpoint
import Control.Applicative

type Rewrite a = a -> Maybe a

data TermF a = Cons String a | Nil deriving (Functor, Show)

type Term = Fix TermF

-- reprocess2 :: Functor f => (f b -> Fix f -> b) -> Fix f -> b
reprocess2 :: (TermF (Maybe Term) -> Rewrite Term) -> Rewrite Term
reprocess2 f t = f (reprocess2 f <$> unFix t) t
Оказалось неудобно, так что я сделал вариант с "соединителем" j. В нашем случае это будет (,):
reprocess3 j f = self
	where self = f . fmap (\x -> j x (self x)) . unFix
Всё равно, кажется, есть пространство для улучшения.

Вот текущая версия реврайта:
fromList = foldr (\x y -> Fix $ Cons x y) (Fix Nil)

x = fromList ["foo", "bar", "foo", "baz"] 

clientRewrite :: Rewrite Term
clientRewrite (Fix (Cons "foo" a)) = Just $ Fix $ Cons "bar" a
clientRewrite (Fix (Cons "bar" a)) = Just $ Fix $ Cons "quux" a
clientRewrite _ = Nothing

rewriteMany x = clientRewrite x >>= rewriteAfterChange where
	rewriteAfterChange x = (clientRewrite x >>= rewriteAfterChange) <|> return x

process Nil = rewriteMany (Fix Nil)
process (Cons a (oldB, newB)) = rewriteMany f <|> fmap (const f) newB where
	f = Fix (Cons a (fromMaybe oldB newB))
result = reprocess3 (,) process x

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