如何实现一个变体，以便它可以建立在返回类型的任何值之上(而不仅仅是大小写)

Question

我把变质作用定义如下：

-- Fixed point of a Functor 
newtype Fix f = In (f (Fix f))

deriving instance (Eq (f (Fix f))) => Eq (Fix f)
deriving instance (Ord (f (Fix f))) => Ord (Fix f)
deriving instance (Show (f (Fix f))) => Show (Fix f)

out :: Fix f -> f (Fix f)
out (In f) = f

-- Anamorphism
type Coalgebra f a = a -> f a

ana :: (Functor f) => Coalgebra f a -> a -> Fix f
ana f = In . fmap (ana f) . f

我现在必须写一个像这样的余代数：

appendListCoAlg :: (ListF' a, ListF' a) -> ListF a (ListF' a, ListF' a)
appendListCoAlg (In (ConsF a as), listb) = ConsF a (as, listb)
appendListCoAlg (In NilF, In (ConsF b bs)) = ConsF b (In NilF, bs)
appendListCoAlg (In NilF, In NilF) = NilF

在这里，必须从“基本情况”(NilF)中构造一个非质体。

我感兴趣的是是否有可能编写ana，这样我就可以列出以下内容：

appendListCoAlg :: (ListF' a, ListF' a) -> ?
appendListCoAlg (In (ConsF a as), listb) = ConsF a (as, listb)
appendListCoAlg (In NilF, **bs**) = **bs**
appendListCoAlg (In NilF, In NilF) = NilF

在这里，我可以返回一个类型为“早期”的值。

Answer 1

ana不允许您这样做。您可以使用apo (尽管它仍然不像可能的那样整洁；Either确实应该在外部)。

apo :: Corecursive t => (a -> Base t (Either t a)) -> a -> t

appendListCoAlg :: [a] -> [a] -> ListF a (Either [a] [a])
appendListCoAlg listb (a : as) = ConsF a (Right as)  -- Right: continue unfolding
appendListCoAlg listb [] = Left <$> project listb    -- Left: stop unfolding

append :: [a] -> [a] -> [a]
append lista listb = apo (appendListCoAlg listb) lista