正如人们在评论中指出的那样,您无法实现有效的Applicative
实例为Map
因为你无法实施pure
以守法的方式。由于同一律,pure id <*> v
= v
, the pure
实现需要维护所有键,同时将映射与函数应用程序相交。您不能对部分映射执行此操作,因为根据参数化,您可能在一个映射或另一个映射中没有用于构造函数的键a -> b
或论证a
你需要制作一个b
在生成的地图中。pure x
需要像那个一样工作ZipList
(它使用repeat
),生成映射的地图every相同值的键x
,但这不可能Map
因为它是有限的。然而,它is可以使用允许无限映射的替代表示,例如基于函数和Eq
.
-- Represent a map by its lookup function.
newtype EqMap k v = EM (k -> Maybe v)
-- Empty: map every key to ‘Nothing’.
emEmpty :: EqMap k v
emEmpty = EM (const Nothing)
-- Singleton: map the given key to ‘Just’ the given value,
-- and all other keys to ‘Nothing’.
emSingleton :: (Eq k) => k -> v -> EqMap k v
emSingleton k v = EM (\ k' -> if k == k' then Just v else Nothing)
-- Insertion: add an entry that overrides any earlier entry
-- for the same key to return ‘Just’ a new value.
emInsert :: (Eq k) => k -> v -> EqMap k v -> EqMap k v
emInsert k v (EM e) = EM (\ k' -> if k == k' then Just v else e k')
-- Deletion: add an entry that overrides any earlier entry
-- for the same key to return ‘Nothing’.
emDelete :: (Eq k) => k -> EqMap k v -> EqMap k v
emDelete k (EM e) = EM (\ k' -> if k == k' then Nothing else e k')
emLookup :: EqMap k v -> k -> Maybe v
emLookup (EM e) = e
instance Functor (EqMap k) where
-- Map over the return value of the lookup function.
fmap :: (a -> b) -> EqMap k a -> EqMap k v
fmap f (EM e) = EM (fmap (fmap f) e)
instance Applicative (EqMap k) where
-- Map all keys to a constant value.
pure :: a -> EqMap k a
pure x = EM (const (Just x))
-- Intersect two maps with application.
(<*>) :: EqMap k (a -> b) -> EqMap k a -> EqMap k b
fs <*> xs = EM (\ k -> emLookup k fs <*> emLookup k xs)
不幸的是,这不仅仅是语义上无限的:正如你添加的或删除键值对,它也grows无限在记忆中!这是因为条目是闭包的链接列表,而不是具体化为数据结构:您只能通过以下方式从映射中删除值adding指示它们被删除的条目,就像版本控制系统中的恢复一样。查找的效率也非常低,查找的效率与键的数量成线性关系,而不是对数关系Map
。对于初中级函数式程序员来说,充其量这只是一个不错的学术练习,只是为了了解如何用函数表示事物。
这里的一个简单替代方案是“默认映射”,它将不存在的键映射到常量值。
data DefaultMap k v = DM v (Map k v)
dmLookup :: (Ord k) => k -> DefaultMap k v -> v
dmLookup k (DM d m) = fromMaybe d (Map.lookup k m)
-- …
然后执行Applicative
很简单:现有键的交集,加上应用默认值的不存在键。
instance Functor (DefaultMap k) where
-- Map over the return value of the lookup function.
fmap :: (a -> b) -> DefaultMap k a -> DefaultMap k b
fmap f (DM d m) = DM (f d) (fmap f m)
instance Applicative (DefaultMap k) where
-- Map all keys to a constant value.
pure x = DM x mempty
-- Intersect two maps with application, accounting for defaults.
DM df fs <*> DM dx xs = DM (df dx) $ Map.unions
[ Map.intersectionWith ($) fs xs
, fmap ($ dx) fs
, fmap (df $) xs
]
DefaultMap
有点不寻常的是你can删除键值对,但只能通过有效地将它们“重置”为其默认值,因为即使删除同一键后,对给定键的查找也始终会成功。虽然你当然可以恢复类似于部分行为的东西Map
using DefaultMap k (Maybe v)
默认为Nothing
以及始终将定义的键映射到的不变量Just
.
I think还有一个instance Monad (DefaultMap k)
,通过同构instance Monad ((->) k)
or instance Monad (Stream k)
,因为喜欢Stream
, a DefaultMap
is always无限——而可能有限ZipList
不能有Monad
实例,因为它必然违反结合律a >=> (b >=> c)
= (a >=> b) >=> c
.