所以,只是为了好玩,我一直在Haskell中使用一个CountedList类型,使用Peano号码 和smart constructors。Haskell计数列表类型
型号安全head
和tail
对我来说看起来很酷。
而且我想我已经达到了什么,我知道该怎么做
{-# LANGUAGE EmptyDataDecls #-}
module CountedList (
Zero, Succ, CountedList,
toList, ofList,
empty, cons, uncons,
head, tail,
fmap, map, foldl, foldr, filter
) where
import qualified List (foldr, foldl, filter)
import Prelude hiding (map, head, foldl, foldr, tail, filter)
data Zero
data Succ n
data CountedList n a = CL [a]
toList :: CountedList n a -> [a]
toList (CL as) = as
ofList :: [a] -> CountedList n a
ofList [] = empty
ofList (a:as) = cons a $ ofList as
empty :: CountedList Zero a
empty = CL []
cons :: a -> CountedList n a -> CountedList (Succ n) a
cons a = CL . (a:) . toList
uncons :: CountedList (Succ n) a -> (a, CountedList n a)
uncons (CL (a:as)) = (a, CL as)
head :: CountedList (Succ n) a -> a
head = fst . uncons
tail :: CountedList (Succ n) a -> CountedList n a
tail = snd . uncons
instance Functor (CountedList n) where
fmap f = CL . fmap f . toList
map :: (a -> b) -> CountedList n a -> CountedList n b
map = fmap
foldl :: (a -> b -> a) -> a -> CountedList n b -> a
foldl f a = List.foldl f a . toList
foldr :: (a -> b -> b) -> b -> CountedList n a -> b
foldr f b = List.foldr f b . toList
filter :: (a -> Bool) -> CountedList n a -> CountedList m a
filter p = ofList . List.filter p . toList
极限(对不起任何抄写错误 - 机器我最初写这个的瓦特/我Haskell编译当前已关闭) 。
我所做的大部分工作都是在没有问题的情况下编译的,但我遇到了ofList
和filter
的问题。我想我理解为什么 - 当我说ofList :: [a] -> CountedList n a
时,我说的是ofList :: forall n . [a] -> CountedList n a
- 创建的列表可以是任何需要的计数类型。我想写的是相当于伪类型ofList :: exists n . [a] -> CountedList n a
,但我不知道如何。
有没有一种解决方法可以让我编写ofList
和filter
函数,就像我想象的一样,还是我已经达到了可以用这个函数完成的极限?我有一个感觉,那就是我错过了existential types。
我不知道为什么有人会downvote这个题。我保持平衡。 – 2010-01-14 16:00:51
看起来像有人点击downvote错误并修复它:目前没有downvote。 – JaakkoK 2010-01-14 16:02:45
事实上,我点击了我手机上的错误按钮,然后失去了网络连接。我希望堆栈溢出将在不久的将来得到一个移动样式表(带有一些更大的箭头):-) – 2010-01-14 16:12:57