haskell - 如何在不使用求和类型或副本的情况下创建异构数据类型列表

Question

用例:一款跨多个消息服务跟踪用户的应用。有一个 TwitterAccount 数据类型，一个 FacebookAccount 数据类型，等等。这些可以很容易地与 Account 总和类型结合在一起，但是下一层层次结构导致了我的问题。

一个 TwitterAccount 有一个 TwitterPost 的列表，一个 FacebookAccount 有一个 FacebookPost 的列表，等等.

我的任务:我希望能够将所有帐户最近 10 天的所有帖子放入一个列表中，并从中提取公共(public)时间和消息正文字段以进行显示。

我失败的方法: 我认为如果 Post 的每个类都实现一个类型类，如 SimplePost 公开函数 messageBody 和 messageTime 这可能会解决我的问题，但我无法创建 [SimpleMessage] 的列表。

我想保持 TwitterAccount 只能包含TwitterPost 等不变性，所以我不能使用 sum -类型。我不想创建对象的副本来执行此操作。

这个问题最好、最干净、最像 Haskell 的设计是什么？

更新这不是答案，但作为 recursion.ninja 和 Helder Pereira 提供的四种解决方案的替代方案，我一直在考虑是否可以使用幻像类型满足我的不变量，Account 和 Post 类型包含所有提供者所需的所有可能信息。然而，元组的使用和笨拙的逻辑意味着它不能很好地扩展；也许这应该在不同的问题中。

{-# LANGUAGE EmptyDataDecls #-}   

-- Some FSharpisms
(|>) = flip ($)
(<|) = ($)
infixr 0 <|


data Twitter
data Facebook
data LinkedIn

data Post a = Post{
    postBody :: String,
    postDate :: UTCTime,
    postForwarded :: Bool,
    postFriendMentions :: [UserName]
    } deriving (Show, Eq)

data Account a = Account {
    accountName  :: String,
    accountPosts :: [Post a]
    } deriving (Show, Eq)

data User = User {
    userName   :: String,
    userTweets :: Account Twitter,
    userFaces  :: Account Facebook,
    userLinks  :: Account LinkedIn
    }

prettyShowUtc :: UTCTime -> String
prettyShowUtc utc = ...

prettyShow :: Post a -> String
prettyShow p = prettyShowUtc (postDate p) ++ " : " ++ show (postBody p)

showOrderedOf2 :: ([Post a], [Post b]) -> [String]
showOrderedOf2 ([], []) = []
showOrderedOf2 (ls, []) = map prettyShow ls
showOrderedOf2 ([], rs) = map prettyShow rs
showOrderedOf2 ((l:ls), (r:rs)) = 
    if postDate l < postDate r
    then prettyShow l : showOrderedOf2 (ls, (r:rs))
    else prettyShow r : showOrderedOf2 ((l:ls), rs)

showOrderedOf3 :: ([Post a], [Post b], [Post c]) -> [String]
showOrderedOf3 ([], [], []) = []
showOrderedOf3 (as, [], []) = map postBody as
showOrderedOf3 ([], bs, []) = map postBody bs
showOrderedOf3 ([], [], cs) = map postBody cs
showOrderedOf3 (as, bs, []) = showOrderedOf2 (as, bs)
showOrderedOf3 ([], bs, cs) = showOrderedOf2 (bs, cs)
showOrderedOf3 (as, [], cs) = showOrderedOf2 (as, cs)
showOrderedOf3 ((a:as), (b:bs), (c:cs)) =
    let (adate, bdate, cdate) = (postDate a, postDate b, postDate c)
        minDate = minimum [adate, bdate, cdate]
    in
    if adate == minDate
    then prettyShow a : showOrderedOf3 (as, (b:bs), (c:cs))
    else (if bdate == minDate
         then prettyShow b : showOrderedOf3 ((a:as), bs, (c:cs))
         else prettyShow c : showOrderedOf3 ((a:as), (b:bs), cs))

createAndShowSample :: IO ()
createAndShowSample =
    let faceAc = Account {...} :: Account Facebook
        twitAc = Account {...} :: Account Twitter
        linkAc = Account {...} :: Account LinkedIn
    in 
    showOrderedOf3 (accountPosts faceAc, accountPosts twitAc, accountPosts linkAc)
        |> intercalate "\n"
        |> putStrLn

Answer 1

您应该将 FaceBookAccount 和 TwitterAccount 抽象为 SocialMediaAccount 的实例

Haskell 代码:

import Control.Applicative ((<$>))
import Data.List
import Data.Ord
import Data.Time

data FaceBookAccount = FaceBookAccount [FaceBookPost]
data TwitterAccount  = TwitterAccount  [TwitterPost]
data FaceBookPost    = FaceBookPost String UTCTime
data TwitterPost     = TwitterPost  String UTCTime

data SocialMediaAccount 
   = SocialMediaAccount 
   { accountPosts :: [SocialMediaPost]
   }
data SocialMediaPost 
   = SocialMediaPost
   { postBody :: String
   , postTime :: UTCTime
   }

class SocialMedia a where
  simpleAccount  :: a -> SocialMediaAccount  

instance SocialMedia FaceBookAccount where
  simpleAccount (FaceBookAccount xs) = SocialMediaAccount $ f <$> xs
    where 
      f (FaceBookPost text time) = SocialMediaPost text time

instance SocialMedia TwitterAccount where
  simpleAccount (TwitterAccount xs) = SocialMediaAccount $ f <$> xs
    where 
      f (TwitterPost text time) = SocialMediaPost text time

getAllMessages :: (SocialMedia a, SocialMedia b) => a -> b -> [SocialMediaPost]
getAllMessages xs ys = sortBy (comparing postTime) 
                     $ extract xs
                    ++ extract ys
  where
    extract :: SocialMedia a => a -> [SocialMediaPost]
    extract = accountPosts . simpleAccount