文章/答案/技术大牛

发布

社区首页 >问答首页 >Haskell -容器容器和实例解密

问Haskell -容器容器和实例解密
EN

Stack Overflow用户

提问于 2013-10-19 18:43:16

回答 1查看 165关注 0票数 0

Haskell新手的简短版本:假设我有一个容器类，容器中包含* -> *类。我希望将其放入另一个容器中，并仍然将第二个容器作为原始类的实例，如

data Container2 a container = Container2 (container a)

instance Container (Conrainer2 a) where ...

但这似乎是不可能的，因为GHC总是生成以下错误：

Kind mis-match

    The first argument of `Container' should have kind `* -> *',
    but `Container2 a' has kind `(* -> *) -> *'

有可能解决这个问题吗？

长版本:我正在使用以下代码对Java中的迭代器接口建模：

module Main where

data NextResult a iter = Stop | More a (iter a)

class Iter iter where
  next :: iter a -> NextResult a iter

-- Convert iterator to a list
toList :: (Iter c) => c a -> [a]
toList iter = case next iter of
  Stop -> []
  More value iter2 -> value : toList iter2 

-- List itself is iterator
instance Iter [] where
  next [] = Stop
  next (x:xs) = More x xs

main = let iter = [1,2,3,4,5] in print $ toList iter

使用GHC7.4.1，它编译并打印预期的1 2 3 4 5。现在，我想定义一个转换迭代器，它从一个函数和一个迭代器构建一个新的迭代器。为此，我增加了以下几行：

data TransformedIter from to iter = TransformedIter (from->to) (iter from)

instance Iter (TransformedIter from to) where
  next (TransformedIter f iter) = case next iter of
    Stop -> Stop
    More value iter2 -> More (f value) (TransformedIter f iter2)

但这产生了错误：

Main.hs:21:16:
    Kind mis-match
    The first argument of `Iter' should have kind `* -> *',
    but `TransformedIter from to' has kind `(* -> *) -> *'
    In the instance declaration for `Iter (TransformedIter from to)'

我试图解决这个问题，但结果总是一种或另一种类型的类型错误。那么，在Haskell，如何模拟这种转变呢？

更新

我误解了实例声明的工作原理。根据下面的建议，我翻转了TransformedIter类型的顺序，最后得到了：

module Main where

data NextResult a iter = Stop | More a (iter a)

class Iter iter where
  next :: iter a -> NextResult a iter

toList :: (Iter c) => c a -> [a]
toList iter = case next iter of
  Stop -> []
  More value iter2 -> value : toList iter2 

instance Iter [] where
  next [] = Stop
  next (x:xs) = More x xs

main = let iter = [1,2,3,4,5] in print $ toList iter

data TransformedIter iter from to = TransformedIter (from->to) (iter from)

instance Iter (TransformedIter iter from) where
  next (TransformedIter f iter) = case next iter of
    Stop -> Stop
    More value iter2 -> More (f value) (TransformedIter f iter2)

然而，这又产生了另一个错误：

Main.hs:22:40:
    No instance for (Iter iter)
      arising from a use of `next'
    In the expression: next iter
    In the expression:
      case next iter of {
        Stop -> Stop
        More value iter2 -> More (f value) (TransformedIter f iter2) }
    In an equation for `next':
        next (TransformedIter f iter)
          = case next iter of {
              Stop -> Stop
              More value iter2 -> More (f value) (TransformedIter f iter2) }

我将实例声明更改为：

instance Iter (Iter iter => TransformedIter iter from) where

这产生了另一个错误：

Main.hs:21:10:
    Illegal instance declaration for `Iter
                                        (Iter iter => TransformedIter iter from)'
      (All instance types must be of the form (T a1 ... an)
       where a1 ... an are *distinct type variables*,
       and each type variable appears at most once in the instance head.
       Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Iter (Iter iter =>
                                           TransformedIter iter from)'

在我加入-XFlexibleInstances之后，我得到了：

Main.hs:21:10:
    Illegal polymorphic or qualified type:
      Iter iter => TransformedIter iter from
    In the instance declaration for `Iter (Iter iter =>
                                           TransformedIter iter from)'

因此，我仍然不知道如何将TransformedIter声明为Iter的一个实例。有什么线索吗？

更新2

使用GADTs GHC扩展，我成功地定义了TransformedIter：

module Main where

data NextResult a iter = Stop | More a (iter a)

class Iter iter where
  next :: iter a -> NextResult a iter

toList :: (Iter c) => c a -> [a]
toList iter = case next iter of
  Stop -> []
  More value iter2 -> value : toList iter2 

instance Iter [] where
  next [] = Stop
  next (x:xs) = More x xs

data TransformedIter iter from to where
  TransformedIter :: Iter iter => 
                     (from->to) -> (iter from) -> TransformedIter iter from to

instance Iter (TransformedIter iter from) where
  next (TransformedIter f iter) = case next iter of
    Stop -> Stop
    More value iter2 -> More (f value) (TransformedIter f iter2)

twice = (*) 2

main = let iter = TransformedIter twice [1,2,3,4,5] in print $ toList iter

它编译并打印预期的2，4，6，8，10，但这个扩展真的需要吗？

haskell

回答 1

Stack Overflow用户

回答已采纳

发布于 2014-11-21 22:15:00

答案是在J. Abrahamson的评论中给出的，现在我终于有了一个不使用任何GHC扩展的版本(我以前在实例声明中使用了错误的上下文语法)：

module Main where

data NextResult a iter = Stop | More a (iter a)

class Iter iter where
  next :: iter a -> NextResult a iter

toList :: (Iter c) => c a -> [a]
toList iter = case next iter of
  Stop -> []
  More value iter2 -> value : toList iter2 

instance Iter [] where
  next [] = Stop
  next (x:xs) = More x xs

data TransformedIter iter from to = TransformedIter (from->to) (iter from)

instance Iter iter => Iter (TransformedIter iter from) where
  next (TransformedIter f iter) = case next iter of
    Stop -> Stop
    More value iter2 -> More (f value) (TransformedIter f iter2)

twice = (*) 2

main = let iter = TransformedIter twice [1,2,3,4,5] in print $ toList iter

打印2 4 6 8 10

票数 1

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/19469661

复制

相似问题

问Haskell -容器容器和实例解密
EN

回答 1

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问Haskell -容器容器和实例解密EN

回答 1

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问Haskell -容器容器和实例解密
EN