顶级 OverloadedLists 文字

Question

我有一个用于重构练习的测试套件，我希望它与 Data.List 和 Data.List.NonEmpty 兼容。练习由一个函数 foo :: [Foo] -> Foo 组成，测试套件有一些

data Case = Case
  { description :: String
  , input :: [Foo]
  , expected :: Foo
  }

cases :: [Case]
cases =
  [ Case { description = "blah blah"
         , input = [Foo 1, Foo 2, Foo 3]
         , expected = Foo 1
         }
  , ...
  ]

为了使测试套件具有 OverloadedLists 的多态性，我尝试了

{-# LANGUAGE OverloadedLists #-}
...

data Case list = Case
  { description :: String
  , input :: list Foo
  , expected :: Foo
  }

cases =
  [ Case { description = "blah blah"
         , input = [Foo 1, Foo 2, Foo 3]
         , expected = Foo 1
         }
  , ...
  ]

但这给了我错误

    • Couldn't match expected type ‘GHC.Exts.Item (list0 Foo)’
                  with actual type ‘Foo’
      The type variable ‘list0’ is ambiguous
      ...
   |
50 |          , input = [Foo 1, Foo 2, Foo 3]
   |                     ^^^^^

我想将 IsList list 约束移动到 Case 数据类型，就像这样

{-# LANGUAGE GADTs #-}
{-# LANGUAGE OverloadedLists #-}
...

data Case list where
  Case :: IsList list => String -> list Foo -> Foo -> Case list

cases =
  [ Case "blah blah" [Foo 1, Foo 2, Foo 3] (Foo 1), ... ]

但这给了我错误

    • Expected kind ‘* -> *’, but ‘list’ has kind ‘*’
    • In the type ‘list Foo’
      In the definition of data constructor ‘Case’
      In the data declaration for ‘Case’
   |        
24 |   Case :: IsList list => String -> list Foo -> Foo -> Case list
   |                                    ^^^^^^^^

我不确定这里最简单的方法是什么。有什么提示吗？

Answer 1

这不起作用的原因是 List (l Foo) => l 的 Item 类型本身不是 Foo。扩展对其进行了抽象，因此它期望列表文字的元素为 Item (l Foo).

类型

然而，您可以添加一个类型约束，表明这些项目确实属于 Foo:

类型

{-# LANGUAGE OverloadedLists #-}

data Case list = Case
  { description :: String
  , input :: list Foo
  , expected :: Foo
  }

cases :: (IsList (l Foo), <b>Item (l Foo) ~ Foo</b>) => [Case l]
cases = [
    Case { description = "blah blah"
         , input = [Foo 1, Foo 2, Foo 3]
         , expected = Foo 1
         }
  ]

这里我们说Item (l Foo)应该和Foo一样。

顶级 OverloadedLists 文字

Top-level OverloadedLists literal

haskell

overloaded-lists