haskell - 顶级 OverloadedLists 字面量

Question

我有一个用于重构练习的测试套件，我希望它与 Data.List 兼容。和 Data.List.NonEmpty .练习包含一个函数 foo :: [Foo] -> Foo并且测试套件有一些

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
         }
  , ...
  ]

使用 OverloadedLists 使测试套件具有多态性， 我试过了

{-# 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]
   |                     ^^^^^

我想移动 IsList list对 Case 的约束数据类型，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因为 Item List (l Foo) => l 的类型本身不是 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.

但是，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), Item (l Foo) ~ Foo) => [Case l]
cases = [
    Case { description = "blah blah"
         , input = [Foo 1, Foo 2, Foo 3]
         , expected = Foo 1
         }
  ]

在这里，我们因此说 Item (l Foo)应与 Foo 相同.