頂級 OverloadedLists 文字

Question

我有一個用於重構練習的測試套件，我希望它與Data.List和Data.List.NonEmpty兼容。 練習包含一個函數foo :: [Foo] -> Foo並且測試套件有一些

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
         }
  , ...
  ]

為了使用OverloadedLists使測試套件具有多態性，我嘗試了

{-# 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
         }
  , ...
  ]

但這給了我錯誤

    • 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]
   |                     ^^^^^

我想將IsList list約束移動到Case數據類型，就像這樣

{-# 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), ... ]

但這給了我錯誤

    • 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
   |                                    ^^^^^^^^

我不確定這里最簡單的方法是什么。 任何提示？

Answer 1

這不起作用的原因是因為List (l Foo) => l的Item類型本身不是Foo 。 該擴展對其進行了抽象，因此它期望您的列表文字的元素為Item (l Foo)類型。

但是，您可以添加一個類型約束，表明這些項目確實是Foo類型：

{-# 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
         }
  ]

因此，我們在這里說Item (l Foo)應該與Foo相同。