带有类型错误的HUnit TestCase

Question

我编写了一个类似LISP的flatten的函数

data NestedList a = Elem a | List [NestedList a]

flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (List xs) = concatMap flatten xs

这两个测试用例运行良好：

test1 = TestCase (assertEqual "test singleton" [5] (flatten (Elem 5)))
test2 = TestCase (assertEqual "test mixed" 
                              [1,2,3,4]
                              (flatten (List [Elem 1, 
                                              List [Elem 2, Elem 3],
                                              Elem 4])))

然而，这一次报告一个类型错误：

test3 = TestCase (assertEqual "test empty" [] (flatten (List [])))

来自REPL的测试运行良好：

*Main> [] == flatten (List [])
True

为什么我会得到一个错误，我应该如何为一个空列表编写一个测试用例？

编辑：这里是确切的错误消息：

Ambiguous type variable `a0' in the constraints:
  (Show a0) arising from a use of `assertEqual'
            at D:\haskell\source.hs:61:19-29
  (Eq a0) arising from a use of `assertEqual'
          at D:\haskell\source.hs:61:19-29
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `TestCase', namely
  `(assertEqual "test empty" [] (flatten (List [])))'

Answer 1

我猜问题是编译器找不出空列表的类型。可能是[Int]，[Char]，任何真正的东西。试着给它一些类型，哪种类型完全不重要。

test3 = TestCase (assertEqual "test empty" ([] :: [Int]) (flatten (List [])))

注在其他情况下，编译器可以推断列表的类型。

Answer 2

正如错误消息已经告诉您的，您必须将类型签名添加到单元测试中的列表中。可能是这样的：

test3 = TestCase (assertEqual "test empty" ([]::[()]) (flatten (List [])))