HSpec无期望未编译

Question

我正在学习Haskell和我写了这个函数：

safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead (x:xs) = Just x

我现在正在尝试用HSpec测试它：

import Test.Hspec

main :: IO ()
main = hspec spec

spec :: Spec
spec =

  describe "safeHead" $
    it "should return Nothing for empty list" $
      safeHead [] `shouldBe` Nothing

但这不能编译：

Error:(14, 19) ghc: No instance for (Eq a0) arising from a use of ‘shouldBe’
    The type variable ‘a0’ is ambiguous
    Note: there are several potential instances:
      instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
      instance Eq a => Eq (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’
      instance Eq Ordering -- Defined in ‘ghc-prim-0.4.0.0:GHC.Classes’
      ...plus 31 others
    In the second argument of ‘($)’, namely
      ‘safeHead [] `shouldBe` Nothing’
    In the second argument of ‘($)’, namely
      ‘it "should return Nothing for empty list"
       $ safeHead [] `shouldBe` Nothing’
    In the expression:
      describe "safeHead"
      $ it "should return Nothing for empty list"
        $ safeHead [] `shouldBe` Nothing

我也尝试过这样做：

safeHead :: (Eq a) => [a] -> Maybe a
safeHead [] = Nothing
safeHead (x:xs) = Just x

但这仍然不能满足以下情况：

Error:(14, 19) ghc: No instance for (Eq a0) arising from a use of ‘shouldBe’
    The type variable ‘a0’ is ambiguous
    Note: there are several potential instances:
      instance (Eq a, Eq b) => Eq (Either a b)
        -- Defined in ‘Data.Either’
      instance Eq Data.Monoid.All -- Defined in ‘Data.Monoid’
      instance forall (k :: BOX) (f :: k -> *) (a :: k).
               Eq (f a) =>
               Eq (Data.Monoid.Alt f a)
        -- Defined in ‘Data.Monoid’
      ...plus 43 others
    In the second argument of ‘($)’, namely
      ‘safeHead [] `shouldBe` Nothing’
    In the second argument of ‘($)’, namely
      ‘it "should return Nothing for empty list"
       $ safeHead [] `shouldBe` Nothing’
    In the expression:
      describe "safeHead"
      $ it "should return Nothing for empty list"
        $ safeHead [] `shouldBe` Nothing

我不知道这有什么问题。如果我尝试像这样的其他测试，它会编译得很好：

    it "should return the head" $ do
      safeHead [1] `shouldBe` Just 1
      safeHead [2,3,4,5,6,1] `shouldBe` Just 2

所以，它是关于Nothing本身的，它不能被等价物比较？那么，如何断言某物返回Nothing呢？还是我的功能太普通了？

附带注意:我看到了与此函数类似的错误：

palindrome :: (Eq a) => [a] -> [a]
palindrome xs = xs ++ reverse xs

当尝试测试空列表时：

palindrome [] `shouldBe` []

在以下几个方面都失败了：

Error:(26, 21) ghc: No instance for (Eq a0) arising from a use of ‘shouldBe’
    The type variable ‘a0’ is ambiguous
    Note: there are several potential instances:
      instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
      instance Eq a => Eq (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’
      instance Eq Ordering -- Defined in ‘ghc-prim-0.4.0.0:GHC.Classes’
      ...plus 32 others
    In a stmt of a 'do' block: palindrome [] `shouldBe` []
    In the second argument of ‘($)’, namely
      ‘do { palindrome [] `shouldBe` [] }’
    In the second argument of ‘($)’, namely
      ‘it
         "should turn a list into a palindrome, so it reads same both forwards and backwards"
       $ do { palindrome [] `shouldBe` [] }’

Answer 1

所以，它本身并不是什么，它不能被等价物比较吗？

Nothing的类型是什么?它是Nothing :: Maybe a。在这种情况下，GHC不喜欢a：“类型变量‘a0’是模棱两可的”。毕竟，shouldBe接受任何可以与(==)进行比较并显示的内容。如果a是Eq的一个实例，Maybe a是Eq的一个实例。GHC不可能知道要使用哪个a，所以需要手动指定它：

  describe "safeHead" $
    it "should return Nothing for empty list" $
      safeHead [] `shouldBe` (Nothing :: Maybe Int)

这不是胁迫，你只是明确了你想要使用的所有可能类型中的哪种。其他例子：

  describe "safeHead" $
    it "should return Nothing for empty list" $ do
      safeHead [] `shouldBe` (Nothing :: Maybe Int)
      safeHead [] `shouldBe` (Nothing :: Maybe ())
      safeHead [] `shouldBe` (Nothing :: Maybe Integer)
      safeHead [] `shouldBe` (Nothing :: Maybe Char)