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我正在学习 Haskell，我已经编写了这个函数：

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

我现在正在尝试使用 HSpec 对其进行测试：

import Test.Hspec

main :: IO ()
main = hspec spec

spec :: Spec
spec =

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

但这无法编译：

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

我也试过这个：

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

但这仍然失败：

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

我不知道这里有什么问题。如果我尝试像这样的其他测试，它编译得很好：

    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它自己的，它不能用相等来比较？你如何断言某些东西会返回Nothing呢？还是我的功能太通用了？

旁注：我看到这个函数有类似的错误：

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

尝试测试空列表时：

palindrome [] `shouldBe` []

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` [] }’
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1 回答 1

7

所以，它是关于 Nothing 本身的东西，它不能被相等比较？

什么Nothing是类型？是Nothing :: Maybe a。而 GHC 不喜欢a这种情况：“类型变量 'a0' 是模棱两可的”。毕竟，shouldBe拿任何可以比较(==)和展示的东西。AndMaybe a是Eqifa的一个实例，是 的一个实例Eq。GHC 不可能知道你想使用哪个，所以你需要手动指定它： a

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

这不是强制，您只是在明确要使用所有可能的类型中的哪一种。其他示例：

  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)
于 2015-06-03T10:12:11.463 回答