haskell - HSpec 无预期编译失败

Question

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

但是编译失败:

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 (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

我也尝试过这个:

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

但是仍然失败:

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

我不知道这里出了什么问题。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

那么，这是关于Nothing本身的问题，它不能通过相等进行比较？那么你如何断言某物返回Nothing呢？或者我的功能太通用了？

旁注:我在这个函数中看到过类似的错误:

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

尝试测试空列表时:

palindrome [] `shouldBe` []

失败的原因是:

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

So, it's something about Nothing itself, that it cannot be compared by equals?

Nothing 的类型是什么？这是没什么::也许是。 GHC 不喜欢这种情况下的 a:“类型变量‘a0’是不明确的”。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 的实例，则 Maybe a 是 Eq 的实例。 GHC 不可能知道您想要使用哪个 a，因此您需要手动指定它:

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

这不是强制，您只是明确要使用所有可能类型中的哪一种。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)