Question

我正在学习Haskell并且我已经写了这个函数：

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

我现在正试图用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

但是无法编译：

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

但是仍然失败了：

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

我不知道这里的问题是什么。如果我尝试其他类似的测试，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，关于Nothing本身的一些事情，它是不能用等于进行比较的？你怎么断言某些东西然后返回Nothing？或者我的功能太通用了吗？

侧面说明：我在这个功能上看到了类似的错误：

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

尝试测试空列表时：

palindrome [] `shouldBe` []

哪个失败了：

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，这是关于Nothing本身的东西，它不能用equals进行比较吗？

Nothing的类型是什么？它是Nothing :: Maybe a。 GHC在这种情况下不喜欢a：“类型变量'a0'是不明确的”。毕竟，shouldBe会将可以与(==)进行比较并显示的任何内容。如果Maybe a是Eq的实例，则a是Eq的实例。 GHC不可能知道你要使用哪个 a，所以你需要手工指定：

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

这不是强制，你只是明确你要使用的所有可能类型的。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)

HSpec没有任何期望未能编译

1 个答案: