我正在学习 Haskell 并且编写了这个函数：
safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead (x:xs) = Just x
我现在尝试用 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
我也尝试过这个：
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
所以，这是关于Nothing本身就不能平等比较吗？你如何断言某些东西会返回Nothing然后？或者我的功能太通用了？
边注：
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
当尝试测试空列表时：
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` [] }’
