測試函數返回一個單子可能

Question

說我有一個功能：

safeHead :: [a] -> Maybe a 
safeHead [] = Nothing 
safeHead xs = Just $ head xs 

和測試：

describe "Example.safeHead" $ do 
    it "returns the head" $ do 
    safeHead [1,2,3] `shouldBe` Just 1 

    it "returns Nothing for an empty list" $ 
    safeHead [] `shouldBe` Nothing 

然而，這會產生：

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 forall (k :: BOX) (s :: k). Eq (Data.Proxy.Proxy s) 
    -- Defined in ‘Data.Proxy’ 
    instance (GHC.Arr.Ix i, Eq e) => Eq (GHC.Arr.Array i e) 
    -- Defined in ‘GHC.Arr’ 
    ...plus 88 others 
In the second argument of ‘($)’, namely 
    ‘safeHead [] `shouldBe` Nothing’ 
In a stmt of a 'do' block: 
    it "returns Nothing for an empty list" 
    $ safeHead [] `shouldBe` Nothing 
In the second argument of ‘($)’, namely 
    ‘do { it "returns the head" 
     $ do { safeHead [...] `shouldBe` Just 1 }; 
     it "returns Nothing for an empty list" 
     $ safeHead [] `shouldBe` Nothing }’ 

爲什麼？我該如何解決它？

Answer 1

shouldBe    :: (Eq a, Show a) => a -> a -> Expectation 
safeHead    ::   [a] -> Maybe a 
[]      ::   [a] 
safeHead []    ::     Maybe a 
Nothing     ::     Maybe a 
shouldBe (safeHead []) :: (Eq a, Show a) => Maybe a -> Expectation 

shouldBe (safeHead []) Nothing ::      Expectation -- but what's `a`? 

正如你所看到的，a完全是不明確的。它可以是具有Show和Eq實例的任何類型。這也是你的錯誤消息的一部分：

 
No instance for (Eq a0) arising from a use of ‘shouldBe’ 
The type variable ‘a0’ is ambiguous 

所以挑一個：

it "returns Nothing for an empty list" $ 
    safeHead [] `shouldBe` (Nothing :: Maybe()) 

當你在它，使用快速檢查驗證safeHead作品像head：

it "returns Just (head xs) for a non-empty list" $ property $ \(NonEmpty xs) -> 
    safeHead xs `shouldBe` (Just (head xs) :: Maybe Integer) 

Answer 2

正如user2407038所評論的，編譯器不知道如何實例化a。他提出的修正可能是最好的 - 你應該明確指定類型a。

但對於完整性，我想指出，有其他的解決方案，extended default rules：

{-# LANGUAGE ExtendedDefaultRules #-} 

describe "Example.safeHead" $ do 
    it "returns the head" $ do 
    safeHead [1,2,3] `shouldBe` Just 1 

    it "returns Nothing for an empty list" $ 
    safeHead [] `shouldBe` Nothing 

擴展修改the standard defaulting rules，包括更多的情況下，例如Eq類型的類。

加入：經過一番思考，我重新考慮了一下我的答案。多態函數的單元測試結果不應該依賴於特定方式來實例化類型變量。所以可能擴展的默認規則是測試中的正確事物？我從來沒有用過它真正的代碼，所以我不能肯定地說，但它絕對值得考慮。