如果在測試中存在`Nothing == Nothing`條件，則HUnit不允許編譯測試用例

Question

我遇到了奇怪的HUnit行爲。如果Nothing == Nothing條件存在於測試中，則不允許編譯測試用例。這裏是我的代碼再現這種行爲：

module TestTest where 

import Control.Exception 
import Control.Monad 
import Test.HUnit 
import Test.AssertError 

testTests = test [ 
    "test A01" ~: "x == x" ~: True ~=? Nothing == Nothing, 
    "test _" ~: "empty test" ~: True ~=? True 
    ] 

runTests :: IO Counts 
runTests = do 
    runTestTT testTests 

嘗試將文件與下面的錯誤此內容ghci回報加載：

[2 of 2] Compiling TestTest   (Test/TestTest.hs, interpreted) 

Test/TestTest.hs:9:49: 
    No instance for (Eq a0) arising from a use of ‘==’ 
    The type variable ‘a0’ is ambiguous 
    Note: there are several potential instances: 
     instance Eq Counts -- Defined in ‘Test.HUnit.Base’ 
     instance Eq Node -- Defined in ‘Test.HUnit.Base’ 
     instance Eq State -- Defined in ‘Test.HUnit.Base’ 
     ...plus 53 others 
    In the second argument of ‘(~=?)’, namely ‘Nothing == Nothing’ 
    In the second argument of ‘(~:)’, namely 
     ‘True ~=? Nothing == Nothing’ 
    In the second argument of ‘(~:)’, namely 
     ‘"x == x" ~: True ~=? Nothing == Nothing’ 
Failed, modules loaded: Test.AssertError. 

注意在同一測試用例條件Just 2 == Just 2工作正常。如果我在ghci中輸入Nothing == Nothing，則按預期返回True。

任何想法爲什麼HUnit可能會這樣？這是一個錯誤還是預期的行爲？

Answer 1

問題是您指定了兩個Nothing s，並且這些都沒有提示a的類型。當然，你可以推斷，對於Nothing這並不重要。但是Haskell並沒有這樣推理：它對「我應該指出什麼(==)函數」感興趣？「。

您可以通過使類型顯式來解決問題。例如：

testTests = test [ 
    "test A01" ~: "x == x" ~: True ~=? (Nothing :: Maybe Int) == Nothing, 
    "test _" ~: "empty test" ~: True ~=? True 
    ]