如何測試導入Control.Monad.Except與Hunit？

Question

如何測試Control.Monad.Except（兩個後衛結果）像一個函數：

foo :: Double -> Double -> Except String Double 
foo x y 
    | x < -10.0 = throwError "Invalid parameter" 
    | otherwise = pure $ x + y 

使用hunit？

Answer 1

寫一些使用runExcept來執行Except動作並使用~?=來檢查其結果的函數非常簡單。

shouldThrow :: Eq e => Except e a -> e -> Test 
m `shouldThrow` e = runExcept m ~?= Left e 

shouldReturn :: Eq a => Except e a -> a -> Test 
m `shouldReturn` x = runExcept m ~?= Right x 

用法示例：

testFoo = TestList [ 
    foo -11 2 `shouldThrow` "Invalid parameter", 
    foo 3 1 `shouldReturn` 4 
    ]