Hspec和QuickCheck-模糊類型變量a0？

Question

我在Haskell中編寫了一個函數，該函數接受任意元素的列表並返回（映射）元組列表。 每個元組都包含原始元素和一個分數，列表中的所有分數都加1（因此，我只需使用1 ``div`` length xs一次計算分數並將其應用於所有元素）。 這是代碼：

uniform :: [a] -> [(a, Int)]
uniform xs = map (\x -> (x, prob)) xs
  where prob = 1 `div` (length xs)

（免責聲明：這實際上是一個稍微簡化的版本，但我產生的行為完全相同，因此希望足夠了。）

我試圖通過使用Hspec和Quickcheck的基於屬性的測試來涵蓋這一點：

spec = do
    describe "uniform" $ do

        it "produces a uniform distribution summing to 1" $ property $
            let totalProbability ((_, p):xs) = p + (totalProbability xs)
            in (\xs -> (totalProbability $ uniform xs) `shouldBe` 1)

但是，當我運行此命令時，出現以下錯誤：

• Ambiguous type variable ‘a0’ arising from a use of ‘property’
  prevents the constraint ‘(Arbitrary a0)’ from being solved.
  Probable fix: use a type annotation to specify what ‘a0’ should be.
  These potential instances exist:
    instance (Arbitrary a, Arbitrary b) => Arbitrary (Either a b)
      -- Defined in ‘Test.QuickCheck.Arbitrary’
    instance Arbitrary Ordering
      -- Defined in ‘Test.QuickCheck.Arbitrary’
    instance Arbitrary Integer
      -- Defined in ‘Test.QuickCheck.Arbitrary’
    ...plus 19 others
    ...plus 62 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
• In the second argument of ‘($)’, namely
    ‘property
       $ let totalProbability ((_, p) : xs) = p + (totalProbability xs)
         in (\ xs -> (totalProbability $ uniform xs) `shouldBe` 1)’
  In a stmt of a 'do' block:
    it "produces a uniform distribution summing to 1"
      $ property
          $ let totalProbability ((_, p) : xs) = p + (totalProbability xs)
            in (\ xs -> (totalProbability $ uniform xs) `shouldBe` 1)
  In the second argument of ‘($)’, namely
    ‘do it "produces a uniform distribution summing to 1"
          $ property
              $ let totalProbability ((_, p) : xs) = ...
                in (\ xs -> (totalProbability $ uniform xs) `shouldBe` 1)’

| 12 | 它“產生一個總和為1的均勻分布”。 ^^^^^^^^^^ ...

我猜想在某個地方我沒有給QuickCheck有關如何生成測試值的足夠信息，但是我不確定從這里去哪里。

任何幫助將不勝感激。

謝謝！

Answer 1

您需要為xs指定類型：這是字符串列表嗎？ 整型？ 布爾值？ 這樣，QuickCheck可以生成該類型的隨機樣本。

您可以編寫，例如：

...
in (\xs -> (totalProbability $ uniform (xs :: [Int])) `shouldBe` 1)