使用HSpec和QuickCheck驗證Data.Monoid屬性

Question

我正在嘗試使用HSpec和QuickCheck來驗證Monoid的屬性（關聯性和標識元素）。 我將驗證特定的實例，但想保留大多數代碼的多態性。 這是幾個小時后我想到的：

module Test where

import Test.Hspec
import Test.QuickCheck
import Data.Monoid

instance (Arbitrary a) => Arbitrary (Sum a) where
    arbitrary = fmap Sum arbitrary

instance (Arbitrary a) => Arbitrary (Product a) where
    arbitrary = fmap Product arbitrary

prop_Monoid_mappend_mempty_x x = mappend mempty x === x

sumMonoidSpec = it "mappend mempty x = x" $ property (prop_Monoid_mappend_mempty_x :: Sum Int -> Property)
productMonoidSpec = it "mappend mempty x = x" $ property (prop_Monoid_mappend_mempty_x :: Product Double -> Property)

main :: IO ()
main = hspec $ do
    describe "Data.Monoid.Sum" $ do
        sumMonoidSpec
    describe "Data.Monoid.Product" $ do
        productMonoidSpec

我想擁有的是多態的

monoidSpec = it "mappend mempty x = x" $ property prop_Monoid_mappend_mempty_x

然后指定實際的Monoid實例（Sum，Product）和類型（Int，Double）。 問題是它不會進行類型檢查。 我不斷

src/Test.hs@18:42-18:50 No instance for (Arbitrary a0) arising from a use of property
The type variable a0 is ambiguous
Note: there are several potential instances:
  instance Arbitrary a => Arbitrary (Product a)
    -- Defined at /home/app/isolation-runner-work/projects/68426/session.207/src/src/Test.hs:10:10
  instance Arbitrary a => Arbitrary (Sum a)
    -- Defined at /home/app/isolation-runner-work/projects/68426/session.207/src/src/Test.hs:7:10
  instance Arbitrary () -- Defined in Test.QuickCheck.Arbitrary
  ...plus 27 others …
src/Test.hs@18:51-18:79 No instance for (Monoid a0)
  arising from a use of prop_Monoid_mappend_mempty_x
The type variable a0 is ambiguous
Note: there are several potential instances:
  instance Monoid () -- Defined in Data.Monoid
  instance (Monoid a, Monoid b) => Monoid (a, b)
    -- Defined in Data.Monoid
  instance (Monoid a, Monoid b, Monoid c) => Monoid (a, b, c)
    -- Defined in Data.Monoid
  ...plus 18 others …

我知道我需要在多態版本中將Monoid約束為Arbitrary，Eq和Show，但我不知道如何。

問題是如何以多態方式表達Monoid的規范並避免代碼重復？

Answer 1

注意property :: Testable prop => prop -> Property的類型property :: Testable prop => prop -> Property 。 類型var prop被刪除，並且如果類型變量不再可用，則實例解析無法進行。 基本上，您要執行的是延遲實例選擇，並且必須在選擇實例之前使類型可用。

一種方法是攜帶一個額外的Proxy prop參數：

-- Possibly Uuseful helper function
propertyP :: Testable prop => Proxy prop -> prop -> Property 
propertyP _ = property 

monoidProp :: forall m . (Arbitrary m, Testable m, Show m, Monoid m, Eq m) 
           => Proxy m -> Property 
monoidProp _ = property (prop_Monoid_mappend_mempty_x :: m -> Property)

monoidSpec :: (Monoid m, Arbitrary m, Testable m, Show m, Eq m) => Proxy m -> Spec
monoidSpec x = it "mappend mempty x = x" $ monoidProp x 

main0 :: IO ()
main0 = hspec $ do
    describe "Data.Monoid.Sum" $ do
        monoidSpec (Proxy :: Proxy (Sum Int))
    describe "Data.Monoid.Product" $ do
        monoidSpec (Proxy :: Proxy (Product Double))

另一種方法是使用一個庫等tagged ，其提供的類型Tagged ，這只是增加了一些幻象類型參數到現有類型：

import Data.Tagged

type TaggedProp a = Tagged a Property 
type TaggedSpec a = Tagged a Spec 

monoidPropT :: forall a. (Monoid a, Arbitrary a, Show a, Eq a) 
            => TaggedProp a
monoidPropT = Tagged (property (prop_Monoid_mappend_mempty_x :: a -> Property))

monoidSpecT :: forall a . (Monoid a, Arbitrary a, Show a, Eq a) => TaggedSpec a
monoidSpecT = Tagged $ it "mappend mempty x = x" 
                          (unTagged (monoidPropT :: TaggedProp a))

main1 :: IO ()
main1 = hspec $ do
    describe "Data.Monoid.Sum" $ do
        untag (monoidSpecT :: TaggedSpec (Sum Int))
    describe "Data.Monoid.Product" $ do
        untag (monoidSpecT :: TaggedSpec (Product Double))

這些解決方案基本上是等效的，盡管在某些情況下，一種或另一種可能會更方便。 由於我對您的用例了解不足，因此將兩者都包括在內。

這兩個-XScopedTypeVariables需要-XScopedTypeVariables 。