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我正在尝试使用 HSpec 和 QuickCheck 来验证 Monoids 的属性（关联性和标识元素）。我将验证特定实例，但希望保持大部分代码多态。这是我几个小时后想出的：

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

我想要的是多态的

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

并稍后指定实际的 Monoid 实例（Sum、Product）和类型（Int、Double）。问题是它不会进行类型检查。我不断得到

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 …

我知道我需要将多态版本中的 Monoid 约束为 Arbitrary、Eq 和 Show，但我不知道如何。

问题是如何以多态的方式表达 Monoid 的规范并避免代码重复？

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1 回答 1

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注意 的类型property :: Testable prop => prop -> Property。类型 varprop被删除，如果类型变量不再可用，则无法进行实例解析。基本上你想要做的是推迟实例选择，并且要做到这一点，你必须使类型可用，直到你选择实例的那一刻。

一种方法是携带一个额外的Proxy prop参数：

-- 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))

另一种方法是使用tagged提供 type 之类的库Tagged，它只是将一些幻像类型参数添加到现有类型：

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))

这些解决方案本质上是等效的，尽管在某些情况下，其中一种可能更方便。由于我对您的用例了解不足，因此我将两者都包括在内。

这两个都只需要-XScopedTypeVariables.

于 2014-10-17T18:02:20.640 回答