Question

我使用hspec和QuickCheck验证Functor实例的仿函数法则。我有功能

functorIdentity :: (Functor f, Eq (f a)) => f a -> Bool

和

functorComposition :: (Functor f, Eq (f c)) => (Fun a b) -> (Fun b c) -> f a -> Bool

然后我使用这样的代码块测试这两个：

testListFunctorness :: IO ()
testListFunctorness =
  hspec $ do
    describe "list" $ do
      it "should obey functor identity" $ do
        property (functorIdentity :: [Int] -> Bool)
      it "should obey functor composition" $ do
        property
          (functorComposition :: (Fun Int String) -> (Fun String Int) -> [Int] -> Bool)

问题是，为了测试不同Functor实例的相同属性，我需要复制除[Int]之外的所有内容：

testMaybeFunctorness :: IO ()
testMaybeFunctorness =
  hspec $ do
    describe "maybe" $ do
      it "should obey functor identity" $ do
        property (functorIdentity :: Maybe Int -> Bool)
      it "should obey functor composition" $ do
        property
          (functorComposition :: (Fun Int String) -> (Fun String Int) -> Maybe Int -> Bool)

感觉我应该能够编写一个在不同Functor个实例中具有某种多态性的表达式，但我甚至无法想到如何开始这样做。

如何方便地将该测试逻辑块重用于多个不同的Functor？

Answer 1

您可以做的是明确将所需类型传递给testFunctorness：

import Data.Proxy

testFunctorness :: forall a. Functor a => Proxy a -> IO ()
testFunctorness _ =
  hspec $ do
    describe "the type" $ do
      it "should obey functor identity" $ do
        property (functorIdentity :: a Int -> Bool)
      it "should obey functor composition" $ do
        property
          (functorComposition :: (Fun Int String) -> (Fun String Int) -> a Int -> Bool)

通话看起来像testFunctorness (Proxy :: Proxy [])。

您需要启用{-# LANGUAGE ScopedTypeVariables #-}，以便函数内的a引用类型签名中的a。然后需要forall让typechecker知道这个a应该是词法范围的。

键入构造函数作为表达式的参数

1 个答案: