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任意类型类的约束中的非类型变量参数

parametric-polymorphism typeclass polymorphism haskell

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yhylord · 技术社区 · 6 年前

第15章的练习 Haskell编程从第一原理 ,我正在写一个 Arbitrary 基于另一个实例 任意的 实例:

module AccumulateRight where

import Data.Semigroup
import Test.QuickCheck

data Validation a b = Fail a | Pass b deriving (Eq, Show)

newtype AccumulateRight a b =
    AccumulateRight (Validation a b) deriving (Eq, Show)

type TestType = AccumulateRight String [Int]

instance Semigroup b => Semigroup (AccumulateRight a b) where
    _ <> (AccumulateRight (Fail x)) = Fail x
    (AccumulateRight (Fail x)) <> _ = Fail x
    (AccumulateRight (Success a)) <> (AccumulateRight (Success b)) =
        AccumulateRight . Success $ a <> b

instance (Arbitrary a, Arbitrary b) => Arbitrary (Validation a b) where
    arbitrary = oneof [Fail <$> arbitrary, Pass <$> arbitrary]

instance Arbitrary (Validation a b) => Arbitrary (AccumulateRight a b) where
    arbitrary = AccumulateRight <$> arbitrary

semigroupAssoc :: (Eq m, Semigroup m) => m -> m -> m -> Bool
semigroupAssoc a b c = (a <> (b <> c)) == ((a <> b) <> c)

type Assoc = TestType -> TestType -> TestType -> Bool

main :: IO ()
main = quickCheck (semigroupAssoc :: Assoc)

但出现以下错误:

    â¢ Non type-variable argument
        in the constraint: Arbitrary (Validation a b)
      (Use FlexibleContexts to permit this)
    â¢ In the context: Arbitrary (Validation a b)
      While checking an instance declaration
      In the instance declaration for âArbitrary (AccumulateRight a b)â
   |
22 | instance Arbitrary (Validation a b) => Arbitrary (AccumulateRight a b) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.

所以我在这里做错什么了吗?为什么不能在这里使用现有数据的typeclass作为约束?

1 回复 | 直到 6 年前

1

4

Daniel Wagner 6 年前

这是一个愚蠢的限制,在人们理解typeclass实现起来有多困难之前就已经实施了。结果证明它很容易支持,所以有一个语言扩展——在错误中提到的——让您这么说。您可以通过添加

{-# LANGUAGE FlexibleContexts #-}

到文件的顶部,作为扩展名,这个文件被认为是完全良性的。但是,在这种情况下,您不应该打开它,而应该只写

instance (Arbitrary a, Arbitrary b) => Arbitrary (AccumulateRight a b)

——毕竟, (Arbitrary a, Arbitrary b) 在什么条件下 Arbitrary (Validation a b) 持有。