Question

对于使用QuickCheck，我正在尝试实现任意实例：

import Test.QuickCheck
import Test.QuickCheck.Arbitrary (class Arbitrary, class Coarbitrary)

data Arith = Lit Int | Add Arith Arith | Neg Arith

derive instance arithEq :: Eq Arith

instance arbitraryFoo :: Arbitrary Arith where
    arbitrary = arbLit <|> arbAdd <|> arbNeg where
        arbAdd = do
            a <- arbitrary
            b <- arbitrary
            pure $ Add a b
        arbNeg = do
            a <- arbitrary
            pure $ Neg a
        arbLit = do
            x <- arbitrary
            pure $ Lit x

但是PureScript告诉了我

The value of arbitraryFoo is undefined here, so this reference is not allowed.

当我根据错误消息更改代码以遵循指南时：

import Test.QuickCheck
import Test.QuickCheck.Arbitrary (class Arbitrary, class Coarbitrary)
import Control.Lazy

instance arbitraryFoo :: Arbitrary Arith where
    arbitrary = fix $ \arb -> let
        arbAdd = do
            a <- arb
            b <- arb
            pure $ Add a b
        arbNeg = do
            a <- arb
            pure $ Neg a
        arbLit = do
            x <- arbitrary
            pure $ Lit x
        in arbLit <|> arbAdd <|> arbNeg

我仍然有一些错误：

RangeError: Maximum call stack size exceeded

那么，我该如何解决？以及为什么Arbitrary无法自动导出？

Answer 1

为防止其生成无限递归结构，您应使生成器sized，然后使resize乘-1，每次使其递归使用。

以下是使用该技术的JSON生成器的示例：https://github.com/purescript-contrib/purescript-argonaut-core/blob/37fc57c722851b1936c438db750aa8195dc630c7/src/Data/Argonaut/Gen.purs。尽管Gen是用MonadGen抽象的，但是如果您想实现Arbitrary，则可以应用相同的原理。

如何在PureScript中快速检查自定义ADT？

1 个答案: