我正在学习 Monad Transformers,其中一个练习要求实现 Monad 实例StateT
。
我想使用以下方法测试我的实现是否符合 Monad 法则validity https://github.com/NorfairKing/validity包,就像checkers
包裹。
问题是,我的Arbitrary
实例无法编译。我看见这个问题 https://stackoverflow.com/a/43415006/839733,但它并没有完全达到我想要的效果,因为测试基本上重复了实现并且不检查法律。
还有这个问题 https://stackoverflow.com/q/38930234/839733,但还没有答案,而且我已经想出了如何测试不涉及函数的 Monad Transformers(比如MaybeT
).
{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE InstanceSigs #-}
module Ch11.MonadT (StT (..)) where
import Control.Monad.Trans.State (StateT (..))
newtype StT s m a = StT (s -> m (a, s))
deriving
(Functor, Applicative)
via StateT s m
instance (Monad m) => Monad (StT s m) where
return :: a -> StT s m a
return = pure
(>>=) :: StT s m a -> (a -> StT s m b) -> StT s m b
StT x >>= f = StT $ \s -> do
(k, s') <- x s
let StT y = f k
y s'
(>>) :: StT s m a -> StT s m b -> StT s m b
(>>) = (*>)
My test:
{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeApplications #-}
module Ch11.MonadTSpec (spec) where
import Ch11.MonadT (StT (..))
import Test.Hspec
import Test.QuickCheck
import Test.Validity.Monad
spec :: Spec
spec = do
monadSpecOnArbitrary @(StTArbit Int [] Int)
-- create wrapper to avoid orphan instance error
newtype StTArbit s m a = StTArbit (StT s m a)
deriving
(Functor, Applicative, Monad)
instance (Arbitrary s, Function s, Arbitrary1 m, Arbitrary a) => Arbitrary (StTArbit s m a) where
arbitrary = do
f <- arbitrary :: Fun s (m (a, s))
StTArbit . StT <$> f
Error:
• Couldn't match type: (a0, s0)
with: s -> m (a, s)
Expected: Gen (s -> m (a, s))
Actual: Gen (a0, s0)
• In the second argument of ‘(<$>)’, namely ‘f’
In a stmt of a 'do' block: StTArbit . StT <$> f