这种runST的推广是否安全？

Question

Control.Monad.ST在base包中包含runST运行严格的状态变换器monad:

runST :: (forall s. ST s a) -> a

但是,我需要一个通用版本runST:

runSTCont :: (forall s . (forall b . ST s b -> b) -> a) -> a
runSTCont f = f $ \m -> runST $ unsafeCoerce m

我的问题是:这是否unsafeCoerse安全使用？(我想是的,因为据我所知,索引的唯一目的s是防止s在结果中泄漏索引值a.runSTCont不能泄漏s索引值的类型,所以应该没问题.)

请注意,runST在来表示runSTCont这样runSTCont至少一般为runST:

runST' :: (forall s. ST s a) -> a 
runST' m = runSTCont $ \runST -> runST m

Answer 1

我不这么认为:

crash = runSTCont crasher where
  crasher :: forall s. (forall b . ST s b -> b) -> Integer
  crasher go =
    let r :: forall a. STRef s (a -> a)
        r = go $ newSTRef id
    in go $ do writeSTRef r (tail . tail)
               f <- readSTRef r
               return $ f (17 :: Integer)

问题是Haskell缺乏价值限制.