为什么不具有不可预测的类型允许异类列表？

Question

我有一个简短的例子：

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ImpredicativeTypes #-}

main = mapM_ print ([1 :: Int, "str", Just 5] :: [forall a. Show a => a])

这有两个原因失败（产生4个错误，但每个元素中有3个错误）：

No instance for (Show (forall a. Show a => a))
  arising from a use of `print'

这是非常令人费解的（我也不理解），另一个：

Could not deduce (a ~ Int)
from the context (Show a)
  bound by a type expected by the context: Show a => a
  at main.cpp:5:18-42
  `a' is a rigid type variable bound by
      a type expected by the context: Show a => a at main.cpp:5:18
In the expression: 1 :: Int

这似乎将上下文强化为Show a => a。为什么它不起作用？ Aren的impredicative类型基本上是这个的门户吗？