我开始理解forall关键字是如何在所谓的“存在类型”中使用的，比如:

data ShowBox = forall s. Show s => SB s

然而，这只是forall用法的一个子集，我根本无法理解它在这样的情况下的用法:

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

或者解释为什么它们不同:

foo :: (forall a. a -> a) -> (Char, Bool)
bar :: forall a. ((a -> a) -> (Char, Bool))

或者整个RankNTypes的东西…

我倾向于使用清晰、无专业术语的英语，而不是学术环境中常见的语言。我试图阅读的大部分解释(我可以通过搜索引擎找到的)都有这些问题:

They're incomplete. They explain one part of the use of this keyword (like "existential types") which makes me feel happy until I read code that uses it in a completely different way (like runST, foo and bar above). They're densely packed with assumptions that I've read the latest in whatever branch of discrete math, category theory or abstract algebra is popular this week. (If I never read the words "consult the paper whatever for details of implementation" again, it will be too soon.) They're written in ways that frequently turn even simple concepts into tortuously twisted and fractured grammar and semantics.

所以…

回到真正的问题上来。有谁能用清楚明了的英语完全解释forall关键字(或者，如果它存在的话，指出一个我错过的如此清晰的解释)，而不假设我是一个沉浸在行话中的数学家吗?