haskell - 理解签名中 `System.ZMQ4.Monadic` 的 `forall`

Question

查看来自 System.ZMQ4.Monadic 的 runZMQ :

这个类型签名是什么意思？

λ: :t runZMQ 
runZMQ
  :: transformers-0.4.2.0:Control.Monad.IO.Class.MonadIO m =>
     (forall z. ZMQ z a) -> m a

特别是，我不理解forall。

Answer 1

来自ZQM docs :

The ZMQ monad is modeled after ST and encapsulates a Context. It uses the uninstantiated type variable z to distinguish different invoctions of runZMQ and to prevent unintented use of Sockets outside their scope. Cf. the paper of John Launchbury and Simon Peyton Jones Lazy Functional State Threads.

所以 z 参数的作用就像 ST s a 中的 s 参数一样。

Haskell Wiki 中对 ST monad 的一种解释:

https://wiki.haskell.org/Monad/ST