haskell - Understanding `System.ZMQ4.Monadic`'s `forall` in Signature -

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looking @ runzmq system.zmq4.monadic:

what meaning of type signature?

λ: :t runzmq  runzmq   :: transformers-0.4.2.0:control.monad.io.class.monadio m =>      (forall z. zmq z a) -> m

in particular, don't understand forall.

from zqm docs:

the zmq monad modeled after st , encapsulates context. uses uninstantiated type variable z distinguish different invoctions of runzmq , prevent unintented use of sockets outside scope. cf. paper of john launchbury , simon peyton jones lazy functional state threads.

so z parameter acts s parameter in st s a.

one explanation of st monad haskell wiki:

https://wiki.haskell.org/monad/st


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