My inability to comprehend Monads

Version1

module Main where

import System.ZMQ4.Monadic (ZMQ, Socket, socket, bind, send, runZMQ, liftIO, Push(..))
import System.ZMQ4 (Sender)

import Data.ByteString.Char8 (pack)

run :: System.ZMQ4.Sender t => Socket z t -> ZMQ z ()
run publisher =
    mapM_ ((send publisher []) . pack) urls

    where
        urls        = [generateUrl x | x <- [0,100..]]
        generateUrl :: Integer -> String
        generateUrl i = "http://www.imdb.com/search/title?count=100&start=" ++ show i

main :: IO ()
main =
     runZMQ $ do
            publisher <- socket Push
            bind publisher "tcp://*:10150"

            run publisher

            liftIO $ putStrLn "Sent urls"

Version2

module Main where

import System.ZMQ4.Monadic (ZMQ, Socket, socket, bind, send, runZMQ, liftIO, Push(..))
import System.ZMQ4 (Sender)

import Data.ByteString.Char8 (pack)

run :: System.ZMQ4.Sender t => Socket z t -> ZMQ z ()
run publisher =
    mapM_ ((send publisher []) . pack) urls

    where
        urls        = [generateUrl x | x <- [0,100..]]
        generateUrl :: Integer -> String
        generateUrl i = "http://www.imdb.com/search/title?count=100&start=" ++ show i

openSender :: System.ZMQ4.Sender t => Int -> Socket z t -> ZMQ z ()
openSender = undefined

main :: IO ()
main =
     runZMQ $ do
            run $ openSender 10150

            liftIO $ putStrLn "Sent urls"


{-

Compiler error:

src/Main.hs:23:19:
    Couldn't match expected type ‘Socket z t0’
                with actual type ‘Socket z0 t1 -> ZMQ z0 ()’
    Probable cause: ‘openSender’ is applied to too few arguments
    In the second argument of ‘($)’, namely ‘openSender 10150’
    In a stmt of a 'do' block: run $ openSender 10150

-}

Version3

module Main where

import System.ZMQ4.Monadic
import Data.ByteString.Char8 (pack)

run :: Sender t => Socket z t -> ZMQ z ()
run publisher =
    mapM_ ((send publisher []) . pack) urls

    where
        urls        = [generateUrl x | x <- [0,100..]]
        generateUrl :: Integer -> String
        generateUrl i = "http://www.imdb.com/search/title?count=100&start=" ++ show i

openSender :: Int -> ZMQ z (Socket z t)
openSender port = undefined

main :: IO ()
main =
     runZMQ $ do
            run =<< openSender 10150
            liftIO $ putStrLn "Sent urls"


{-

But this generates a compiler error:

src/Main.hs:21:13:
    No instance for (Sender t0) arising from a use of ‘run’
    The type variable ‘t0’ is ambiguous
    Note: there are several potential instances:
      instance Sender Dealer -- Defined in ‘System.ZMQ4’
      instance Sender Pair -- Defined in ‘System.ZMQ4’
      instance Sender Pub -- Defined in ‘System.ZMQ4’
      ...plus 7 others

Maybe this is because we left the openSender function undefined?

-}