Check whether a port is open in Haskell - OUTDATED; newer version here: https://gist.github.com/nh2/0a1442eb71ec0405a1e3ce83a467dfde

IsPortOpen.hs

{-# LANGUAGE ScopedTypeVariables #-}

module IsPortOpen where

import           Control.Exception (bracket, catchJust)
import           Network (PortID(PortNumber), PortNumber, connectTo)
import           System.IO (hClose)
import           System.IO.Error (ioeGetErrorType, isDoesNotExistErrorType)


isPortOpen :: String -> PortNumber -> IO Bool
isPortOpen host portNo = do
  let connectOrThrow :: IO ()
      connectOrThrow =
        bracket
          (connectTo host (PortNumber portNo))
          (hClose)
          (\_ -> return ())

  catchJust
    (\(err :: IOError) ->
      -- Unfortunately IOError doesn't give us the exact `errno` so we
      -- cannot precisely match `ECONNREFUSED`.
      -- `isDoesNotExistErrorType` is the closest Haskell approximation
      -- because `ECONNREFUSED` gets mapped to `NoSuchThing` which
      -- gets matched with `isDoesNotExistErrorType`.
      if isDoesNotExistErrorType (ioeGetErrorType err) then Just () else Nothing)
    (connectOrThrow >> return True) -- on success
    (\() -> return False) -- on failure

This example is outdated now because it uses the deprecated Socket module.

See this newer version for a better version, that also solves the Unfortunately IOError doesn't give us the exact errno problem.