Using the Reader monad with QuickCheck / monadicIO

Question

I'd like to pass an integer as a CLI argument to a Haskell program that makes use of QuickCheck / monadicIO. That integer is going to be used inside the assert to make the tests customizable. The problem is that once I parse the integer value in main, I don't know how to pass it inside of the monadicIO call without using something as ugly as an IORef. I would think that an elegant solution might be the Reader monad, but I couldn't find a solution to make it work, seen as quickCheck is rigid in its arguments. Any ideas?

Later Edit 1: As requested, I'm attaching the actual code I'm trying this on, and failing. The commented-out lines represent my failed attempt. Background: the test suite is intended to exercise a very simple remote endpoint that computes the SHA512 of the randomized input generated by QuickCheck. The remote endpoint is Python/Flask based.

Later Edit 2 in response to @user2407038: I could make propHasExpectedLengthCeiling take an additional argument of type Int, but quickCheck would generate random values for it, and that's not what I want happening. My goal is to use the maxSegmentLengthCeiling that I'm taking in from the command-line arguments and use it in let testPassed = actualMaxSegmentLength <= maxSegmentLengthCeiling inside of the monadicIO block. Right now maxSegmentLengthCeiling is specified as a top-level value, which means I have to recompile the code every time I change the value. I don't yet have any code that involves IORef because that's a last resort and the essence of my question is how to avoid going the IORef route.

import qualified Data.ByteString.Lazy.Char8 as LC

import Control.Applicative     ( (<$>) )
import Data.Function           ( on )
import Data.List               ( groupBy )
import Data.Char               ( isDigit )
--import Safe                    ( headMay
--                               , readMay
--                               )
--import System.Environment      ( getArgs )
import Network.HTTP.Conduit    ( simpleHttp )
import Test.QuickCheck         ( Arbitrary
                               , Property
                               , arbitrary
                               , choose
                               , frequency
                               , quickCheckWith
                               , stdArgs
                               , vectorOf
                               )
import Test.QuickCheck.Test    ( Args
                               , maxSuccess
                               )
import Test.QuickCheck.Monadic ( assert
                               , monadicIO
                               , run
                               )

newtype CustomInput = MkCustomInput String deriving Show

instance Arbitrary CustomInput where
  arbitrary =
    let
      genCustomInput = vectorOf 20
                       $ frequency [ (26, choose ('0','9'))
                                   , (10, choose ('a','z'))
                                   ]
    in
      MkCustomInput <$> genCustomInput

maxSegmentLengthCeiling :: Int
maxSegmentLengthCeiling = 22

urlPrefix :: String
urlPrefix = "http://192.168.2.3:5000/sha512sum/"

propHasExpectedLengthCeiling :: CustomInput -> Property
propHasExpectedLengthCeiling (MkCustomInput input) = monadicIO $ do
  testPassed <- run $ do
    response <- simpleHttp $ urlPrefix ++ input
    let stringResponse = LC.unpack response
    let brokenDownStringResponse = groupBy ( (==) `on` isDigit ) stringResponse
    let actualMaxSegmentLength = maximum $ map length brokenDownStringResponse
    let testPassed = actualMaxSegmentLength <= maxSegmentLengthCeiling
    putStrLn ""
    putStrLn ""
    putStrLn $ "Input:       " ++ input
    putStrLn $ "Control sum: " ++ stringResponse
    putStrLn $ "Breakdown:   " ++ show brokenDownStringResponse
    putStrLn $ "Max. length: " ++ show actualMaxSegmentLength
    putStrLn $ "Ceiling:     " ++ show maxSegmentLengthCeiling
    putStrLn $ "Test result: " ++ if testPassed then "Pass" else "Fail"
    putStrLn ""
    putStrLn ""
    return testPassed
  assert $ testPassed

customArgs :: Args
customArgs = stdArgs { maxSuccess = 1000000 }

--readMayAsInt :: String -> Maybe Int
--readMayAsInt = readMay

main :: IO ()
main =
--main = do
--  cliArgs <- getArgs
--  let ceilingInputMay = headMay cliArgs >>= readMayAsInt
--  maxSegmentLengthCeiling <- case ceilingInputMay of
--                               (Just lengthCeiling) -> return lengthCeiling
--                               Nothing              -> error "No valid number given"
  quickCheckWith
    customArgs
    propHasExpectedLengthCeiling

Answer 1

Make maxSegmentLengthCeiling a parameter to propHasExpectedLengthCeiling :

propHasExpectedLengthCeiling :: Int -> CustomInput -> Property

and invoke it as

main = do 
  [n] <- getArgs
  quickCheckWith customArgs (propHasExpectedLengthCeiling (read n))