Verb the Noun generator in Haskell

vtn.hs

import Data.Char (toUpper)
import Control.Monad (liftM, mapM_)
import System.Random

readLines :: String -> IO [String]
readLines = liftM lines . readFile

boundaries :: [a] -> (Int, Int)
boundaries xs = (0, length xs - 1)

randomElement :: StdGen -> [a] -> (StdGen, a)
randomElement generator xs = let (index, generator') = randomR (boundaries xs) generator
                                 x = xs !! index
                                 in (generator', x)

randomPair :: StdGen -> [a] -> [b] -> (StdGen, (a, b))
randomPair generator xs ys = let (generator', x) = randomElement generator xs
                                 (generator'', y) = randomElement generator' ys
                                 in (generator'', (x, y))

randomPairs :: StdGen -> [a] -> [b] -> [(a, b)]
randomPairs generator xs ys = (x, y) : morePairs
                              where (generator', (x, y)) = randomPair generator xs ys
                                    morePairs = randomPairs generator' xs ys
 
title :: String -> String
title (x:xs) = (toUpper x):xs

main = do
  verbs <- readLines "verbs.txt"
	nouns <- readLines "nouns.txt"
	generator <- newStdGen
	let pairs = take 100 $ randomPairs generator verbs nouns
	mapM_ (\(verb, noun) -> putStrLn $ (title verb) ++ " the " ++ (title noun)) pairs